LTC4020
1
4020fd
For more information www.linear.com/LTC4020
VIN (V)
5
EFFICIENCY (%)
POWER (W)
100 100
90
80
70
60
50
40
30
20
10
0
95
90
85
80
75 10 15
4020 TA01b
20 3025
VOUT = 14V
EFFICIENCY
INPUT POWER
P(LOSS)
Typical applicaTion
FeaTures DescripTion
55V Buck-Boost
Multi-Chemistry
Battery Charger
The LT C
®
4020 is a high voltage power manager provid-
ing PowerPath™ instant-on operation and high efficiency
battery charging over a wide voltage range. An onboard
buck-boost DC/DC controller operates with battery and/or
system voltages above, below, or equal to the input voltage.
The LTC4020 seamlessly manages power distribution
between battery and converter outputs in response to load
variations, battery charge requirements and input power
supply limitations.
The LTC4020 battery charger can provide a constant-current/
constant-voltage charge algorithm (CC/CV), constant-
current charging (CC), or charging with an optimized 4-step,
3-stage lead-acid battery charge profile. Maximum converter
and battery charge currents are resistor programmable.
The IC's instant-on operation ensures system load power
even with a fully discharged battery. Additional safety
features include preconditioning for heavily discharged
batteries and an integrated timer for termination and
protection.
applicaTions
L, LT , LT C , LT M , Linear Technology and the Linear logo are registered trademarks and
PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents, including 7583113 and 8405362.
n Wide Voltage Range: 4.5V to 55V Input, Up to 55V
Output (60V Absolute Maximums)
n Synchronous Buck-Boost DC/DC Controller
n Li-Ion and Lead-Acid Charge Algorithms
n ±0.5% Float Voltage Accuracy
n ±5% Charge Current Accuracy
n Instant-On for Heavily Discharged Batteries
n Ideal Diode Controller Provides Low Loss
PowerPath When Input Power is Limited
n Input Voltage Regulation for High Impedance Input
Supplies and Solar Panel Peak Power Operation
n Onboard Timer for Protection and Termination
n Bad Battery Detection with Auto-Reset
n NTC Input for Temperature Qualified Charging
n Binary Coded Open-Collector Status Pins
n Low Profile (0.75mm) 38-Pin 5mm × 7mm QFN
Package
n Portable Industrial and Medical Equipment
n Solar-Powered Systems
n Military Communications Equipment
n 12V to 24V Embedded Automotive Systems
Buck-Boost DC/DC Converter Controller with PowerPath Battery Charger Accepts
Inputs from 4.5V to 55V and Produces Output Voltages Up to 55V
5V to 30V 6-Cell Lead-Acid Supply/Charger
Maximum Power Efficiency vs VIN
(Application Circuit on Page 37)
LTC4020
RNTC
RSENSEB
RSENSEA
BUCK-BOOST
DC/DC CONVERTER PowerPath BATTERY CHARGER
4020 TA01a
VIN VOUT
RCS
BG1
TG1
SENSBOT
SENSTOP
SENSVIN
BG2
TG2
VFBMAX
CSP
CSN
BGATE
VFB
NTC
SENSGND
LTC4020
2
4020fd
For more information www.linear.com/LTC4020
pin conFiguraTionabsoluTe MaxiMuM raTings
PVIN, SENSVIN ............................................. –0.3 to 60V
BST1, BST2 .................................................. –0.3 to 66V
SW1, SW2 ........................................................–2 to 60V
SENSVIN – PVIN ........................................... –0.3 to 60V
BST1 – SW1, BST2 – SW2 ............................. –0.3 to 6V
SENSVIN – SENSTOP, SENSBOT –
SENSGND ....................................................–0.3 to 0.3V
CSP, CSN ..................................................... –0.3 to 60V
CSP – CSN ...................................................–0.3 to 0.3V
STAT1, STAT2, SHDN ................................... –0.3 to 60V
VFBMAX, VINREG, VFB, VFBMIN, BAT, FBG ....... –0.3 to 60V
MODE ............................................................. –0.3 to 6V
Status Pin Currents:
STAT1, STAT2 ......................................................5mA
Operating Junction Temperature
Range (Note 2) ....................................... –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
(Note 1)
13 14 15 16
TOP VIEW
39
SGND
UHF PACKAGE
38-LEAD PLASTIC QFN (5mm × 7mm)
17 18 19
38 37 36 35 34 33 32
24
25
26
27
28
29
30
31
8
7
6
5
4
3
2
1TG1
BST1
SGND
SENSGND
SENSBOT
SENSTOP
SENSVIN
RT
SHDN
VIN_REG
MODE
STAT1
TG2
BST2
SGND
VC
ITH
VFBMAX
ILIMIT
CSOUT
CSP
CSN
BGATE
BAT
SW1
BG1
PVIN
PGND
INTVCC
BG2
SW2
STAT2
TIMER
RNG/SS
NTC
VFB
FBG
VFBMIN
23
22
21
20
9
10
11
12
TJMAX = 125°C, θJA = 34°C/W, θJC = 2°C/W
EXPOSED PAD (PIN 39) IS SGND, MUST BE SOLDERED TO PCB
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC4020EUHF#PBF LTC4020EUHF#TRPBF 4020 38-Lead (5mm × 7mm) Plastic QFN –40°C to 125°C
LTC4020IUHF#PBF LTC4020IUHF#TRPBF 4020 38-Lead (5mm × 7mm) Plastic QFN –40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Buck-Boost Switching Converter
VIN Operating Voltage Range PVIN; SENSVIN l4.5 55 V
UVLO VIN Supply UVLO (Rising)
VIN Supply UVLO Hysteresis
DC/DC Functions Enabled
VIN Falling
l3.6 4.0
0.4
4.4 V
V
SENSVIN Supply UVLO (Rising)
SENSVIN UVLO Hysteresis
INTVCC Enabled
SENSVIN Falling
3.4
0.3
V
V
BST Supplies UVLO (Rising)
BST Supplies UVLO Hysteresis
BST1 – SW1; BST2 – SW2;
SW1, SW2 = 0V
l3.0 3.3
0.4
3.8 V
V
elecTrical characTerisTics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). PVIN = SENSVIN = CSP = CSN = BAT = 20V, SHDN = 2V,
C(TG1, BG1, TG2, BG2) = 1000pF, VRNG/SS = 2V.
orDer inForMaTion
(http://www.linear.com/product/LTC4020#orderinfo)
LTC4020
3
4020fd
For more information www.linear.com/LTC4020
elecTrical characTerisTics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). PVIN = SENSVIN = CSP = CSN = BAT = 20V, SHDN = 2V,
C(TG1, BG1, TG2, BG2) = 1000pF, VRNG/SS = 2V.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
INTVCC Boost Refresh Supply Voltage ILOAD = 5mA l4.85 5.0 5.15 V
Boost Refresh Supply Dropout PVIN = 4.5V; IINTVCC = 5mA 4.46 V
Boost Refresh Supply Short-Circuit Current
Limit
VINTVCC = 0V l85 150 mA
IPVIN PVIN Operating Current Note 3; ITH = 0V l1.6 3 mA
Shutdown Current VSHDN = 0 l3 6 µA
ISENSVIN SENSEVIN Operating Current l0.25 0.5 mA
Shutdown Current VSHDN = 0 l25 60 µA
ISENSTOP SENSETOP Operating Current 32.5 µA
Shutdown Current VSHDN = 0 0.1 µA
VFBMAX DC/DC Converter Reference Charging Terminated l2.7 2.75 2.8 V
SHDN IC Enable Threshold (Rising)
Threshold Hysteresis
l1.175 1.225
100
1.275 V
mV
SHDN Pin Bias Current 10 nA
VSENS DC/DC Converter Inductor Current Limit
(Average Value)
VSENSVIN – VSENSTOP;
VSENSGND – VSENSBOT
l45 50 60 mV
Reverse Current Inhibit
(Average Value)
VITH Falling (TG2 Disabled)
VITH Rising (TG2 Enabled)
l0 2
6
mV
mV
ILIMIT Inductor Current Limit Programming VILIMIT = 0.5V; VILIMIT/VSENS(MAX) 20 V/V
ILIMIT Pin Bias Current l47.5 50 52.5 µA
ITH Error Amp Current Limit VFBMAX = 0, VITH = 1.3V 8 µA
Error Amp Transconductance VFBMAX = 2.75V; VITH = 1.3V 95 umho
VC High Side Current Sense Transconductance (VSENSVIN – VSENSETOP) to IVC
VC = 1.8V
200 umho
Low Side Current Sense Transconductance (VSENSGND – VSENSEBOT) to IVC
VC = 1.8V
200 umho
DCMAX Maximum Duty Cycle BG2: tON • fOl70 80 %
fOSwitching Frequency RRT = 100k
RRT = 50k
RRT = 250k
l235 250
500
100
265 kHz
kHz
kHz
tON Minimum On Time BG2 l150 250 ns
tOFF Minimum Off Time TG1 l300 500 ns
tTR Gate Drive Transition Time TG1, BG1, TG2, BG2 5 ns
tNOL Gate Drive Non-Overlap time (TG1 – SW1) to BG1,
(TG2 – SW2) to BG2
75 ns
LTC4020
4
4020fd
For more information www.linear.com/LTC4020
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Battery Charger
VBAT Charger Output Voltage Range l55 V
VFB Float Reference
Auto Recharge Voltage
Precondition Threshold (Rising)
Precondition Hysteresis
CC/CV Charging (MODE = 0V)
% of Float Reference
% of Float Reference
l
l
l
2.4875
2.475
96.5
68
2.5
97.5
70
85
2.5125
2.525
98.5
72
V
V
%
%
mV
Absorption Reference
Float Reference
Bulk Charge Threshold (Falling)
Precondition Threshold (Rising)
Precondition Hysteresis
Lead-Acid Charging (MODE = INTVCC)
% of Absorption Reference
% of Absorption Reference
% of Absorption Reference
l
l
l
l
2.4875
2.475
91.5
86
68
2.5
92.5
87.5
70
85
2.5125
2.525
93.5
89
72
V
V
%
%
%
mV
Voltage Reference CC Charging (MODE = - NC -)
l
2.4875
2.475
2.5 2.5125
2.525
V
V
VIN_REG Input Regulation Reference % of Float (CC/CV), Safety (CC), or
Absorption (LA) Reference
l98 100 102 %
VFBMIN Instant-On Reference % of Float (CC/CV) or Absorption (LA)
Reference
l84 85 86 %
C/10 Detection Enable (Rising)
Hysteresis (Falling)
2.175
20
V
mV
Instant-On Charge Current Reduction
Threshold
VCSN – VBAT; Note 4 0.45 V
C/10 Detection Enable
C/10 Detection Hysteresis
VCSN – VBAT Falling
VCSN – VBAT Rising
1.05
150
V
mV
Charge Current Reduction Gain ΔVCS(MAX)/Δ(VCSN – VBAT);Note 4 –33 mV/V
IBATQ Battery Bias Currents with PowerPath
Switcher Disabled
ICSP + ICSN + IBAT l9 18 µA
CSN, CSP Charger Current Sense Pin Operating Bias
Currents
ICSP = ICSN; Charging Enabled 40 µA
Charger Current Sense Limit Voltage VCSP – VCSN l47.5 50 52.5 mV
Charger Current Sense Termination Voltage
(C/10)
VCSP – VCSN; MODE = 0V l3 5 7 mV
Charger Current Sense Precondition Voltage VCSP – VCSN; VFB = 1.5 l1.5 3 4.5 mV
Sense Input UVLO
UVLO Hysteresis
VCSP Rising (Charging Enabled)
VCSP Falling (Charging Disabled)
l1.6 1.75
100
1.9 V
mV
CSOUT Offset VCSP = VCSN l0.225 0.25 0.290 V
Gain ΔVCSOUT / Δ(VCSP – VCSN)l19 20 21 V/V
RNG/SS Current Limit Programming VRNG/SS = 0.5V; VRNG/SS/VCS(MAX) l18 20 22 V/V
NTC NTC Range Limit (High)
NTC Range Limit (Low)
NTC Range Hysteresis
VNTC Rising
VNTC Falling
% of VNTC(H,L)
l
l
1.30
0.27
1.35
0.3
20
1.40
0.33
V
V
%
INTC NTC Pin Bias Current VNTC = 0.8V l47.5 50 52.5 µA
NTC Disable Current INTC Pin Current (Falling) 3.5 µA
NTC Disable Current Hysteresis 2 µA
VBGATE Gate Clamp Voltage VCSN – VBGATE l7 9.5 12 V
C/10 Detection Enable (Falling)
C/10 Detection Enable Hysteresis
VCSN < 7V 0.425
0.125
V
V
elecTrical characTerisTics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). PVIN = SENSVIN = CSP = CSN = BAT = 20V, SHDN = 2V,
C(TG1, BG1, TG2, BG2) = 1000pF, VRNG/SS = 2V.
LTC4020
5
4020fd
For more information www.linear.com/LTC4020
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
BGATE BGATE Pull-Down Current Charging Enabled 15 µA
BGATE Pull-Up Current Charging Disabled; VCSN – VBGATE = 2V 15 µA
BGATE Standby Pull-Down Current VSHDN = 0V 120 µA
Ideal Diode Pull-Down Current Charging Disabled; VBAT – VCSN = 0.5V 500 µA
Ideal Diode Forward Voltage VBAT – VCSN; VCSN Measured Through
100Ω Series Resistor
l5 14 20 mV
TIMER Timer High Threshold 1.5 V
Timer Low Threshold 1.0 V
C/10 Mode Threshold (Rising) l0.4 0.5 0.6 V
C/10 Mode Hysteresis 225 mV
Timer Source/Sink Current VTIMER = 1.25V l8.5 10 11.5 µA
VSTAT(L) Status Pins Enabled Voltage ISTAT1 = 1mA; ISTAT2 = 1mA
ISTAT1 = 5mA; ISTAT2 = 5mA
l
l
0.15
0.75
0.4
2.5
V
V
IVFBMIN Instant-On Feedback Bias Current 10 nA
IVFB Feedback Pin Bias Current 10 nA
IVIN_REG Input Regulation Bias Current 10 nA
IFBG Pin Current (Disabled) VSHDN = 0V; VFBG = 55V 10 nA
RNTC NTC Minimum Disable Resistance l250 400 kΩ
RFBG FBG Resistance to SGND IFBG = 1mA l20 50 Ω
elecTrical characTerisTics
The l denotes the specifications which apply over the specified operating
junction temperature range, otherwise specifications are at TA = 25°C (Note 2). PVIN = SENSVIN = CSP = CSN = BAT = 20V, SHDN = 2V,
C(TG1, BG1, TG2, BG2) = 1000pF, VRNG/SS = 2V.
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC4020 is tested under pulsed load conditions such that
TJ ≈ TA. The LTC4020E is guaranteed to meet specifications from
0°C to 85°C junction temperature. Specifications over the –40°C to
125°C operating junction temperature range are assured by design,
characterization, and correlation with statistical process controls. The
LTC4020I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The junction temperature (TJ) is calculated from
the ambient temperature (TA) and power dissipation (PD) according
to the formula TJ = TA + (PD • θJA). Note that the maximum ambient
temperature consistent with these specifications is determined by specific
operating conditions in conjunction with board layout, the rated package
thermal resistance and other environmental factors. This IC includes
overtemperature protection that is intended to protect the device during
momentary overload. Junction temperature will exceed 125°C when
overtemperature protection is active. Continuous operation above the
specified maximum operating junction temperature may impair device
reliability.
