LTC4041
1
Rev A
For more information www.analog.com
TYPICAL APPLICATION
FEATURES DESCRIPTION
2.5A Supercapacitor
Backup Power Manager
The LTC
®
4041 is a complete supercapacitor backup sys-
tem for 2.9V to 5.5V supply rails. It contains a high cur-
rent step-down DC/DC converter to charge a single super-
capacitor or two supercapacitors in series. When input
power is unavailable, the step-down regulator operates
in reverse as a step-up regulator to backup the system
output from the supercapacitor(s).
The LTC4041’s adjustable input current limit function
reduces charge current to protect the input supply from
overload while an external disconnect switch isolates the
input supply during backup. When the input supply drops
below the adjustable PFI threshold, the 2.5A boost regula-
tor delivers power from the supercapacitor to the system
output.
An optional input overvoltage protection (OVP) circuit
protects the LTC4041 from high voltage damage at the VIN
pin. An internal supercapacitor balancing circuit maintains
equal voltages across each supercapacitor and limits the
maximum voltage of each supercapacitor to a pre-deter-
mined value. The LTC4041 is available in a low profile
(0.75mm) 24-Lead 4mm × 5mm QFN package.
Single Supercapacitor 3.3V Backup Application Complete Backup Event with
a Single 10F Supercapacitor
APPLICATIONS
n 2.5A Step-Down Supercapacitor Charger and 2.5A
Step-Up Backup Supply
n 6.5A Switches for 2.5A Backup from One
Supercapacitor or Two in Series
n Input Current Limit Prioritizes Load over
ChargeCurrent
n Input Disconnect Switch Isolates Input During Backup
n Automatic Seamless Switch-Over to Backup Mode
n Internal Supercapacitor Balancer (No External
Resistors)
n Programmable Charge Current and Charge Voltage
n Input Power Fail Indicator
n System Power Good Indicator
n Optional OVP Circuitry Protects Device to >60V
n Constant Frequency Operation
n Thermally Enhanced 24-Lead 4mm × 5mm
QFN Package
n Ride-Through “Dying Gasp” Supplies
n High Current Ride-Through 3V to 5V UPS
n Power Meters/Industrial Alarms
n Servers/Solid State Drives All registered trademarks and trademarks are the property of their respective owners. Protected
by U.S. Patents, including 6522118, 6570372, 6700364, 8139329.
12mΩ
1.07M
340k
2.2µH
100µF
2.2µF
121k
75k
10F
VIN
PFO
CLN
IGATE
VSYS
BSTFB
RSTFB
SW
SCAP
CHGEN
BSTEN
CAPSEL
CAPFLT
CAPFB
SYSGD
CAPGD
IMON
GND
PFI
OVSNS
CPF
PROG
LTC4041
3.3V
INPUT
SUPPLY
3.3V
SYSTEM
OUTPUT
SUPERCAP
PINS NOT USED
IN THIS CIRCUIT:
BAL
1nF
1k
4041 TA01a
P
BACKUP
= 3.3W = 3.3V @ 1A
C
SYS
= 100µF
3.8 seconds
600ms/DIV
V
IN
1V/DIV
V
SYS
1V/DIV
V
SCAP
1V/DIV
V
SYSGD
2V/DIV
0V
0V
4041 TA01b
Document Feedback
LTC4041
2
Rev A
For more information www.analog.com
PIN CONFIGURATIONABSOLUTE MAXIMUM RATINGS
VIN (Transient) t < 1ms, Duty Cycle < 1% ..... –0.3V to 7V
VIN (Steady State), SCAP, BAL, CLN,
VSYS, BSTFB, PFI, CPF, CAPFB, CAPFLT,
PFO, SYSGD, OVSNS, IMON ............................–0.3V to 6V
BSTEN, CHGEN, CAPGD, RSTFB,
CAPSEL ........... –0.3V to [Max (VIN, VSCAP, VSYS) +0.3V]
IOVSNS .................................................................. ±10mA
ICAPGD, IPFO, ISYSGD ...............................................10mA
IPROG ....................................................................–1.1mA
Operating Junction Temperature Range
(Notes 2, 3) ............................................ –40°C to 125°C
Storage Temperature Range .................. –65°C to 150°C
(Note 1)
8 9
TOP VIEW
UFD PACKAGE
24-LEAD (4mm × 5mm) PLASTIC QFN
TJMAX = 125°C, θJA = 43°C/W, θJC = 3.4°C/W
EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB
10 11 12
24 23 22 21 20
6
5
4
3
2
1
VSYS
PROG
IMON
CHGEN
BSTEN
VIN
CLN
PFI
BSTFB
CPF
OVSNS
IGATE
PFO
CAPGD
VSYS
SCAP
SW
SW
CAPSEL
CAPFLT
BAL
RSTFB
SYSGD
CAPFB
7
14
15
16
17
18
19
13
25
GND
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC4041EUFD#PBF LTC4041EUFD#TRPBF 4041 24-Lead (4mm × 5mm × 0.75mm)
Plastic QFN –40°C TO 125°C
LTC4041IUFD#PBF LTC4041IUFD#TRPBF 4041 24-Lead (4mm × 5mm × 0.75mm)
Plastic QFN –40°C TO 125°C
Consult ADI Marketing for parts specified with wider operating temperature ranges.