Note 3: ICC does not include switching currents. VBST1 = VBST2 = VINTVCC
and VSW1 = VSW2 = 0V for testing.
Note 4: See Typical Performance Characteristics
LTC4020
6
4020fd
For more information www.linear.com/LTC4020
Typical perForMance characTerisTics
Shutdown Current vs Temperature
(IPVIN + ISENSVIN + ISENSTOP)
INTVCC Short-Circuit Current
Limit vs Temperature
Maximum Charge Current
(Percent of Programmed ICSMAX)
vs RNG/SS Voltage
Instant-On: Maximum Charge
Current (Percent of ICSMAX) vs
VCSN–BAT
IBATQ (IBAT + ICSN + ICSP) vs VBAT
PowerPath Switcher Disabled
IBATQ (IB AT + ICSN + ICSP) vs
Temperature PowerPath Switcher
Disabled
VFLOAT(CC/CV) or, VABSORB(Lead-Acid)
Reference vs Temperature
VFLOAT(Lead-Acid) Reference vs
Temperature VFBMAX Reference vs Temperature
TEMPERATURE (°C)
VOLTAGE (V)
2.80
2.79
2.78
2.77
2.76
2.75
2.74
2.73
2.72
2.71
2.70 50205–10–25–40 80 95 110
4020 G03
12535 65
TEMPERATURE (°C)
STANDBY MODE CURRENT (µA)
50
45
40
35
30
15
25
20
60–40–30–20–10 010 80 90
4020 G04
50403020 70
TEMPERATURE (°C)
INTVCC CURRENT LIMIT (mA)
155
154
153
152
151
150
149
148
147
146
145 50205–10–25–40 80 95 110
4020 G05
12535 65
RNG/SS VOLTAGE (V)
0
MAXIMUM CHARGE CURRENT (%)
100
90
70
50
80
60
40
30
20
10
00.4 0.8 1.0
4020 G06
1.20.2 0.6
VBAT (V)
0
IBATQ (µA)
11.0
10.5
10.0
8.0
9.5
9.0
8.5
7.0
6.5
7.5
6.0 10 30 4535 40 50
4020 G08
555 15 2520
TEMPERATURE (°C)
–40
VOLTAGE (V)
2.36
2.35
2.34
2.33
2.32
2.31
2.30
2.29
2.28
2.27
2.26
2.25 20–10 80 95 110
4020 G02
125535 50–25 65
TEMPERATURE (°C)
–40
VOLTAGE (V)
2.525
2.520
2.510
2.500
2.515
2.505
2.495
2.490
2.485
2.480
2.475 20 35 50–10 80 95 110
4020 G01
1255–25 65
VCSN–BAT (V)
0
MAXIMUM CHARGE CURRENT (%)
100
90
70
50
80
60
40
30
20
10
00.5 11.25
4020 G07
2.251.5 1.75 2.520.25 0.75
TEMPERATURE (°C)
IBATQ (µA)
11.0
10.5
10.0
9.5
8.0
9.0
8.5
60–40–30–20–10 010 80 90
4020 G09
50403020 70
VBAT = 20V
TA = 25°C, unless otherwise noted.
LTC4020
7
4020fd
For more information www.linear.com/LTC4020
Typical perForMance characTerisTics
Ideal Diode VF vs Battery Voltage Ideal Diode VF vs Temperature
IINTVCC (mA)
0
VINTVCC (V)
4.5
4.4
4.2
4.1
4.3
4.0 10 30 40
4020 G12
5020
25°C
125°C
PVIN = 4.5V
V(CSP-CSN) (mV)
0
VCSOUT (V)
1.3
1.2
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
1.1
0.2 10 30 40
4020 G13
5020
BST Refresh Regulator Dropout
INTVCC vs IINTVCC
CSOUT vs CSP-CSN
Switching Frequency vs Input
Voltage
Switching Frequency vs
Temperature
IRNG/SS, ILIMIT, INTC, vs
Temperature
Feedback References vs Input
Voltage INTVCC vs VIN
VIN (V)
5
FEEDBACK REFERENCES (V)
2.80
2.75
2.65
2.60
2.55
2.50
2.45
2.40
2.35
2.30
2.70
2.25 1510 3530 4540 50
4020 G14
552520
VFBMAX
VFLOAT(CC/CV;ABSORB)
VFLOAT(LEAD-ACID)
VIN (V)
5
VINTVCC (V)
5.10
5.08
5.04
5.02
5.00
4.98
4.96
4.94
4.92
5.06
4.90 1510 3530 4540 50
4020 G15
552520
IINTVCC = 5mA
TEMPERATURE (°C)
–40
IRNG/SS, ILIMIT, INTC (µA)
52.5
52.0
51.0
50.5
50.0
49.5
49.0
48.5
48.0
51.5
47.5 –25 5 20 35 50 65 80 11095
4020 G18
125–10
VBAT (V)
0
IDEAL DIODE VF (mV)
16.0
15.5
15.0
13.5
13.0
12.5
14.5
14.0
12.0 10 25 30 35 40 45 50
4020 G10
555 15 20
TEMPERATURE (°C)
–40
IDEAL DIODE VF (mV)
24
22
20
16
14
12
18
10 –10 35 50 65 80 95 110
4020 G11
125–25 5 20
VBAT = 20V
TA = 25°C, unless otherwise noted.
VIN (V)
0
SWITCHING FREQUENCY (kHz)
255
254
252
251
250
249
248
247
246
253
245 10 30 40 50
4020 G16
6020
RRT = 100kΩ
TEMPERATURE (°C)
–40
SWITCHING FREQUENCY (kHz)
255
254
252
251
250
249
248
247
246
253
245 –25 5 20 35 50 65 80 11095
4020 G17
125–10
VIN = 20V
RRT = 100kΩ
LTC4020
8
4020fd
For more information www.linear.com/LTC4020
pin FuncTions
TG1 (Pin 1): VIN side (step-down) primary switch FET
gate driver output.
BST1 (Pin 2): Boosted supply rail for VIN side (step-down)
switch FETs. Connect 1µF capacitor from this pin to SW1.
Connect 1A Schottky diode cathode to this pin, anode to
INTVCC pin.
SENSGND (Pin 4): Kelvin connection for PGND used for
SENSBOT current sense reference.
SENSBOT (Pin 5): Ground Referred Current Sense Amplifier
Input. Inductor current is monitored via a PGND referenced
current sense resistor (RSENSEB), typically in series with
the source of the VIN side synchronous switch FET. Kelvin
connect this pin to the associated sense resistor. Inductor
current is limited to a maximum average value (ILMAX), and
corresponds to 50mV across this sense resistor during
normal operation.
RSENSEB = 0.05/ILMAX
Set RSENSEB = RSENSEA, as described in SENSTOP pin
description. A filter capacitor (CSENSB) is typically con-
nected from SENSBOT to SENSGND for noise reduction.
CSENSB ~ 1nS/RSENSEB
See Applications Information section.
SENSTOP (Pin 6): VIN Referred Current Sense Amplifier
Input. Inductor current is monitored via a VIN referenced
current sense resistor (RSENSEA), typically in series with
the drain of the VIN side primary switch FET. Kelvin con-
nect this pin to the associated sense resistor. Inductor
current is limited to a maximum average value (ILMAX),
and corresponds to 50mV across this sense resistor dur-
ing normal operation.
RSENSEA = 0.05/ILMAX
Set RSENSEA = RSENSEB, as described in SENSBOT pin
description.
SENSVIN (Pin 7): Kelvin connection for input supply (VIN)
used for SENSTOP current sense reference. Input power
supply pin for most internal low current functions. Typical
pin bias current is 0.25mA.
RT (Pin 8): System Oscillator Frequency Control Pin.
Connect resistor (RRT) from this pin to ground. Resistor
value can range from 50kΩ (500khz) to 500kΩ (50khz).
RRT = 100kΩ yields 250kHz operating frequency. See
Applications Information.
SHDN (Pin 9): Precision Threshold Shutdown Pin. En-
able threshold is 1.225V (rising), with 100mV of input
hysteresis. When in shutdown, all charging functions are
disabled and input supply current is reduced to 27.5µA.
Typical SHDN pin input bias current is 10nA.
VIN_REG (Pin 10): Input Voltage Regulation Reference.
Battery charge current is reduced when the voltage on
this pin falls below 2.5V. Connecting a resistor divider
from VIN to this pin enables programming of minimum
operational VIN voltage for the battery charging function.
This is used to program the peak power voltage for a
solar panel, or to help maintain a minimum voltage on
a poorly regulated input supply. This pin should not be
used to program minimum operational VIN voltage with
low impedance supplies. Should the input supply begin to
collapse, the LTC4020 reduces the DC/DC converter input
power such that programmed minimum VIN operational
voltage is maintained. If the voltage regulation feature is
not used, connect the VIN_REG pin to VIN or INTVCC. Typical
VIN_REG pin input bias current is 10nA. See Applications
Information.
MODE (Pin 11): Charger Mode Control Pin. Short this pin
to ground to enable a constant-current/constant-voltage
(CC/CV) charging algorithm. Connect this pin to pin INTVCC
to enable a 4-step, 3-stage lead-acid charging algorithm.
Float this pin to force a constant-current (CC) charging
function. See Applications Information section.
STAT1 (Pin 12): Open-collector status output, typically
pulled up through a resistor to a supply voltage. This status
pin can be pulled up to voltages as high as 55V when the
pin is disabled, and can sink currents up to 1mA when logic
low (<0.4V). Pull down currents as high as 5mA (absolute
maximum) are supported for higher current applications,
such as lighting LEDs.
If the LTC4020 is configured for a CC/CV charging algo-
rithm, the STAT1 pin is pulled low while battery charge
currents exceed 10% of the programmed maximum (C/10).
The STAT1 pin is also pulled low during NTC faults. The
STAT1 pin becomes high impedance when a charge cycle
LTC4020
9
4020fd
For more information www.linear.com/LTC4020
is terminated or when charge current is below the C/10
threshold.
If the LTC4020 is configured for a CC charging algorithm,
the STAT1 pin is pulled low during the entire charging
cycle. The STAT1 pin becomes high impedance when the
charge cycle is terminated.
If the LTC4020 is configured for a lead-acid charging
algorithm, the STAT1 pin is used as a charge cycle stage
indicator pin, and pulled low during the bulk and absorp-
tion charging stages. The pin is high impedance during the
float charging period and during NTC or bad battery faults.
See Applications Information section.
STAT2 (Pin 13): Open-collector status output, typically
pulled up through a resistor to a supply voltage. This sta-
tus pin can be pulled up to voltages as high as 55V when
disabled, and can sink currents up to 1mA when enabled
(<0.4V). Pull down currents as high as 5mA (absolute
maximum) are supported for higher current applications,
such as lighting LEDs.
If the LTC4020 is configured for a CC/CV charging algo-
rithms, the STAT2 pin is pulled low during NTC faults or
after a bad battery fault occurs.
If the LTC4020 is configured for a CC charging algorithms,
the STAT2 pin is pulled low during NTC faults.
If the LTC4020 is configured for a lead-acid charging
algorithm, the STAT2 pin is used as a charge cycle stage
indicator pin, and pulled low during the bulk and float
charging stages. The pin is high impedance during the
absorption charging stage and during NTC or bad bat-
tery faults.
See Applications Information section.
TIMER (Pin 14): End-Of-Cycle Timer Programming Pin.
If a timer based charging algorithm is desired, connect
a capacitor (CTIMER) from this pin to ground. If no timer
functions are desired, connect this pin to ground.
End-of-cycle time (in hours) is programmed with the value
of CTIMER following the equation:
TEOC = CTIMER • 1.46 x 107;
During CC/CV or lead-acid charging algorithms, a bad bat-
tery fault is generated if the battery voltage does not reach
the precondition threshold voltage within 1/8 of TEOC, or:
TPRE = CTIMER • 1.82 x 106.
A 0.2µF capacitor is typically used for CC/CV charging,
which generates a 2.9 hour timer TEOC, and a precondition
limit time of 22 minutes. A 0.47µF capacitor is typically
used for a lead-acid charger, which generates a 6.8 hour
absorption stage safety timeout.
RNG/SS (Pin 15): Battery Charge Current Programming
Pin. This pin allows dynamic adjustment of maximum
charge current, and can be used to employ a soft-start
function.
Setting the voltage on the RNG/SS pin reduces maximum
charge current from the value programmed. The maximum
charge current is reduced to the fraction of programmed
current (as per the sense resistor, RCS) corresponding
to the voltage set on the pin (in volts). This pin has an
effective range from 0 to 1V.
For example, with 0.5V RNG/SS on the pin, the maximum
charge current will be reduced to 50% of the programmed
value set by the sense resistor values.
50µA is sourced from the RNG/SS pin, so maximum
charge current can be programmed by connecting a single
resistor (RRNG/SS) from RNG/SS to ground, such that the
voltage dropped across the resistor is equivalent to the
desired pin voltage:
VRNG/SS = 50µA • RRNG/SS
Soft-start functionality can be implemented by connecting
a capacitor (CRNG/SS) from RNG/SS to ground, such that
the time required to charge the capacitor is the desired
soft-start interval (TSS1). The voltage that corresponds to
full programmed inductor current on the RNG/SS pin is
1V, so the relation for this capacitor reduces to:
CRNG/SS = 50µA • TSS1
pin FuncTions
LTC4020
10
4020fd
For more information www.linear.com/LTC4020
The RNG/SS pin is pulled low during periods when charg-
ing is disabled, including NTC faults, bad battery faults,
and normal charge cycle termination. This allows for a
graceful start after faults and when initiating new charge
cycles, should soft-start functionality be implemented.
Both a soft-start capacitor and a programming resistor
can be implemented in parallel.
RNG/SS voltage can also be manipulated using an active
device, such as employing a pull-down transistor to dis-
able charge current or to dynamically servo maximum
charging current. Because this pin is internally pulled to
ground during fault conditions, active devices with low-
impedance pull up capability cannot be used.
See Applications Information section.
NTC (Pin 16): Battery Temperature Monitor Pin. Connect
a 10kΩ, β = 3380 NTC thermistor from this pin to ground.
The NTC pin is the input to the negative temperature coef-
ficient temperature monitoring circuit. This pin sources
50µA, and monitors the voltage created across the 10kΩ
thermistor. When the voltage on this pin is above 1.35V
(0°C) or below 0.3V (40°C), charging is disabled and an
NTC fault is signaled at the STAT1 and STAT2 status pins.
If the internal timer is being used, the timer is paused,
suspending the charging cycle until the NTC fault condi-
tion is relieved. There is approximately 5°C of temperature
hysteresis associated with each of the temperature thresh-
olds. The NTC function remains enabled while thermistor
resistance to ground is less than 250kΩ. If this function
is not desired, leave the NTC pin unconnected or connect
a 10k resistor from the NTC pin to ground. The NTC pin
contains an internal clamp that prevents excursions above
2V, so the pin must not be pulled high with a low imped-
ance source. A low impedance element can be used to
pull the pin to ground.
VFB (Pin 17): Battery Voltage Feedback Pin. Battery volt-
ages are programmed through a feedback resistor divider
placed from the BAT pin to FBG.
During CC/CV charging, the battery voltage references are:
Float Voltage (VFLOAT) = 2.5V
Trickle Charge Voltage (VTRK) = 1.75V
Auto-Restart Voltage (VRESTART) = 2.4375
During lead-acid charging, the battery voltage references
are:
Absorption Voltage (VABSOR) = 2.5V
Float Charge Voltage (VF LT ) = 2.3125V
Trickle Charge Voltage (VTRK) = 1.75V
Bulk Recharge Voltage (VBULK) = 2.1875V
With RFB1 connected from BAT to VFB and RFB2 connected
from VFB to FBG, the ratio of (RFB1/RFB2) for the desired
programmed battery float voltage (CC/CV charging) or
absorption voltage (lead-acid charging) follows the relation:
RFB1/RFB2 = (VFLOAT/ABSORB)/2.5 – 1 (V)
FBG (Pin 18): Voltage Feedback Divider Return. This pin
contains a low impedance path to signal ground, used
as the ground reference for voltage monitoring feedback
resistor dividers. When VIN is not present or the LTC4020
is in shutdown, this pin becomes high impedance, elimi-
nating current drain from the battery associated with the
feedback resistor dividers.
VFBMIN (Pin 19): Minimum voltage feedback pin for instant-
on operation. Minimum DC/DC converter output voltage
(VOUTMIN) is programmed using this pin for instant-on
functionality. VOUTMIN is programmed through a feedback
resistor divider placed from the CSP pin to FBG.
If the battery voltage is below the voltage level programmed
using this pin, the LTC4020 controls the external Pow-
erPath FET as a linear pass element, allowing the DC/DC
converter output to achieve the minimum programmed
voltage. Maximum battery charge current is reduced as
the voltage across the PowerPath FET increases to control
power dissipation.
The internal VFBMIN voltage reference is 2.125V. With a
resistor (RMIN1) connected from CSP to VFBMIN, and a
resistor (RMIN2) connected from VFBMIN to FBG, the ratio
of these resistors for the desired minimum converter
output voltage follows the relation:
RMIN1/RMIN2 = (VOUT(MIN)/2.125) – 1
pin FuncTions
LTC4020
11
4020fd
For more information www.linear.com/LTC4020
Using the same resistor values for battery voltage pro-
gramming, or RFB1 = RMIN1 and RFB2 = RMIN2, yields an
instant-on voltage that is 85% of VFLOAT (CC/CV charging)
or VABSORB (lead-acid charging):
VOUT(MIN) = 0.85 • VFLOAT/ABSORB
BAT (Pin 20): Battery Voltage Monitor Pin. This pin serves as
the positive reference for the LTC4020 ideal diode function.
If a system load occurs that is large enough to collapse
the DC/DC converter output while charging is terminated
or disabled, and the battery is disconnected (PowerPath
FET is high impedance), the ideal diode function engages
the PowerPath. This function powers the system load from
the battery, and modulates the PowerPath FET gate such
that the system output voltage is maintained with 14mV
across the PowerPath FET, provided the voltage drop due
to RDS(ON) < 14mV. This allows large loads to be accom-
modated without excessive power dissipation in the body
diode of the PowerPath FET.
BGATE (Pin 21): PowerPath FET Gate Driver Output.
This pin is controls the multiple functions of the Power-
Path FET.
This pin is pulled low during a normal charging cycle,
minimizing the FET series impedance between the DC/
DC converter output and the battery.
The BGATE pin is also forced low when the DC/DC converter
is disabled, maintaining a low impedance connection to
power the system from the battery.
When BGATE is pulled low, CSP BGATE is limited inter-
nally to 9.5V, so if CSP > 9.5V, BGATE is maintained by
an internal clamp at CSP – BGATE = 9.5V. The BGATE
pin must be near ground or at the clamp voltage for C/10
detection to occur.
If the battery voltage is lower than the instant-on threshold
(see VFBMIN), BGATE servos the PowerPath FET imped-
ance such that a voltage drop between the CSN pin and
the BAT pin is created while battery charging continues. If
the VCSN – VBAT voltage exceeds 0.4V, maximum charge
current is reduced to decrease power dissipation in the
PowerPath FET.
When the DC/DC converter is enabled, but the battery
charge cycle has terminated, BGATE is pulled high to
disconnect the battery from the converter output. The
battery is also disconnected in the same manner during
NTC faults. The ideal diode function is active during these
periods, however, so if a system load occurs that is larger
than what the DC/DC converter can accommodate, the
battery can supply the required current, and the BGATE
pin will be servo controlled to force a voltage drop of only
14mV across the PowerPath FET. The ideal diode function
is disabled during bad battery faults.
If a PowerPath FET is not being used, such as with a lead-
acid charging application, connect a 0.1nF capacitor from
BGATE to CSN.
CSN (Pin 22): Battery Charger Current Sense Negative
Input. Connect this pin to the negative terminal of the
battery charge current sense resistor (RCS) through a
100Ω resistor. Connect a filter capacitor between this
pin and the CSP pin for ripple reduction. See Applications
Information section.
The value of the sense resistor is related to the maximum
battery charge current (ICSMAX):
RCS = 0.05/ICSMAX
This pin also serves as the negative reference for the
LTC4020 ideal diode function (see BAT).
CSP (Pin 23): Battery Charger Current Sense Positive
Input. Connect this pin to the positive terminal of the
battery charge current sense resistor (RCS) through a
100Ω resistor. Connect a filter capacitor between this
pin and the CSN pin for ripple reduction. See Applications
Information section.
The value of the sense resistor is related to the maximum
battery charge current (ICSMAX) such that:
RCS = 0.05/ICSMAX
CSOUT (Pin 24): Current Sense Amplifier Output and
Charge Current Monitor. Connect 100pF capacitor to
ground.
pin FuncTions
LTC4020
12
4020fd
For more information www.linear.com/LTC4020
Pin output impedance is 100kΩ, so any loading for moni-
tors must be high impedance. The sense output voltage
follows the relation:
VCSOUT = 0.25 + 20 • (VCSP – VCSN)
CSOUT is only active while battery charger functions are
operating. CSOUT pin voltage is pulled to 0V after charge
cycle termination or during fault conditions.
ILIMIT (Pin 25): Switched Inductor Maximum Current
Programming Pin. This pin allows dynamic adjustment
of DC/DC converter maximum average inductor current,
and can be used to employ a soft-start function.
Setting the voltage on the ILIMIT pin reduces maximum
average inductor current from the value programmed.
The inductor current limit is reduced to the fraction of
programmed current (as per the sense resistors) corre-
sponding to the voltage set on the pin (in volts). This pin
has an effective range from 0 to 1V.
For example, with 0.5V on the pin, the maximum inductor
current will be reduced to 50% of the programmed value
set by the sense resistor values.
50µA is sourced from this pin, so maximum inductor cur-
rent can be programmed by connecting a single resistor
(RILIMIT) from ILIMIT to ground, such that the voltage
dropped across the resistor is equivalent to the desired
pin voltage:
VILIMIT = 50µA • RILIMIT
Soft-start functionality can be implemented by connecting
a capacitor (CILIMIT) from ILIMIT to ground, such that the
time required to charge the capacitor is the desired soft-
start interval (TSS2). The voltage that corresponds to full
inductor current on the ILIMIT pin is 1V, so the relation for
this capacitor reduces to:
CILIMIT = 50µA • TSS2
ILIMIT voltage can also be manipulated using an active
device, such as employing a pull-down transistor to dis-
able DC/DC converter current or to dynamically servo
maximum current. Because this pin is internally pulled to
ground during portions of the converter power-up cycle,
active devices with low impedance pull-up capability can-
not be used.
VFBMAX (Pin 26): DC/DC Converter Output Feedback Pin.
Maximum DC/DC converter output voltage (VOUTMAX) is
programmed using this pin. When a battery charge cycle
is terminated or disabled, and the battery is disconnected
(PowerPath FET is high impedance), the converter output
will servo to this maximum voltage.
The internal VFBMAX voltage reference is 2.75V. With a
resistor (RMAX1) connected from CSP to VFBMAX and a
resistor (RMAX2) connected from VFBMAX to FBG, the ratio
of RMAX1/RMAX2 for the desired maximum DC/DC converter
output voltage follows the relation:
RMAX1/RMAX2 = (VOUTMAX/2.75) – 1
Using the same resistor values for battery voltage pro-
gramming, or RFB1 = RMAX1 and RFB2 = RMAX2, yields an
instant-on voltage that is 110% of VFLOAT (CC/CV charging)
or VABSORB (lead-acid charging):
VOUTMAX = 1.1 • VFLOAT/ABSORB
ITH (Pin 27): DC/DC Converter Voltage Loop Compensation
Pin. See Applications Information section for compensa-
tion component selection details.
VC (Pin 28): DC/DC Converter Current Loop Compensation
Pin. See Applications Information section for compensa-
tion component selection details.
BST2 (Pin 30): Boosted supply rail for VOUT side (step-
up) switch FETs. Connect a 1µF capacitor from this pin
to SW2. Connect a 1A Schottky diode cathode to this pin,
anode to INTVCC pin.
TG2 (Pin 31): VOUT side (step-up) synchronous switch
FET gate driver output.
SW2 (Pin 32): Switched node for step-up switches.
Connect the switched inductor to this pin. Connect the
primary switch FET drain and synchronous switch FET
source to this pin.
pin FuncTions
LTC4020
13
4020fd
For more information www.linear.com/LTC4020
BG2 (Pin 33): VOUT side (step-up) primary switch FET
gate driver output.
INTVCC (Pin 34): Boosted Driver Refresh Supply. This
supply is regulated to 5V and is current limited to 150mA.
Connect a 2.2µF ceramic capacitor from this pin to PGND.
Boosted supply refresh diode anodes are connected to
this pin. Using this pin to power external 5V circuitry is
not recommended.
PGND (Pin 35): Switch high current return path for step-up
primary and step-down synchronous switches.
PVIN (Pin 36): High Current Input Supply Pin. Connect
10µF decoupling capacitor from this pin to PGND. The PVIN
pin provides input supply current for the INTVCC internal
5V linear regulator.
BG1 (Pin 37): VIN side (step-down) synchronous switch
FET gate driver output.
SW1 (Pin 38): Switched node for step-down switches.
Connect the switched inductor to this pin. Connect the
primary switch FET source and synchronous switch FET
drain to this pin.
SGND (Pins 3, 29, Exposed Pad 39): Signal Ground Refer-
ence. Connect to the output decoupling capacitor negative
terminal and battery negative terminal. The exposed pad
(39) must be soldered to PCB ground (SGND) for electrical
connection and rated thermal performance.
pin FuncTions
LTC4020
14
4020fd
For more information www.linear.com/LTC4020
block DiagraM
DC/DC Controller Block Diagram
A
SWITCH
DRIVERS
B
NOL
PVIN
FEEDBACK
REFERENCE
(2.5V)
SHUTDOWN/REFERENCES
PWM CONTROL
COUNTER
(128 CYCLES)
INTERNAL
ENABLE
2mV
(VSENS)
SHDN
RT
VOLTAGE/CURRENT SENSE
SENSVIN
SENSTOP
VFBMAX
2.75V
1V
ILIMIT
SENSBOT
SENSGND
ITH VC
INTVCC
BST1
TG1
SW1
INTVCC
BG1
PGND
×2.04 ×2
1.225V +
–
–
+
+
OSCILLATOR
FREQUENCY
SET
–
+
–
+
–
+
200µ
EA-CA
Ω
+
–
200µ
EA-CB
Ω
+
–
95µ
EA-V
Ω
+
–
5k
5k
gm = 10µ
VCBUCK
VCBOOST
1V CLAMP/
DETECT
S
R
Q
S
R
Q
CLK
4020 BDa
RESET
128
INTVCC
BG2
PGND
BST2
TG2
SW2
SGND
C
D
NOL
LTC4020
15
4020fd
For more information www.linear.com/LTC4020
50µA
0.7µA
100k
1.75V
VFLOAT
2.5V
TRICKLE
+
–
+
–
+
–
+
–
+
–
–
+
–
+
240µ
Ω
+
+
–
+
–
50µA
V_40C2V
VREF
C/10
CC/CV
TRICKLE
BULK
RESTART
V_0C
270µ
Ω
+
–
270µ
Ω
+
–
+
0.1V
9.3µA 0.45V 14mV
INSTANT-ON
FOLDBACK
DISABLE
CHARGER
1.75V
A(V) = 20
2.075V
C/10
BULK
2.125V
RESTART
2.4375V
TIMER/
OSCILLATOR
TERMINATION
AND CONTROL
LOGIC
+
–
+
–
+
–
CHARGER
ENABLED
IDEAL
DIODE
INSTANT
ON
CSOUT
RNG/SS
VFB
VIN_REG
NTC
TIMER
4020 BDb
CSP
CSN
BGATE
VFBMIN
BAT
ITH
STAT1
STAT2
MODE
FBG
IC
ACTIVE
block DiagraM
Battery Charger Block Diagram
LTC4020
16
4020fd
For more information www.linear.com/LTC4020
operaTion
Functional Overview
The LTC4020 is an advanced high voltage power manager
and multi-chemistry battery charger designed to efficiently
transfer power from a variety of sources to a system power
supply rail and a battery.
The LTC4020 contains a step-up/step-down DC/DC
controller that allows operation with battery and system
voltages that are above, below, or equal to the input volt-
age (VIN). A precision threshold shutdown feature allows
incorporation of input voltage UVLO functionality using
a simple resistor divider. When in low current shutdown
mode, the IC input supply bias is reduced to only 27.5µA.
The LTC4020 charger is programmable to produce opti-
mized charging profiles for a variety of battery chemistries.
The LTC4020 can provide a constant-current/constant-
voltage charge characteristic with either C/10 or timed
termination for use with lithium based battery systems,
a constant-current characteristic with timed termination,
or an optimized 4-step, 3-stage lead-acid charge profile.
Maximum battery charge current is programmable using
a sense resistor, and a charge current range adjust pin
allows dynamic adjustment of maximum charge current. A
switcher core current limit adjust pin also allows dynamic
limiting of power available to the system by virtue of limit-
ing maximum current in the DC/DC converter inductor.
The LTC4020 preconditions heavily discharged batteries
by reducing charge current to one-fifteenth of the pro-
grammed maximum. Once the battery voltage climbs above
an internally set threshold, the IC automatically increases
maximum charging current to the full programmed value. A
bad battery detection function signals a fault and suspends
charging should a battery not respond to preconditioning.
Battery temperature is monitored using a thermistor
measurement system. This feature monitors battery
temperature during the charging cycle, suspending the
charge cycle and signaling a fault condition if the battery
temperature moves outside a safe charging range of 0°C
to 40°C. The charge cycle automatically resumes when
the temperature returns to that safe charging range.
Instant-on PowerPath architecture ensures that an applica-
tion is powered immediately after an external voltage is
applied, even with a completely dead battery, by prioritiz-
ing power to the application. Since the controller output
(VOUT) and the battery (BAT) are sometimes decoupled,
the LTC4020 includes an ideal diode controller, which
guarantees that ample power is always available to VOUT
if there is insufficient power available from the DC/DC
converter. Should there be no input power available (VIN),
the LTC4020 makes a low impedance connection from the
battery to VOUT though the PowerPath FET. Battery life
is maximized during periods of input supply disconnect
by reducing the LTC4020 battery standby current to less
than 10µA.
The LTC4020 contains two digital open-collector outputs
that provide charger status and signal fault conditions.
These binary coded pins signal battery charging, standby
or shutdown modes, battery temperature faults, and bad
battery faults.
DC/DC Converter Operation
(See Block Diagrams)
The LTC4020 uses a proprietary average current mode
DC/DC converter architecture.
As shown in Figure 1, when VIN is higher than VOUT dur-
ing step-down (buck) operation, switches A (driven by
pin TG1) and B (driven by pin BG1) perform the PWM
required for accommodating power conversion. Ideally,
switch D (driven by pin TG2) would conduct continuously
and switch C (driven by pin BG2) would stay off, making
PWM switching action much like that in a synchronous buck
topology. Switch D uses a bootstrapped driver, however,
so switch C conducts for a minimum on time of 150nS
each cycle to refresh the driver and switch D is disabled
to accommodate this refresh time. A 75ns non-overlap
period, separates the conduction of the two switches,
preventing shoot-through currents.
When VIN is lower than VOUT during step-up (boost) op-
eration, switches C and D perform the PWM required for
accommodating power conversion. Ideally, switch A would
conduct continuously and switch B would stay off, making
LTC4020
17
4020fd
For more information www.linear.com/LTC4020
operaTion
PWM switching action much like that in a synchronous
boost topology. Since switch A also uses a bootstrapped
drive, however, the B switch conducts for 100nS during
this refresh period. A 75ns non-overlap period, separates
the conduction of the two switches, preventing shoot-
through currents.
If VIN is close to VOUT, the controller operates in 4-switch
(buck-boost) mode, where both switch pairs PWM si-
multaneously to accommodate conversion requirements.
The LTC4020 senses the DC/DC converter output voltage
using a resistor divider feedback network that drives the
VFBMAX pin. The difference between the voltage on the
VFBMAX pin and an internal 2.75V reference is converted
into an error current by the voltage loop transconductance
amplifier (EA-V). This error current is integrated by a
compensation network to produce a voltage on pin ITH.
The ITH compensation network is designed to optimize
the response of the converter to changes in load current
while the converter is in regulation. At regulation, the ITH
pin will servo to a value that corresponds to the average
inductor current of the DC/DC converter.
Inductor current is monitored through two like value
sense resistors, placed in series with each of the VIN side
converter switches, A and B. The sum of these sensed
currents yields a reasonably accurate continuous repre-
sentation of inductor current.
The voltage produced on the ITH pin is translated into
an offset at the input of the two current sense amplifiers.
The difference between this offset voltage and the sum
of the voltages is sensed on the SENSTOP and SENSBOT
pins, then is converted to error currents by the current
sense transconductance amplifiers (EA-CA and EA-CB).
These error currents are summed and integrated by a
compensation network to produce a voltage on the pin VC.
This compensation network is designed to optimize the
response of the converter duty cycle to required changes
in inductor current.
The VC pin voltage is compared to an internally generated
ramp, and the output of this comparison controls the duty
cycle of the charger’s switches.
Figure 1 shows a simplified diagram of the four power
switches and their connections to the IC, inductor, VIN,
VOUT, ground, and current sense elements.
Figure 1. Converter Switch Diagram
Reverse current protection is accomplished through dis-
abling the VOUT side synchronous switch (D) during initial
power-up, when the converter is in step-up duty cycle
limit, and when ITH falls to a voltage that corresponds
to <1/25 of programmed ILMAX. Once these conditions
subside, the D switch remains disabled for an additional
128 clock cycles.
Figure 2. Operating Regions vs Duty Cycle (DC)
A
B
VOUT
VIN
RSENSEB
TG2
SW2SW1
BG2
TG1
BG1
SENSEBOT
SENSETOP
DB*
*OPTIONAL
DD*D
C
4020 F01
RSENSEA
DC MAX
(BOOST)
DC MAX
(BUCK)
DC MIN
(BOOST)
DC MIN
(BUCK) 4020 F02
C/D PWM – A MIN OFF, B MIN ON
4-SWITCH PWM
A/B PWM – C MIN ON, D MIN OFF
LTC4020
18
4020fd
For more information www.linear.com/LTC4020
operaTion
Battery Charger Operation
(See Block Diagrams)
During the majority of a normal battery charge cycle, the
LTC4020 makes a low impedance connection between
the battery and the DC/DC converter output through the
PowerPath FET, as in Figure 3. This PFET is controlled by
the LTC4020 through modulation of the BGATE pin, which
is connected to the FET gate. When charging is disabled,
the FET is disabled, disconnecting the battery from the
converter output by pulling the gate of the PowerPath FET
high via the BGATE pin. The converter output is regulated
by VFBMAX while the FET is disabled. When normal charger
operation resumes, the gate is pulled low. As the BGATE pin
is a slow-moving node, C/10 detection is disabled until the
BGATE pin approaches its normal operating voltage, which
prevents premature C/10 detection during reconnection of
the battery. The slow movement of BGATE can also cause
the converter output to regulate to VFBMAX for a short time
during start-up until the FET is enabled. This FET is also
linearly controlled during low battery conditions to enable
the instant-on function, where the converter output can be
separated from a heavily discharged battery to power the
rest of the system before the battery voltage responds to
charging. C/10 detection is also disabled when the charger
is operating in instant-on mode. This FET is also automati-
cally configured as a 14mV ideal diode, which provides a
low loss path from the battery to the output when system
loads require power from the battery while the battery is
disconnected from the converter output.
The battery charger takes control of the DC/DC converter
operation by modulating the ITH pin voltage in response to
sensed battery charge current, battery voltage, and input
voltage. The converter thus provides exactly the amount
of power required to satisfy both the system load and
battery charger requirements.
Battery charge current is monitored via an external sense
resistor connected to the pins CSP and CSN. The voltage
across this resistor is amplified internally by a factor of
20, which is output onto pin CSOUT. This output voltage
rides on top of a constant 250mV offset. The CSOUT pin
voltage drives an internal transconductance amplifier that
servos the DC/DC converter’s ITH pin voltage in response
to the current requirements of a charging battery. CSOUT
voltage is also used internally as a charge current monitor
to detect <C/10 current thresholds.
Battery voltage is monitored via the VFB pin. This voltage
drives a transconductance amplifier that servos the DC/DC
converter ITH pin voltage in response to voltage developed
on a charging battery. The transition from constant-current
(CC) to constant-voltage (CV) charging modes is also
detected using this transconductance amplifier. The VFB
voltage is used for all battery voltage monitor thresholds,
each being defined as a percentage of the internal 2.5V
reference voltage.
Input voltage regulation is implemented via the VIN_REG pin
for use with poorly regulated or high impedance supplies.
This pin drives a transconductance amplifier that reduces
the ITH pin voltage in response to voltage sensed on the
VIN_REG pin falling through 2.5V. This transconductance
amplifier remains active even while battery charging is
disabled, so the input regulation feature continues to
operate regardless of the state of a charge cycle.
The LTC4020 contains an internal charge cycle timer that
is used for time based control of a charge cycle. This func-
tion is enabled by connecting a capacitor to the TIMER
pin. Grounding this pin disables all timer functions. The
timer is used to terminate a successful CC or CC/CV charge
cycle after a programmed end-of-cycle (TEOC) time. This
timer is also used to transition a lead-acid charger to float
charging if charge current does not fall adequately dur-
ing the absorption phase of the charge cycle within the
programmed TEOC time.
Figure 3. Battery Charger PowerPath Diagram
RCS
VOUT
SYSTEM
BATTERY
CSP
CSN
BGATE
BAT
VFB
VFBMIN
4020 F03
LTC4020
19
4020fd
For more information www.linear.com/LTC4020
operaTion
Use of the timer function also enables bad battery detection
during CC/CV or lead-acid charging. This fault condition is
achieved if the battery does not respond to preconditioning
(VFB < 1.75V), such that the charger remains in (or enters)
precondition mode after one-eighth of the programmed
TEOC time. A bad battery fault halts the charging cycle, and
the fault condition is reported on the status pins.
CC/CV Charging Overview (MODE = 0V)
To program the LTC4020 for CC/CV charging, connect the
MODE pin to ground. This mode is commonly used for
Li-Ion, Li-Polymer, and LiFePO4 battery charging.
If the voltage on the VFB pin is below 1.75V, the LTC4020 en-
gages precondition mode, which provides low level charge
currents to gently increase voltage on heavily discharged
batteries. During preconditioning, the maximum charge
current is reduced to one-fifteenth of the programmed
value as set by RCS, the battery charge current program-
ming resistor. Full charge current capability is restored
once the voltage on VFB rises above 1.75V. Full charge
current capability remains until the VFB pin approaches
the 2.5V float voltage. This is the constant-current (CC)
portion of the charge cycle.
When the voltage on the VFB pin approaches the 2.5V
float voltage, the charger transitions into constant-voltage
(CV) mode, and charge current is reduced from the maxi-
mum programmed value. If timer termination is used, the
safety timer period starts when CV mode is initiated, and
the charge cycle will terminate when the timer achieves
end-of-cycle (TEOC). This timer is typically programmed
to achieve TEOC in three hours, but can be configured
for any amount of time by setting an appropriate timing
capacitor value (CTIMER).
During CV mode, the required charge current is steadily
reduced as the battery voltage is maintained such that the
voltage on the VFB pin remains close to 2.5V. If the char-
ger is configured for C/10 termination, when the battery
charge current falls below one-tenth of the programmed
maximum current (<C/10), the charge cycle will terminate
and the charger indicates not charging on the status pins.
When timer termination is used, the charger continues to
operate with charging current less than one-tenth of the
programmed maximum current. The STAT1 status pin,
however, responds to the <C/10 current level regardless
of termination scheme, so the IC will indicate a not charg-
ing status when the charging current is below the C/10
current level. The charge cycle will continue, however,
and the charger will source <C/10 current into the bat-
tery. Programmed float voltage is maintained while the
charger tops-off the battery with low currents until the
programmed TEOC time has elapsed, at which time the
charge cycle will terminate, charge current flow into the
battery will be disabled, and the battery will be discon-
nected from the converter output.
After termination, if the battery discharges such that
the voltage on the VFB pin drops to 2.4375V, or 97.5%
of the programmed float voltage, a new charge cycle is
automatically initiated.
TYPICAL CC/CV CHARGE CYCLE VOLTAGES
(PER CELL)
Li-Ion LiFePO4
Precondition 2.94V 2.52V
Float 4.2V 3.6V
Recharge 4.095V 3.51V
Lead-Acid Charging Overview (MODE = INT_VCC)
To program the LTC4020 for lead-acid charging, connect
the MODE pin to the INTVCC pin. The LTC4020 supports
a 4-step, 3-stage lead-acid charging profile.
The first step of the charging profile provides low level
charge current to gently increase voltage on heavily dis-
charged batteries. If the voltage on VFB is below 1.75V,
which corresponds to just over 10V for a 6-cell (12V)
battery, the maximum charge current is reduced to one-
fifteenth of the programmed value as set by RCS. Once
the VFB voltage rises above 1.75V, full charge current
capability is restored, and the bulk charging stage begins.
The bulk charging stage of the charge profile, which is
the first stage of 3-stage battery charging, is a constant-
current charging stage, with the maximum programmed
charge current forced into the battery. This continues until
the battery voltage rises such that the VFB pin approaches
the 2.5V absorption reference voltage.
LTC4020
20
4020fd
For more information www.linear.com/LTC4020
operaTion
As the bulk charging stage completes and the voltage on
the VFB pin rises to approach 2.5V, the charger transi-
tions into the absorption stage, which is the 2nd stage of
3-stage battery charging. During the absorption stage,
the required charge current is steadily reduced as the
battery voltage approaches the absorption voltage. This is
a constant-voltage charging stage, as the battery voltage
is maintained such that the VFB pin remains close to the
2.5V absorption reference voltage. It is during this stage
that the battery stored charge increases to 100% capac-
ity. The 2.5V absorption reference typically corresponds
to 14.4V for a 6-cell battery.
When the absorption stage charge current is reduced
to one-tenth of the programmed maximum current, the
charger will initiate the third stage in the charge profile,
the float charging stage. The safety timer can be used with
a lead-acid charger to limit the duration of the absorption
stage of the charging profile. The timer is initiated at the
start of the absorption stage, and forces the charger into
float if the charge current does not fall to the required
one-tenth of the programmed maximum current during
the absorption stage before the timer reaches TEOC. A
0.47µF capacitor on the TIMER pin is typically used, which
generates a 6.8 hour absorption stage safety timeout.
Once the float charging stage is initiated, the battery
reference voltage is reduced to 92.5% of the absorption
voltage, or 2.3125V. The battery voltage is maintained at
a voltage corresponding to this reference voltage, and
maximum charge current is reduced to one-fifteenth of
the programmed maximum. This level corresponds to
13.3V for a 6-cell battery.
Once float charging is achieved, the LTC4020 charger re-
mains active and will attempt to maintain the float voltage
on the battery indefinitely. During float charging, if a load
on the battery exceeds the maintenance charge current
of one-fifteenth of the programmed maximum, the bat-
tery voltage will begin to discharge. If a load discharges
the battery such that the voltage on VFB falls to 2.1875V,
corresponding to 12.6V for a 6-cell battery, the LTC4020
restarts the full 3-stage charging cycle by reinitiating the
bulk charging stage. Bulk charging is engaged by resetting
the internal VFB reference to the 2.5V absorption voltage
reference and increasing the charge current capability to
the programmed maximum.
TYPICAL LEAD-ACID CHARGE CYCLE
VOLTAGES (12V System)
Precondition 10.1V
Absorption 14.4V
Float 13.3V
Bulk Restart 12.6V
CC Charging Overview (MODE = NC)
To program the LTC4020 for CC charging, leave the MODE
pin unconnected. This mode can be used for charging
NiCd and NiMH batteries, supercap charging, or in any
other application where a timed current source is desired.
CC mode can also be used when the voltage dependent
precondition mode is not desired.
In CC mode, the LTC4020 will maintain full programmed
charge current capability for the duration of the timer
period. The trickle charge function is disabled, although
maximum charge current will be reduced during lower deck
operation if there is excessive voltage (>0.3V) imposed
across the PowerPath FET. The charger will terminate the
charge cycle and the PowerPath FET will become high-
impedance once timer EOC is reached.
While the charge cycle is designed to be voltage inde-
pendent, a maximum VBAT voltage can be programmed
corresponding to VFB = 2.5V, allowing constant-voltage
functionality at that level if desired.
Once the timer reaches TEOC and the charge cycle ter-
minates, input power or SHDN must be cycled to initiate
another charge cycle. If the timer function is disabled
(TIMER = 0V), the current source function remains active
indefinitely.
Note: For nickel-chemistry batteries (e.g. NiCd or NiMH),
the possibility of overcharging must be considered. A
typical method is to charge with low currents for a long
period of time. NiCd and NiMH batteries can absorb a
C/300 charge rate indefinitely. Shorter duration charging
is possible using a timed current source charge algorithm.
It is recommended to ensure a depleted battery before
charging, then subsequently charge the battery to no more
than 125% capacity. For example, a depleted 2000mAh
NiMH battery is charged with 2.5A for one hour.
LTC4020
21
4020fd
For more information www.linear.com/LTC4020
DC/DC CONVERTER SECTION
Output Voltage Programming
The LTC4020 DC/DC converter maximum output volt-
age, or voltage safety limit, is set by an external feedback
resistive divider, providing feedback to the VFBMAX pin.
This divider sets the output voltage that the converter
will servo to when the PowerPath FET is high impedance,
which occurs after charge cycle termination or during a
charge cycle fault.
applicaTions inForMaTion
PGND. Both nodes on the sense resistor must be Kelvin
connected to the IC, via the pins SENSBOT and SENSGND.
Both of these sense resistors must be of equal value, and
that value programs the switched inductor maximum
average current in the DC/DC converter inductor (ILMAX)
such that:
RSENSEA =RSENSEB =
0.05
I
LMAX
When the converter is stepping down, or operating in buck
mode, the inductor current will be roughly equivalent to
the converter output current. Input supply current (IIN) will
be less than the inductor current (IL), such that:
IL~IIN •VIN
VOUT
When the converter is stepping up, or operating in boost
mode, the inductor current will be roughly equivalent to
the converter input current. Inductor current (IL) will be
greater than output current (IOUT), such that:
IL~IOUT •VOUT
VIN
Overcurrent Detection
The LTC4020 also contains an overcurrent detection
circuit that monitors the low side current sense resistor,
or SENSBOT–SENSGND input. Should that circuit detect
a voltage on that input that is less than –150mV or greater
than 100mV, or roughly 3x the maximum average current,
all of the switches are disabled for four (4) clock cycles.
Parasitic inductances on non-ideal layouts and or body-
diode commutation charge can cause voltage spikes
across the sense resistor at the beginning of synchronous
FET conduction. The LTC4020 overcurrent circuitry is
somewhat resistant to these leading edge spikes but, in
some cases, the overcurrent circuit can be prematurely
triggered. This is identified by the repeated 4-cycle
switch off-time that occurs should premature triggering
occur. Placing a ceramic capacitor across the SENSBOT–
SENSGND input pins near the IC will generally eliminate
Figure 4. VOUT Safety Limit Programming
The resultant feedback signal is compared with the internal
2.75V voltage reference by the converter error amplifier.
The output voltage is given by the equation:
VOUT = 2.75V •1+ RMAX1
RMAX2
where RMAX1 and RMAX2 are defined as in Figure 4.
The values for RMAX1 and RMAX2 are typically the same as
those used for the divider that programs battery voltage
(to the VFB pin; see Battery Charger section), to yield a
DC/DC converter maximum regulation voltage, or safety
limit, that is 10% higher than the battery charge voltage.
RSENSEA, RSENSEB: DC/DC Converter Current
Programming
The LTC4020 performs inductor current sensing via two
resistors connected in series with the VIN side switches
(see Figure 1). The high side sense resistor (RSENSEA) is
connected between VIN and the drain of the top side switch
FET (A). Both nodes on the sense resistor must be Kelvin
connected to the IC via the pins SENSVIN and SENSTOP.
Likewise, the low side sense resistor (RSENSEB) is connected
between the source of the bottom side switch FET (B) and
4020 F04
VOUT
RMAX1
RMAX2
VFBMAX
LTC4020
LTC4020
22
4020fd
For more information www.linear.com/LTC4020
premature triggering by high pass filtering the current sense
signal. Setting the τ of the effective filter in the range of
1ns is generally sufficient to shunt errant signals, such that:
CSENSB ~
1ns
R
SENSEB
Programming Switching Frequency
The RT frequency adjust pin allows the user to program
the LTC4020 DC/DC converter operating frequency from
50KHz to 500KHz.
Higher frequency operation is desirable for smaller ex-
ternal inductor and capacitor values, but at the expense
of increased switching losses and higher gate drive cur-
rents. Higher frequencies may also not allow sufficiently
high or low duty cycle operation due to minimum on/off
time constraints. Lower operating frequencies require
larger external component values, but result in reduced
switching losses yielding higher conversion efficiencies.
Operating frequency (fO) is set by choosing an appropriate
frequency setting resistor (RRT), connected from the RT
pin to ground. This resistor is required for operation; do
not leave this pin open. For a desired operating frequency,
RRT follows the relation:
RRT =100kΩ •fO
250kHz
– 1.0695
Input Supply Decoupling
The LTC4020 is typically biased directly from the charger
input supply through the PVIN and SENSVIN pins. This
supply provides large switched currents, so a high quality,
low ESR decoupling capacitor is recommended to mini-
mize voltage glitches on the VIN supply. Placing a smaller
ceramic capacitor (0.1µF to 10µF) close to the IC in parallel
with the input decoupling capacitor is also recommended
for high frequency noise reduction. The SENSVIN pin is a
Kelvin connection from the VIN supply at the primary VIN
side switch FET (A); separate decoupling for that pin is
not recommended. The charger input supply decoupling
capacitor (CVIN) absorbs all input switching ripple current
in the charger, so it must have an adequate ripple current
rating. RMS ripple current (ICVIN(RMS)) is highest during
step down operation, and follows the relation:
ICVIN(RMS) ~IMAX •DC•1
DC –
1
,
which has a maximum at DC = 0.5, or VIN = 2 • VOUT, where:
ICVIN(RMS) =
I
MAX
2
The simple worst-case of ½ • IMAX is commonly used for
design, where IMAX is the programmed inductor current
limit.
Bulk capacitance (CIN(BULK)) is a function of desired input
ripple voltage (ΔVIN). For step-down operation, CIN(BULK)
follows the relation:
CIN(BULK)≥IMAX •
V
OUT(MAX)
VIN(MIN)
•
1
ΔVIN •fO
,
where fO is the operating frequency, VOUT(MAX) is the DC/
DC converter maximum output voltage and VIN(MIN) is
the regulation voltage corresponding to 2.5V on VIN_REG.
If the input regulation feature is not being used, use the
minimum expected input operating voltage.
If an application does not require step-down operation,
during step-up operation, input ripple current is equivalent
to inductor ripple current (ΔIMAX), so CIN(BULK) follows
the relation:
CIN(BULK) =
ΔI
MAX
ΔV
IN
•f
O
Figure 5. RT vs Operating Frequency
applicaTions inForMaTion
RT (kΩ)
50
OPERATING FREQUENCY (kHz)
600
500
400
300
200
100
0250150 350 400 450
4020 F05
500200100 300
LTC4020
23
4020fd
For more information www.linear.com/LTC4020
ILIMIT Pin
Maximum average inductor current can be dynami-
cally adjusted using the ILIMIT pin as described in the Pin
Description section. Active servos can also be used to
impose voltages on the ILIMIT pin, provided they can only
sink current. Active circuits that source current cannot be
used to drive the ILIMIT pin.
using whichever relation yields the largest inductor value
for LMIN:
If VIN > VOUT (step-down conversion):
LMIN =
VOUT •1– VOUT / VIN(MAX)
(
)
fO•∆IMAX •IMAX
If VIN < VOUT (step-up conversion):
LMIN =VIN •1– VIN / VOUT(MAX)
(
)
fO•∆IMAX •IMAX
For step-down conversion, use the maximum expected
operating voltage for VIN(MAX). If the expected VOUT
operating range (typically from VFBMIN = 2.125V to VFBMAX
= 2.75V) includes VIN(MAX)/2, use that value for VOUT. If the
entire operating range is below VIN(MAX)/2, use the value
corresponding to VFBMAX = 2.75V. If the entire operating
range is above VIN(MAX)/2, use the value corresponding
to VFBMIN = 2.125V.
For step-up conversion, use the maximum output volt-
age (typically corresponding to pin VFBMAX = 2.75V) for
VOUT(MAX). If the expected VIN operating range includes
VOUT(MAX)/2, use that value for VIN. If the entire input
operating range is below VOUT(MAX)/2, use the maximum
operating voltage for VIN. If the entire input operating range
is above VOUT(MAX)/2, use the minimum input operating
voltage for VIN.
Magnetics vendors typically specify inductors with
maximum RMS and saturation current ratings. Select an
inductor that has a saturation current rating at or above
1.25 • IMAX, and an RMS rating above IMAX.
Output Decoupling
During periods when the LTC4020 DC/DC converter output
is not connected to the battery through the PowerPath
FET, the system load is driven directly by the converter.
The converter creates large switched currents, so a high
quality, low ESR decoupling capacitor is recommended
to minimize voltage glitches on the VOUT supply. Placing
a smaller ceramic decoupling capacitor (0.1µF to 10µF)
in parallel with the output decoupling capacitor is also
recommended for high frequency noise reduction. The
VOUT decoupling capacitor (CVOUT) absorbs the majority of
applicaTions inForMaTion
Figure 6. Using the ILIMIT Pin for Digital Control
of Maximum Average Inductor Current
Figure 7. Driving the ILIMIT Pin with a Current Sink
Active Servo Amplifier
4020 F06
10k
LOGIC HIGH = HALF CURRENT
ILIMIT
LTC4020
4020 F06
SERVO
REFERENCE
ILIMIT
LTC4020
–
+
Inductor Selection
The primary criterion for inductor value selection in an
LTC4020 charger is the ripple current created in that induc-
tor. Once the inductance value is determined, an inductor
must also have a saturation current equal to or exceeding
the maximum peak current in the inductor.
An inductor value (L) is calculated given the maximum
desired amount of ripple current (ΔIMAX). Maximum
inductor ripple current should generally be in the range
of 0.2 to 0.5 (as a fraction of maximum average inductor
current, IMAX). When stepping down, ripple current gets
larger with increased VIN, and is maximized when VOUT =
VIN/2. When stepping up, ripple current gets larger with
increased VOUT, and is maximized when VIN = VOUT/2. The
inductor value must be chosen using the greatest expected
operational difference between these values.
A minimum inductor value for a given maximum ripple
current and operating frequency (fO) can be determined
LTC4020
24
4020fd
For more information www.linear.com/LTC4020
the converter ripple current, so it must have an adequate
ripple current rating. RMS ripple current (IΔRMS) is highest
during step up operation, and follows the relation:
IΔRMS ~IMAX •DC•1
DC –1
,
having a maximum at DC = 0.5, or VOUT = 2 • VIN, where:
ICVOUT(RMS) =
I
MAX
2
.
The simple worst-case of ½ • IMAX is commonly used for
design, where IMAX is the programmed inductor current
limit.
Bulk capacitance is a function of desired output ripple
voltage (ΔVOUT), and follows the relations:
For step-up operation:
COUT(BULK) ≥IMAX •
V
OUT(MAX)
– V
IN(MIN)
VOUT(MAX)
•
1
ΔVOUT •fO
,
where VOUT(MAX) is the DC/DC converter safety limit, and
VIN(MIN) is the VIN regulation threshold. If the VIN regula-
tion feature is not being used, use the minimum expected
operating voltage.
For step-down operation, output ripple current is equivalent
to inductor ripple current (ΔIMAX), so COUT(BULK) follows
the relation:
COUT(BULK) ≥
ΔI
MAX
ΔV
OUT
•f
O
,
Switch FET Selection
The LTC4020 requires four external N-channel power
MOSFETs, as shown in Figure 1.
Specified parameters used for power MOSFET selection
are: breakdown voltage (VBR(DSS)), threshold voltage
(VGS(TH)), on-resistance (RDS(ON)), reverse transfer ca-
pacitance (CRSS), and maximum inductor current (ILMAX).
The drive voltage is set by the INTVCC supply pin, which is
typically 5V. Consequently, logic-level threshold MOSFETs
must be used in LTC4020 applications.
Transition Losses (PTR): During maximum power opera-
tion, all 4 switches change state once per oscillator cycle,
so the maximum switching transient power losses (PTR)
remain constant over condition.
PTR(A, B) ≈ (k)(VIN)2 (ILMAX)(CRSS)(fO)
PTR(C, D) ≈ (k)(VOUT)2 (ILMAX)(CRSS)(fO)
PTR(A, B) is the transition loss for the VIN side switch FETs
A and B, and PTR(C,D) is the transition loss for VOUT side
switch FETs C and D, with the switch FETs designated as
in Figure 1. The constant k, which accounts for the loss
caused by reverse recovery current, is inversely propor-
tional to the gate drive current and has a empirical value
approximated by k = 1 in LTC4020 applications. ILMAX is
the converter maximum inductor current as programmed
by the two sense resistors.
Conductive Losses (PON): Conductive losses are propor-
tional to switch duty cycle. The average conductive losses
in a switch at maximum inductor current (ILMAX)is:
PON = ILMAX2 • ρT • RDS(ON) • (TON • fO)
where ρT is a normalization factor (unity at 25°C) ac-
counting for the significant variation in on-resistance
with temperature. For a maximum junction temperature
of 125°C, using a value of ρT = 1.5 is reasonable.
If VIN > VOUT (step-down conversion):
PON(A) = ILMAX2 • ρT • RDS(ON(A)) • (VOUT/VIN)
PON(B) = ILMAX2 • ρT • RDS(ON(B)) • (1 – VOUT/VIN)
PON(C+D) = ILMAX2 • ρT • RDS(ON(C, D))
If VIN < VOUT (step-up conversion):
PON(A+B) = ILMAX2 • ρT • RDS(ON(A, B))
PON(C) = ILMAX2 • ρT • RDS(ON(C)) • (1 – VIN/VOUT)
PON(D) = ILMAX2 • ρT • RDS(ON(D)) • (VIN/VOUT)
Optional Schottky Diode (Db, Dd) Selection
Schottky diodes can be placed in parallel with the syn-
chronous FETs B and D, as shown in Figure 1 as Db and
Dd. These diodes conduct during the dead time between
the conduction of the power MOSFET switches and are
intended to prevent the body diode of synchronous switches
from storing charge.
applicaTions inForMaTion
LTC4020
25
4020fd
For more information www.linear.com/LTC4020
To maximize effectiveness of the diodes, the inductance
between the switches and the synchronous switches must
be minimized, so the diodes should be placed adjacent to
their corresponding FET switch.
The Dd diode also reduces power dissipation in the D switch
during periods of reverse current inhibit operation, during
which time the D switch is disabled. Load currents are low
during reverse inhibit, and diode Db only conducts during
switch dead times, so both can have current ratings well
below the DC/DC converter inductor current maximum.
Typically, a diode with an average current rating at or above
one-tenth of ILMAX is adequate, provided the diode has an
instantaneous current rating that exceeds the maximum
inductor current, or ILMAX + ½ ΔIMAX.
Db reverse voltage rating must exceed VIN. Dd reverse
voltage rating must exceed VOUT.
INTVCC LDO Output, and BST1 and BST2 Supplies
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCC pin. An
internal 5V low dropout regulator (LDO) supplies INTVCC
power from the PVIN pin. INTVCC is decoupled to PGND
using a 2.2µF ceramic capacitor.
The BST1 and BST2 bootstrapped supply pins power
internal high side FET gate drivers, which output to pins
TG1 and TG2. BST1 provides switch gate drive above the
input power supply voltage for switch FET A, and BST2
provides switch gate drive above the output power supply
voltage for switch FET D, as designated in Figure 1. These
boosted supply pins allows the use of NFET switches for
increased conversion efficiency. These bootstrapped sup-
plies are regenerated through external Schottky diodes
from the INTVCC pin.
Connect two low leakage 1A Schottky diode anodes to the
INTVCC pin. Connect one Schottky cathode to the BST1
pin. This diode must be rated for reverse voltage standoff
exceeding the maximum input supply voltage. Connect the
other diode cathode to the BST2 pin. This diode must be
rated for reverse voltage standoff exceeding the converter
safety limit output, VOUT(MAX).
Connect a ceramic capacitor from the BST1 pin to the
SW1 pin and another from BST2 pin to the SW2 pin.
The value of these two capacitors should be at least 50
times greater than the equivalent total gate capacitance
of the corresponding switch FET. Total FET gate charge
(QG(TOT)) is typically specified at a specific gate-source
voltage (VGS(Q)). Using those parameters, the required
boost capacitor values (CBST) follow the relation:
CBST > 50 • QG(TOT)/VGS(Q)
CBST = 1µF is typically adequate for most applications.
During low load operation, start-up, and nonoverlap
periods, inductor current is conducted by the silicon
body diode of the synchronous FET. This diode stores a
significant amount of charge, so when the primary switch
turns on for the next switch cycle, reverse recovery current
is conducted by the main switch to discharge this diode.
The resultant short-duration current spike can be orders of
magnitude greater than the inductor current itself, resulting
in an extremely fast dV/dt on the switched node. Conse-
quently, parasitic inductance associated with the switch
FET packaging and/or less-than-ideal layout can induce a
voltage spike of 10 or more volts at the leading edge of a
switching cycle. This can be particularly problematic on
the step-up side of the inductor, as these voltage spikes
are negative, and can cause a build-up of voltage on the
BST2-SW2 capacitor. This would generally occur when
the step-up synchronous switch (D) is disabled, such as
during low load operation and during start-up. If voltage
build-up on the boosted supply proves excessive, it could
potentially violate absolute maximum voltage ratings of the
IC and cause damage. This effect can be greatly reduced
by implementing a Schottky diode across the step-up
synchronous FET, shown as DD in Figure 1, which reduces
reverse recovery charge in the synchronous FET body
diode. A low current 6V Zener (0.1A) in parallel with the
BST2-SW2 capacitor will also effectively shunt any errant
charge and prevent excessive voltage build-up.
External Power for BST1 and BST2 Supplies
Power for the top and bottom MOSFET drivers can be
supplied by an external supply, provided that a precision
5V supply is available (±5%).
The INTVCC internal supply is a linear regulator, which
transfers current from the VIN pin. As such, power dis-
sipation can be excessive with high VIN pin voltages and/
applicaTions inForMaTion
LTC4020
26
4020fd
For more information www.linear.com/LTC4020
Figure 9. Connection of External 5V Regulator for Reduced
Internal Power Dissipation
Figure 8. INTVCC Pass Element SOA (Safe Operating Area)
QG(TOT)ABCD • fO (mA)
0
VIN (V)
60
50
40
30
20
10
04020 60 70 80
4020 F08
903010 50
SOA
4020 F09
LTC4020
VIN
SENSVIN
INTVCC
VIN
VIN
(5V)
5VOUT
or large gate drive requirements. The power dissipation
in the linear pass element (PINTVCC) is:
PINTVCC = (VIN – 5V) • QG(TOT)ABCD • fO,
where QG(TOT)ABCD is the sum of all four switch total gate
charges, and fO is the LTC4020 switching frequency.
In this configuration, the INTVCC pin cannot collapse
when the LTC4020 is in shutdown. As a result of the pin
bias being maintained during shutdown, current will flow
into the INTVCC pin, increasing input supply current. The
total shutdown current flowing into the INTVCC pin in this
configuration is approximately 150µA.
BATTERY CHARGER SECTION
Battery Charge Voltage Programming
The LTC4020 uses an external feedback resistive divider
from the BAT pin to ground to program battery voltages.
This divider provides feedback to the VFB pin, and sets
the final voltage that the battery charger will achieve at
the end of a charge cycle. The feedback reference of 2.5V
corresponds to the battery float voltage during CC/CV
mode charging (MODE = 0V).
applicaTions inForMaTion
Figure 11. Battery Voltage Programming
Figure 10. Connection of Low-Voltage Input Supply
If desired operation places the internal 5V regulator out
of the allowable SOA region, deriving gate drive power
externally is required.
For driving the LTC4020 with an external 5V regulator,
connect the PVIN and INTVCC pins to that regulator output
as shown in Figure 9. The SENSVIN pin remains connected
to the input supply.
4020 F10
LTC4020
VIN
SENSVIN
INTVCC
VIN
(4.5V TO 5.5V)
4020 F11
RFB1
(BATTERY)
RFB2
VFB
VBAT
LTC4020
For operation with tightly regulated low voltage input sup-
plies (4.5V to 5.5V), the LTC4020 internal gate drivers and
BST refresh functions can be powered directly by the input
supply, eliminating the requirement for a 5V regulator to
supply the INTVCC pin. Connect the input supply to the
PVIN, INTVCC, and SENSVIN pins, as shown in Figure 10.
The resultant feedback signal is compared with the internal
2.5V voltage reference by the converter error amplifier.
The output voltage is given by the equation:
V(FLOAT(CC/CV) = 2.5V 1+ RFB1
RFB2
where RFB1 and RFB2 are defined as in Figure 11.
If charging in CC mode (MODE = -NC-), RFB1 and RFB2
corresponding to VFB = 2.5V programs a maximum VBAT
voltage, if constant-voltage functionality at that level if
desired.
LTC4020
27
4020fd
For more information www.linear.com/LTC4020
During lead-acid charging (MODE = INTVCC), the absorption
mode voltage corresponds to 2.5V on the VFB pin. Battery
float voltage (maintenance) corresponds to 2.3125V on the
VFB pin, or 92.5% of the absorption voltage. These volt-
ages typically correspond to 14.4V and 13.3V respectively
for a 6-cell (12V) battery.
The values for RFB1 and RFB2 are typically the same as
those used for the divider that programs the converter
safety limit (converter output to the VFBMAX pin; see
DC/DC Converter section), which yields a DC/DC converter
maximum regulation voltage, or safety limit, that is 10%
higher than the maximum battery charge voltage.
Common Battery Types:
Normalized RFB1 Resistor Values (RFB2 = 1)
BATTERY TYPE VOLTAGE RFB1
1-Cell LiFePO43.6V Float 0.44
1-Cell Li-Ion 4.2V Float 0.68
2-Cell LiFePO47.2V Float 1.88
2-Cell Li-Ion 8.4V Float 2.36
6-Cell Lead-Acid 12V Battery 4.76
3-Cell LiFePO410.8V Float 3.32
3-Cell Li-Ion 12.6V Float 4.04
4-Cell LiFePO414.4V Float 4.76
6-Cell LiFePO421.6V Float 7.64
12-Cell Lead-Acid 24V Battery 10.52
RCS: Battery Charge Current Programming
The LTC4020 senses battery charge current using a sense
resistor that is connected between the CSP and CSN
pins. Maximum average battery charge current (ICSMAX)
is programmed by setting the value of this current sense
resistor. The resistor value is selected so the desired
maximum charge current through that sense resistor
creates a 50mV drop, or:
RCS =
0.05V
ICSMAX
For example, for a maximum average charge current of
5A, use a 0.01Ω sense resistor.
PowerPath FET Function and Instant-On
The LTC4020 controls an external PMOS with its gate
connected to the BGATE pin. This PowerPath FET controls
current flow to and from the battery.
During a normal battery charge cycle, the BGATE pin is
pulled low (clamped at VGS
= 9.5V), which operates the FET
as a low impedance connection from the DC/DC converter
output to the battery, effectively shorting the battery to the
converter output. This minimizes power dissipation from
charge current passing thorough the FET. When there is
no VIN power or when the IC is in shutdown, LTC4020
connects the battery to the converter output by holding
the BGATE pin low, again effectively shorting the battery
to the converter output. This minimizes power dissipation
while the output is powered by the battery.
The LTC4020 controls the PowerPath FET to perform
instant-on operation when a charge cycle is initiated
into a heavily discharged battery. If the battery voltage is
below a programmed minimum operational output volt-
age, corresponding to VFBMIN = 2.125V, the PowerPath
FET is configured as a linear regulator, allowing the DC/
DC converter output to rise above the battery voltage
while still providing charge current into the battery.
During instant-on operation, the BGATE pin is driven by the
LTC4020 to maintain the minimum programmed voltage
on the PowerPath FET source, the FET acting as a high
impedance current source, providing charge current to
the battery, independent of the battery voltage.
applicaTions inForMaTion
Figure 12. Instant-On DC/DC Converter Output vs Battery
Voltage Characteristics
VBAT (V)
4020 F12
VOUT
VFBMIN = 2.125V
VBAT
LTC4020
28
4020fd
For more information www.linear.com/LTC4020
Figure 13. VOUT Instant-On Programming
Figure 14. Instant-On Charger Current Sense Limit Reduction
The resultant feedback signal is compared with the internal
2.125V voltage reference by a dedicated instant-on error
amplifier, the output of which servos the BGATE pin. The
output voltage is given by the equation:
VOUT = 2.125V 1+ RMIN1
RMIN2
where RMIN1 and RMIN2 are defined as in Figure 13.
When the DC/DC converter is operating, but the battery
charger is not in a charging cycle, the PowerPath FET is
automatically configured as an ideal diode between the BAT
pin (anode) and the CSN pin (cathode). The ideal diode
function allows the battery to remain disconnected from the
converter output while the converter is supplying power,
but also allows the battery to be efficiently engaged for ad-
ditional power should a load exceed the DC/DC converter’s
capability. This ideal diode circuit regulates the external
FET to achieve low loss conduction, maintaining a voltage
drop of 14mV across from the BAT pin to the CSN pin,
provided the battery current load though the ideal diode
does not exceed 14mV/RDS(ON). With larger currents, the
FET will behave like a fixed value resistor equal to RDS(ON).
In certain applications, the PowerPath function is not
required. For example, lead-acid chargers do not termi-
nate (they remain in float charging mode indefinitely),
so the battery need never be disconnected from the
output, provided the instant-on feature is not desired.
The PowerPath FET can be eliminated in these applica-
tions by tying the CSN side of the sense resistor to BAT,
connecting VFBMIN to ground, and connecting a 100pF
capacitor from the BGATE pin to CSN. See Typical Ap-
plication Circuits section.
RNG/SS: Dynamic Current Limit Adjust
Maximum charge current can be dynamically adjusted
using the RNG/SS pin as described in the Pin Description
section. Active servos can also be used to impose voltages
on the RNG/SS pin, provided they can only sink current.
Active circuits that source current cannot be used to drive
the RNG/SS pin.
applicaTions inForMaTion
Figure 15. Using the RNG/SS Pin for Digital Control of
Maximum Charge Current
4020 F15
10k
LOGIC HIGH = HALF CURRENT
RNG/SS
LTC4020
4020 F13
VOUT
RMIN1
RMIN2
VFBMIN
LTC4020
The values for RMIN1 and RMIN2 are typically the same as
those used for the divider that programs battery voltage
(to the VFB pin; see Battery Charger section), to yield a
DC/DC converter minimum operational regulation voltage
corresponding to 85% of the battery charge voltage.
During instant-on operation, if the drain-to-source voltage
across the PowerPath FET (VCSN – VBAT) exceeds 0.45V,
the maximum charge current is automatically reduced.
Maximum charge current is reduced linearly across the
range of 0.45V < VCSN – VBAT < 1.95V to one-fifteenth of
the current programmed by the battery charger sense
resistor, RCS. This reduction in charge current helps to
prevent excessive power dissipation in the PowerPath FET.
VCSN–BAT (V)
0
MAXIMUM CHARGE CURRENT (%)
100
90
70
50
80
60
40
30
20
10
00.5 11.25
4020 F14
2.251.5 1.75 2.520.25 0.75
LTC4020
29
4020fd
For more information www.linear.com/LTC4020
RNG/SS: Soft-Start
Soft-start functionality is also supported by the RNG/SS
pin. 50µA is sourced from the RNG/SS pin, so connect-
ing a capacitor from the RNG/SS pin to ground (CRNG/SS)
creates a linear voltage ramp. The maximum charge current
follows this voltage, thus increasing the charge current
capability from zero to the full programmed value as the
capacitor gets charged from 0 to 1V. The value of CRNG/SS
is calculated based on the desired time to full current (TSS)
following the relation:
CRNG/SS = 50µA • TSS
The RNG/SS pin is pulled to ground internally when charg-
ing is terminated so each new charging cycle begins with
a soft-start cycle. RNG/SS is also pulled to ground during
bad battery and NTC fault conditions.
Status Pins
The LTC4020 reports charger status through two open
collector outputs, the STAT1 and STAT2 pins. These pins
can accept voltages as high as 55V when disabled, and
can sink up to 5mA when enabled.
If the LTC4020 is configured for a CC/CV charging algo-
rithm, the STAT1 pin is pulled low while battery charge
currents exceed 10% of the programmed maximum (C/10).
The STAT1 pin is also pulled low during NTC faults. The
STAT2 pin is pulled low during NTC faults or after a bad
battery fault occurs. The STAT1 pin becomes high imped-
ance when a charge cycle is terminated or when charge
current is below the C/10 threshold, and the STAT2 pin
remains high impedance if no fault conditions are present.
If the LTC4020 is configured for a CC charging algorithm,
the STAT1 pin is pulled low during the entire charging
cycle, and the STAT2 pin is pulled low during NTC faults.
The STAT1 pin becomes high impedance when the charge
cycle is terminated.
If the LTC4020 is configured for a lead-acid charging
algorithm, the STAT1 and STAT2 pins are used as charge
cycle stage indicator pins. The STAT1 pin is pulled low
during the bulk and absorption charging stages and is high
impedance during the float charging period and during NTC
or bad battery faults. The STAT2 pin is pulled low during
bulk and float charging stages, and is high impedance
during the absorption charging stage and during NTC or
bad battery faults.
The STAT1 and STAT2 status pins are binary coded, and
signal following the table below, where ON indicates pin
pulled low, and OFF indicates pin high impedance:
applicaTions inForMaTion
STATUS PINS STATE
STAT1 STAT2
CC/CV
(MODE = 0V)
LEAD-ACID
(MODE = INTVCC)
CC
(MODE = -NC-)
OFF OFF Not Charging — Standby or
Shutdown Mode, ICS < C/10
Not Charging — NTC/Bad Battery Fault
or Shutdown
Not Charging — Standby or
Shutdown Mode, ICS < C/10
OFF ON Bad Battery Fault Float Charge Not Used
ON OFF Charging Cycle OK: Trickle Charge or
ICS > C/10
Absorption Charge Charge Cycle OK
ON ON NTC Fault Bulk Charge NTC Fault
Figure 17. Using the RNG/SS Pin for Soft-Start
4020 F17
CRNG/SS
RNG/SS
LTC4020
Figure 16. Driving the RNG/SS Pin with a Current Sink Active
Servo Amplifier
4020 F16
SERVO
REFERENCE
RNG/SS
LTC4020
–
+
LTC4020
30
4020fd
For more information www.linear.com/LTC4020
TIMER: C/10 Termination
The LTC4020 supports a low current based termination
scheme. This termination mode is engaged by shorting
the TIMER pin to ground.
When in CC/CV charge mode, a battery charge cycle ter-
minates when the current output from the charger falls to
below one-tenth the maximum charge current, or ICSMAX,
as programmed with RCS. The C/10 threshold current
corresponds to 5mV across RCS.
During lead-acid charging, the LTC4020 initiates float
charging when the absorption stage charge current is re-
duced to one-tenth of the programmed maximum current.
When charging in CC mode, the current source function
remains active indefinitely.
There is no provision for bad battery detection if C/10
termination is used.
TIMER: Timed Functions
The LTC4020 supports timer based functions, where bat-
tery charge cycle control occurs after a specific amount
of time elapses. Timer termination is engaged when a
capacitor (CTIMER) is connected from the TIMER pin to
ground. CTIMER for a desired end-of-cycle time (TEOC)
follows the relation:
CTIMER = TEOC • 6.87 x 10–2 (µF)
where TEOC is hours.
A typical timer TEOC for Li-Ion charge cycle termination
is three hours, which requires a 0.2µF timer capacitor.
The timer cycle starts when the charger transitions from
constant-current to constant-voltage charging, thus, ter-
mination at the end of the timer cycle only occurs if the
charging cycle was successful. When timer termination is
used, the STAT1 status pin is pulled low during a charging
cycle until the battery charge current falls below the C/10
threshold. The STAT1 pin stays high impedance with charge
currents below C/10, but the charger continues to top off
the battery until timer TEOC, when the LTC4020 terminates
the charging cycle and the PowerPath FET disconnects the
battery from the DC/DC converter output.
During lead-acid charging, the timer acts as an absorp-
tion mode safety timer. Normally, the LTC4020 initiates
float charging when the absorption stage charge current
is reduced to one-tenth of the programmed maximum
current, however, the maximum duration of absorption
charging is limited by the timer. If the charge current does
not fall to one-tenth of the programmed maximum current
by TEOC, the LTC4020 forces the battery charger to begin
float mode charging. A typical timer TEOC for lead-acid
charging is six to eight hours, which is accommodated
by a 0.47µF timer capacitor.
When charging in CC mode, after charge termination, once
the timer reaches TEOC and the charge cycle terminates,
input power or SHDN must be cycled to initiate another
battery charge cycle.
A bad battery detection function is available during
CC/CV or lead-acid charging. This fault condition is achieved
if the battery does not respond to preconditioning (VFB <
1.75V), such that the charger remains in (or enters) pre-
condition mode after one-eighth of the programmed TEOC
time. A bad battery fault halts the charging cycle, and the
fault condition is reported on the status pins. The bad battery
fault remains active until the battery voltage rises above the
precondition threshold, or until power or SHDN is cycled.
Battery Temperature Qualified Charging: NTC
The LTC4020 can accommodate battery temperature moni-
toring by using an NTC (negative temperature co-efficient)
thermistor close to the battery pack. The temperature
monitoring function is enabled by connecting a 10kΩ,
β = 3380 NTC thermistor from the NTC pin to ground. If
the NTC function is not desired, leave the pin unconnected.
The NTC pin sources 50µA, and monitors the voltage
dropped across the 10kΩ thermistor. When the voltage
on this pin is above 1.35V (0°C) or below 0.3V (40°C),
the battery temperature is out of range, and the LTC4020
triggers an NTC fault. The NTC fault condition remains until
the voltage on the NTC pin corresponds to a temperature
within the 0°C to 40°C range. Both hot and cold thresholds
incorporate hysteresis that corresponds to 5°C.
If higher operational charging temperatures are desired,
the temperature range can be expanded by adding series
resistance to the 10k NTC resistor. Adding a 910Ω resistor
will increase the effective HOT temperature threshold to
45°C. The effect of this additional resistance on the COLD
threshold is negligible.
applicaTions inForMaTion
LTC4020
31
4020fd
For more information www.linear.com/LTC4020
During an NTC fault, charging is halted and an NTC fault
is indicated on the status pins. If timer termination is en-
abled, the timer count is suspended and held until the fault
condition is relieved. The RNG/SS pin is also pulled low
during this fault, to accommodate a graceful restart, in the
event that a soft-start function is being incorporated (see
Dynamic Charge Current Adjust and Soft-Start section).
DC/DC Converter: External Compensation and
Filtering Components
The LTC4020 average current mode architecture employs
two integrating compensation nodes. The current setting
loop is compensated at the output of the current sense
amplifier on the VC pin, generally with a series R-C net-
work (RVC, CVC). The voltage generated on the VC pin is
compared with an internal ramp, providing control of the
converter duty cycle.
The voltage loop is compensated at the output of the error
amplifier on the ITH pin, generally with a series R-C net-
work (RITH, CITH). The voltage on the ITH pin is imposed
onto the current sense amplifier, setting the current level
to which the current loop will servo.
While determining compensation components, the
LTC4020 should initially be configured to eliminate any
functional contribution from the battery charger section.
This can be easily accomplished by connecting the NTC pin
to ground, which disables all battery charging functions
and puts the PowerPath FET into a high impedance state.
The current loop compensation (VC pin) transfer function
crossover frequency is typically set to approximately one-
half of the switching frequency; the voltage loop compen-
sation (ITH pin) transfer function crossover frequency is
typically set to approximately one-tenth of the switching
frequency.
Compensation values must be tested at high and low
input voltage operational limits, and also VIN ~ VOUT, so
that stable operation during all switching modes (buck,
boost, buck-boost) is verified.
If a network analyzer is not available for determining
compensation values, use procedures as outlined in
Application Note 19 for adjusting compensation. Appli-
cation Note 19 can be found at http://www.linear.com/
docs/4176.
VC pin compensation:
1. VNTC = VFBMAX = 0V
2. Fix VIN at typical voltage.
3. Fix VOUT at VFB regulation voltage. A charged battery,
battery simulator, or a 2-quadrant power supply can
be used for VOUT.
4. Impose 1V to 1.5V square wave (1kHz) on ITH pin
5. Monitor inductor current using current probe
6. Adjust compensation values as per AN19 until response
is critically damped
ITH pin compensation:
1. VNTC = 0V (disables charger)
2. Bring to regulation (VFBMAX = 2.75V)
3. Step load current on output (25% to 75% of IMAX)
4. Monitor VOUT voltage
5. Adjust compensation values as per AN19 until response
is critically damped and settled in ~10 to 25 cycles
6. VNTC = 0.8 (enable charger)
7. Exercise battery charger and verify stability in all modes
Battery Charger Functions: Filtering Components
Voltage Regulation Loop (VFB):
The charger voltage regulation loop monitors battery
voltage, and as such is controlled by a very slow moving
node. Battery ESR, however, can produce significant AC
voltages due to ripple currents, which can cause unstable
operation. This ESR effect can be reduced by adding a
capacitor to the VFB input, producing a low frequency pole.
applicaTions inForMaTion
Figure 18. VFB Ripple Suppression
4020 F18
VBAT
RFB1
RFB2
VFB
LTC4020
CVFB
LTC4020
32
4020fd
For more information www.linear.com/LTC4020
Figure 19. CSN/CSP Ripple Suppression
Current Sense Regulation Loop (CSN, CSP):
The charger current regulation loop monitors and regu-
lates battery charge current. Ripple voltage on the DC/DC
converter output, however, gets directly imposed across
the charger sense resistor, and can produce significant
ripple currents. Large ripple currents can corrupt low level
current sensing, and can also cause unstable operation.
This ripple current effect can be greatly reduced by adding
a capacitor (CCS) across the CSN and CSP pins, producing
a low frequency pole with the two 100Ω resistors that are
required for those pins. The filter frequency is typically set
to reduce voltage ripple across the current sense inputs
at the CSP and CSN pins to less than 1mVPP.
The ripple-reduction filter on the current sense inputs cre-
ates a phase shift in the charger current loop response,
which can result in instability. A resistor (RCSZ) in series
with CCS creates a zero that can be employed to recover
phase margin. This zero setting resistor will reintroduce
ripple error, so RCS should be minimized. CSOUT can be
coupled into ITH for a similar feedforward zero with RCS = 0.
Current sense information, or differential voltage at the CSN
to CSP pins, is amplified by a factor of 20 then output on
pin CSOUT. This signal is compared to a reference voltage
that is proportional to the maximum charge current at the
input of a transconductance amplifier, which creates an
error current that modulates the ITH compensation pin.
A feedforward zero can be employed to recover phase
margin by putting a capacitor from the CSOUT pin to the
ITH pin (CCSOUT). The output impedance of the CSOUT
pin is ~100kΩ, so if compensation requirements are ap-
propriate, the CCSOUT capacitor can perform double-duty
as both the primary pole ITH capacitor along with a 100kΩ
zero-setting resistance, and as feed forward coupling from
CSOUT to ITH.
Instant-On/Ideal Diode Regulation Loop (BGATE):
The instant-on function regulates the voltage across the
PowerPath FET by servoing the voltage at the BGATE pin.
Gate capacitance of the PowerPath FET is typically suf-
ficient to stabilize this loop. Additional capacitance can be
added to the BGATE pin (CBGATE) to stabilize the current
foldback loop during instant-on operation if necessary:
applicaTions inForMaTion
Figure 20. Instant-On/Ideal Diode Compensation
4020 F20
100Ω
VBAT
BAT
BGATE
CSN
LTC4020 CBGATE
RCS
4020 F19
100Ω
VBAT
CSN
CSP
LTC4020
100Ω
CCS
RCSZ
RCS
Layout Considerations
The LTC4020 is typically used in designs that involve
substantial switching transients. The switch drivers on the
IC are designed to drive large capacitances and, as such,
generate significant transient currents themselves. Supply
bypass capacitor locations must be carefully considered
to avoid corrupting the signal ground reference (SGND)
used by the IC. Typically, high current paths and transients
from the input supply and any local drive supplies must
be kept isolated from SGND, to which sensitive circuits
such as the error amp reference and the current sense
circuits are referred.
Effective grounding can be achieved by considering switch
current in the ground plane, and the return current paths of
each respective bypass capacitor. The VIN bypass return,
INTVCC bypass return, and the sources of the ground-
referred switch FETs carry PGND currents. SGND originates
at the negative terminal of the VOUT bypass capacitor, and
is the small signal reference for the LTC4020.
Do not be tempted to run small traces to separate ground
paths. A good ground plane is important as always, but
PGND referred bypass elements must be oriented such
that transient currents in these return paths do not corrupt
the SGND reference.
LTC4020
33
4020fd
For more information www.linear.com/LTC4020
During the dead time between synchronous switch and
main switch conduction, the body diode of the synchronous
FET conducts inductor current. Commutating the body
diode requires a significant charge contribution from the
main switch during initiation of main switch, creating a
current spike in the main switch. At the instant the body
diode commutates, a current discontinuity is created
between the inductor and main switch, with parasitic
inductance causing the switch node to transition in
response to this discontinuity. High currents and exces-
sive parasitic inductance can generate extremely fast δV/
δt times during this transition. These fast δV/δt transi-
tions can sometimes cause avalanche breakdown in the
synchronous FET body diode, generating shoot-through
currents via parasitic turn-on of the synchronous FET.
Layout practices and component orientations that minimize
parasitic inductance on the switched nodes is critical for
reducing these effects.
Orient power path components such that current paths in
the ground plane do not cross through signal ground areas.
Power ground currents are controlled on the LTC4020
via the PGND pin, and this ground references the high
current synchronous switch drive components, as well
as the local INTVCC supply. It is important to keep PGND
and SGND voltages consistent with each other. Separat-
ing these grounds with thin traces is not recommended.
When a ground referenced switch FET is turned off, gate
drive currents return to the LTC4020 PGND pin from the
switch FET source. The BOOST supply refresh surge cur-
rents also return through this same path. The switch FETs
must be oriented such that these PGND return currents
do not corrupt the SGND reference.
applicaTions inForMaTion
The high δi/δt loop formed by the switch MOSFETs and
the input capacitor (CVIN) should have short wide traces
to minimize high frequency noise and voltage stress
from inductive ringing. Surface mount components are
preferred to reduce parasitic inductances from component
leads. Switch path currents can be controlled by orient-
ing switch FETs, the switched inductor, and input and
output decoupling capacitors in close proximity to each
other. Locate the INTVCC, BST1, and BST2 decoupling
capacitors in close proximity to the IC. These capacitors
carry the switch FET gate drive currents. Locate the small
signal components away from high frequency switching
nodes (TG1, BG1, TG2, BG2, SW1, SW2, BST1, BST2,
and INTVCC). High current switching nodes are oriented
across the top of the LTC4020 package to simplify layout
and prevent corruption of the SGND reference.
Locate the output and battery charger feedback resistors
in close proximity to the LTC4020 and minimize the length
of the high impedance feedback nodes.
The SENSVIN and SENSTOP traces should be routed
together and SENSBOT and SENSGND should be routed
together. Keep these traces as short as possible, and avoid
corruption of these lines by high current switching nodes.
The LTC4020 packaging has been designed to efficiently
remove heat from the IC via the exposed pad on the
backside of the package. The exposed pad is soldered to
a copper footprint on the PCB. The exposed pad is electri-
cally connected to SGND, so a good connection to a PCB
ground plane effectively reduces the thermal resistance
of the IC case to ambient air.
Please refer to LTC Application Note 136, which discusses
guidelines, techniques, and considerations for switching
power supply PCB design and layout: http://www.linear.
com/docs/42146.
LTC4020
34
4020fd
For more information www.linear.com/LTC4020
applicaTions inForMaTion
LTC4020 Constant-Current/Constant-Voltage (CC/CV) Charging Diagram
POWER
AVAILABLE?
INDICATE NOT
CHARGING
BGATE PULLED
LOW
VFB > 2.5 – ε?
VFB > 1.75?
YES
NO
NO
NO NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
YES
4020 CD01
YES
YES
YESYES
YES
YES
VFB < 1.75?
VFB = 2.4375V
NTC
OUT-OF-RANGE?
CHARGE AT
CONSTANT-CURRENT
INDICATE CHARGING
TRICKLE CHARGE
(7%)
TIMER ACTIVE?
BGATE PULLED HIGH
INDICATE BAD
BATTERY FAULT
INDICATE NTC
FAULT
ENABLE IDEAL
DIODE FUNCTION
TIMERS ACTIVE?
PAUSE TIMERS
CHARGE TO FIXED
VOLTAGE
TIMER ACTIVE?
RUN SAFETY TIMER
SAFETY TIMER
AT EOC?
STOP CHARGING
INDICATE NOT
CHARGING
STOP CHARGING
IBAT < C/10?
IBAT < C/10?
START
CLEAR LOW BATTERY
AND SAFETY TIMERS
RUN LOW BATTERY
TIMER
BGATE PULLED LOW
INDICATE NOT
CHARGING
INDICATE CHARGING
TERMINATED
VFB < 2.5 – ε?
INDICATE NOT
CHARGING
ENABLE IDEAL DIODE
FUNCTION
YES
YES
YES
YES
YES
STOP CHARGING
NO TIMER AT 1/8
EOC?
YES
VFB = 2.4375? NO
YES
LTC4020
35
4020fd
For more information www.linear.com/LTC4020
applicaTions inForMaTion
LTC4020 Lead-Acid Charging Diagram
POWER
AVAILABLE?
INDICATE NOT
CHARGING
BGATE PULLED
LOW
SET ABSORPTION
REFERENCE (2.5V)
VFB < 2.5 – ε?
VFB < 1.75?
VFB > 1.75?
YES
NO
NO NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
YES
4020 CD02
NO
YES
YES
YES
YES
YES
YES
VFB < 2.125?
VFB = 2.4375V
NTC
OUT-OF-RANGE?
CHARGE AT
CONSTANT-CURRENT
BGATE PULLED LOW
SET ABSORPTION
REFERENCE (2.5V)
TIMER ACTIVE?
STOP CHARGING
BGATE PULLED HIGH
INDICATE NOT
CHARGING
INDICATE NOT
CHARGING
ENABLE IDEAL
DIODE FUNCTION
TIMERS ACTIVE?
PAUSE TIMERS
CHARGE TO FIXED
VOLTAGE
TIMER ACTIVE?
RUN SAFETY TIMER
SAFETY TIMER
AT EOC?
SET FLOAT
REFERENCE
(2.3125V)
INDICATE FLOAT
CHARGING
STOP CHARGING
IBAT < C/10?
IBAT < C/10?
START
CLEAR LOW BATTERY
AND SAFETY TIMERS
TRICKLE
CHARGE (7%)
INDICATE BULK
CHARGING
INDICATE
ABSORPTION
CHARGING
INDICATE BULK
CHARGING
FLOAT
REFERENCE SET
(2.3125)?
YES
YES
YES
NO YES
YES
YES
YES
RUN LOW
BATTERY TIMER
TIMER AT 1/8
EOC?
LTC4020
36
4020fd
For more information www.linear.com/LTC4020
applicaTions inForMaTion
LTC4020 Constant-Current Charging Diagram
POWER
AVAILABLE?
INDICATE NOT
CHARGING
BGATE PULLED
LOW CLEAR TIMER
NO
NO
NO
4020 CD02
SAFETY TIMER
AT EOC?
NTC
OUT-OF-RANGE?
CHARGE AT
CONSTANT-CURRENT
BGATE PULLED LOW
TIMER ACTIVE?
INDICATE NTC
FAULT
ENABLE IDEAL
DIODE FUNCTION
TIMERS ACTIVE?
PAUSE TIMER
STOP CHARGING
START
INDICATE CHARGING
RUN TIMER
YES
YES
YES
NO
NO YES
YES
INDICATE NOT
CHARGING
ENABLE IDEAL
DIODE FUNCTION
STOP CHARGING
LTC4020
37
4020fd
For more information www.linear.com/LTC4020
Typical applicaTions
5V to 30V to 6-cell lead-acid PowerPath charger/system supply. 6A inductor current limit with
2.5A battery charge current limit. Instant-on functionality incorporated for battery voltages below 12.25V,
14.4V absorption voltage, 13.3V float voltage,and 15.6V maximum output voltage (Instant-On and NTC fault only).
Status pins light LEDs for visible charge-state monitoring.
RSENSEA
0.008
Si7272DP
Si7272DP
Si7272DP
CMSH3-40MA
Si7272DP
RSENSEB
0.008
56µF
×2
VIN
5V TO 30V
LTC4020
SGNDBACK
PGND
4.7µF
SBR0560S1
R
T
, 130k
PVIN
BG1
SW1
TG1
BST1
SGND
SENSGND
95.3k
2nF 2nF
SENSBOT
SENSTOP
SENSVIN
RT
SHDN
VIN_REG
MODE
STAT1
STAT2
TIMER
RNG_SS
6-CELL LEAD-ACID (12V) 4020 TA02
INTVCC
BG2
SW2
TG2
BST2
SGND
VC
ITH
VFBMAX
ILIMIT
CSOUT
CSP
CSN
BGATE
BAT
VFBMIN
FBG
VFB
NTC
1µF
284k
100k
1µF
0.033µF
33k
2.7k
15µH
XAL1010-153MEB
SBR0560S1
BZX84C6V2L
680pF
100Ω
Si7135DP
R
CS
0.02Ω
100Ω
1µF
20k 95.3k
R
NTC
10k
20k
0.33µF
4.7µF
0.1µF
56µF
×3
VOUT
2.7k
+
+
LTC4020
38
4020fd
For more information www.linear.com/LTC4020
Typical applicaTions
15V to 55V to 6-cell Li-Ion PowerPath charger/system supply. 6A inductor current limit with 2.5A battery charge current limit.
Instant-on functionality for battery voltages below 20.4V, 24V charge termination voltage, and 26.4V maximum output voltage.
Status pins light LEDs for visible charge-state monitoring.
RSENSEA
0.008
Si7960DP
Si7960DP Si7960DP
MBRS360
Si7960DP
RSENSEB
0.008
4.7µF
×2
VIN
15V TO 55V
LTC4020
SGNDBACK
PGND
10µF
SBR0560S1
R
T
, 100k
PVIN
BG1
SW1
TG1
BST1
SGND
100pF
SENSGND
SENSBOT
SENSTOP
SENSVIN
RT
SHDNSHDN
VIN_REG
MODE
STAT1
STAT2
TIMER
RNG_SS
4020 TA03
INTVCC
BG2
SW2
TG2
BST2
SGND
VC
ITH
VFBMAX
ILIMIT
CSOUT
CSP
CSN
BGATE
BAT
VFBMIN
FBG
VFB
NTC
56µF
×2
0.1µF
10nF
1µF
56k
XAL1010-153MEB
15µH
SBR0560S1
BZX84C6V2L
680pF
68pF
47k
100Ω
R
CS
0.02Ω
Si7461DP
215k
4.7µF
×4
56µF
×2
VOUT
24V AT
3.5A MAX
0.033µF
2.7k
2.7k
0.2µF
105k
1µF
100Ω
24.9k
6-CELL Li-ION (24V)
R
NTC
10k
+
24.9k
2.2µF
4.7Ω
215k
1nF
10k
+
LTC4020
39
4020fd
For more information www.linear.com/LTC4020
Typical applicaTions
9V to 55V to 9-cell lead-acid (18V) charger/system supply with no PowerPath.
External 5V regulator for boosted supplies. 5A inductor current limit with 1.67A battery charge current limit.
21.5V absorption voltage output, 19.9V float voltage output.
RSENSEA
0.01
RSENSEB
0.01
Si7960DP Si7272DP
CMSH3-40MA
Si7272DP
Si7850DP
56µF
×2
VIN
9V TO 55V
LTC4020
SGNDBACK
PGND
4.7µF
SBR0560S1
R
T
, 100k
0.033µF
PVIN
BG1
SW1
TG1
BST1
SGND
SENSGND
SENSBOT
SENSTOP
SENSVIN
RT
SHDN
VIN_REG
MODE
STAT1
STAT2
TIMER
RNG_SS
9-CELL LEAD-ACID (18V) 4020 TA04
INTVCC
BG2
SW2
TG2
BST2
SGND
VC
ITH
VFBMAX
ILIMIT
CSOUT
CSP
CSN
BGATE
BAT
VFBMIN
FBG
VFB
10k
NTC
1µF
1.5nF
1µF
249k
100k
43k
43k
15µH
SBR0560S1
BZX84C6V2L
680pF
100Ω
1µF
0.1µF
4.7µF
VOUT
56µF
×3
LT3010-5
VIN OUT
SHDN SENSE
GND
+
XAL1010-153MEB
100Ω
R
CS
0.03Ω
100pF
0.33µF
75.9k
+
LTC4020
40
4020fd
For more information www.linear.com/LTC4020
5.00 ±0.10
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE
OUTLINE M0-220 VARIATION WHKD
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
PIN 1
TOP MARK
(SEE NOTE 6)
37
1
2
38
BOTTOM VIEW—EXPOSED PAD
5.50 REF
5.15 ±0.10
7.00 ±0.10
0.75 ±0.05
R = 0.125
TYP
R = 0.10
TYP
0.25 ±0.05
(UH) QFN REF C 1107
0.50 BSC
0.200 REF
0.00 – 0.05
RECOMMENDED SOLDER PAD LAYOUT
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
3.00 REF
3.15 ±0.10
0.40 ±0.10
0.70 ±0.05
0.50 BSC
5.5 REF
3.00 REF 3.15 ±0.05
4.10 ±0.05
5.50 ±0.05 5.15 ±0.05
6.10 ±0.05
7.50 ±0.05
0.25 ±0.05
PACKAGE
OUTLINE
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
PIN 1 NOTCH
R = 0.30 TYP OR
0.35 × 45° CHAMFER
UHF Package
38-Lead Plastic QFN (5mm × 7mm)
(Reference LTC DWG # 05-08-1701 Rev C)
package DescripTion
Please refer to http://www.linear.com/product/LTC4020#packaging for the most recent package drawings.
LTC4020
41
4020fd
For more information www.linear.com/LTC4020
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
revision hisTory
REV DATE DESCRIPTION PAGE NUMBER
A 01/14 Changed VIN to PVIN
Modified ISENSTOP Operating Current spec
Modified Error Amp Transconductance spec
Changed C/10 Detection Enable Units
Modified C/10 Detection Hysteresis spec
Changed Conditions for Gate Clamp Voltage
Changed Conditions for BGATE tests
Changed Conditions for Pin Current (Disabled) spec
Changed cathode to anode for BST1
Changed anode to cathode for BST1
Modified equations for TEOC and TPRE and associated TIMER text
Modified equations for RFB1/RFB2 and RMIN1/RMIN2
Changed cathode to anode for BST2
Changed anode to cathode for BST2
Modified Error Amplified Transconductance
Modified step-up and step-down equations in Switch FET section
Modified CTIMER equation and associated text
Modified Typical Applications circuit
Modified Typical Application circuit to 12-cell
2 to 5
3
3
4
4
4
5
5
8
8
9
10
12
12
14
24
30
38
42
B 09/14 Added “(Application Circuit on Page 37)” to efficiency curve title
Added Ω unit to RFBG specification
Changed “INTVCC Short Circuit Current Limit vs Temperature” curve y-axis units to ‘mA’
Added text to the end of the NTC (Pin 16) section
Corrected formula: (VOUTMAX/2.75) - 1
Changed BST1 on lower-right of block diagram to BST2; Insert “(VSENS)” below ‘2mV’ near VC pin
Added 2V Zener diode symbol from NTC pin (cathode) to ground (anode)
Added “average” to first line; change “Charge” to “Average Inductor” in Figure 6 title
Changed “inductor” to “charge”
Added ground symbol to bottom of IC symbol (backside connection)
Flipped PMOS symbol vertically (Si7461DP); add ground symbol to bottom of IC symbol (backside connection)
Added ground symbol to bottom of IC symbol (backside connection)
Moved connection of BZX84C6V2L anode from BG2 to SW2 (diode between BST2 and SW2); add ground symbol to
bottom of IC symbol (backside connection)
1
5
6
10
12
14
15
23
28
37
38
39
42
C 09/15 Added pin names to Typical Application IC drawing
Added text to end of SENSBOT (5) pin description section
Changed text in RNG/SS section: “Inductor” to “Charge”
Changed ILIMIT text, “...pin, so maximum charge current..." to “...pin, so maximum inductor current..."
Changed Operation section to “a 0.47µF capacitor on the TIMER pin is typically used, which generates a 6.8 hour
absorption stage safety timeout”
Changed “CSENSBOT,SENSGND” to “CSENSB”
Changed “...BGATE pin to ground” to “...BGATE pit to CSN”
Changed “CSN” to “CSN”. Replaced “RCS with RCSZ” in the text and in Figure 19. Replaced “RSENSE” with RCS in
Figure 19
Replaced “RSENSE” with “RCS” in schematic
1
8
9
12
20
22
28
32
37-39, 42
D 04/16 Modified bulk capacitance equation 24
LTC4020
42
4020fd
For more information www.linear.com/LTC4020
© LINEAR TECHNOLOGY CORPORATION 2013
LT 0416 REV D • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC4020
relaTeD parTs
Typical applicaTion
Remote 24V to 55V (48V system) input to 12-cell Li-Ion (48V) PowerPath charger/system supply. 5A inductor current limit with 2.5A
battery charge current limit. Minimum VIN is 24V as input regulation limits voltage loss due to line impedance. Battery termination
voltage is 48V with maximum output voltage of 52.8V. Instant-on functionality limits minimum regulated output voltage to 40.8V.
PART NUMBER DESCRIPTION COMMENTS
LTC3789 High Efficiency, Synchronous, 4 Switch Buck-Boost Controller Improved LTC3780 with More Features
LT3845 High Voltage Synchronous Current Mode Step-Down Controller For Medium/High Power, High Efficiency Supplies
LT3650 High Voltage 2A Monolithic Li-Ion Battery Charger 3mm × 3mm DFN-12 and MSOP-12 Packages
LT3651 High Voltage 4A Monolithic Li-Ion Battery Charger 4A Synchronous Version of LT3650 Family
LT3652/LT3652HV Power Tracking 2A Battery Chargers Multi-Chemistry, Onboard Termination
LTC4009 High Efficiency, Multi-Chemistry Battery Charger Low Cost Version of LTC4008, 4mm × 4mm QFN-20
LTC4012 High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control Similar to LTC4009 Adding PowerPath Control
LT3741 High Power, Constant Current, Constant Voltage, Step-Down Controller Thermally Enhanced 4mm × 4mm QFN and 20-Pin TSSOP
LT8705 80VIN/VOUT Sync Buck-Boost 4-SW Controller Single Inductor, TSSOP-38 and 5mm × 7mm QFN-38
RSENSEA
0.01
Si7850DP
Si7850DP
Si7850DP
Si7850DP
RSENSEB
0.01
VIN
24V TO 55V
LTC4020
SGNDBACK
PGND
R
T
, 100k
PVIN
BG1
SW1
TG1
BST1
SGND
SENSGND
SENSBOT
SENSTOP
SENSVIN
RT
SHDN
VIN_REG
MODE
TIMER
RNG_SS
12-CELL Li-ION (48V) 4020 TA05
INTVCC
BG2
SW2
TG2
BST2
SGND
VC
ITH
VFBMAX
ILIMIT
CSOUT
CSP
CSN
BGATE
BAT
VFBMIN
FBG
VFB
NTC
56µF
×2
1µF
191k
10k
12k
STAT1
STAT2
1µF
SBR0560S1
4.7µF
33k
100Ω
R
CS
0.02Ω
Si7465DP
SBR0560S1
BZX84C6V2L
680pF
100Ω
365k
1.5nF
2nF
0.33µF
356k
20k
20k
1µF
R
NTC
10k
0.1µF
4.7µF
56µF
×2
VOUT
IHLP-5050FD-5A
MBRS360
22µH
0.033µF
+
+