Tape and reel specifications. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
LTC4041
3
Rev A
For more information www.analog.com
The l denotes the specifications which apply over the specified operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3) VIN = VSYS = 5V, VSCAP = 2.5V, RPROG = 2k, unless otherwise noted.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN Input Voltage Range l2.9 5.5 V
VSCAP Supercapacitor Voltage Range (Backup
Boost Input)
l5.4 V
Quiescent Current in Charger Mode with
Charging Complete and Backup Boost
Active (CAPSEL = 1)
VIN and VSYS Total Quiescent Current 800 1600 µA
SCAP Quiescent Current 13 26 µA
Quiescent Current in Charger Mode with
Charging Complete and Backup Boost in
Sleep (CAPSEL = 1)
VIN and VSYS Total Quiescent Current 275 550 µA
SCAP Quiescent Current 13 26 µA
Quiescent Current in Backup Mode with
Backup Boost in Sleep
(VIN = 0V, CAPSEL = 1)
VSYS Quiescent Current l75 150 µA
SCAP Quiescent Current l1 2 µA
Quiescent Current in Shutdown
(CHGEN = BSTEN = CAPSEL = 1, VSYS = 0V)
VIN Quiescent Current 5.5 11 µA
SCAP Quiescent Current l0 1 µA
Buck Supercapacitor Charger
VCAPFB CAPFB Pin Servo Voltage l0.788 0.80 0.812 V
ICAPFB CAPFB Pin Input Leakage Current –50 0 50 nA
ICHG Regulated Supercapacitor Charge Current RPROG = 2k, VSCAP >1V 950 1000 1050 mA
VSYS-to-VSCAP Differential Undervoltage
Lockout Threshold
(VSYS – VSCAP) Falling
(VSYS – VSCAP) Rising
30
100
50
150
70
200
mV
mV
VPROG PROG Pin Servo Voltage 800 mV
hPROG Ratio of Charge Current to PROG Pin Current 2500 mA/mA
Input Current Limit Threshold Voltage VIN – VCLN
l
23.5
22
25
25
26.5
28
mV
mV
AIMON Input Current Limit Amplifier Gain Ratio of VIMON to (VIN – VCLN) 32 V/V
CLN Input Bias Current VCLN = VIN 300 nA
VRECHRG Recharge Threshold Voltage As a Percentage of the Regulated VSCAP 96.2 97.5 98.8 %
End-of-Charge Indication PROG Pin Average Voltage 100 mV
CAPGD Rising Threshold As a Percentage of the Regulated VSCAP 90 92.5 95 %
Hysteresis As a Percentage of the Regulated VSCAP 2.5 %
fOSC(BUCK) Step-Down Converter Switching Frequency VSCAP>1V 2.0 2.25 2.5 MHz
RP(BUCK) High Side Switch On-Resistance 130 mΩ
RN(BUCK) Low Side Switch On-Resistance 120 mΩ
ILIM(BUCK) PMOS Switch Current Limit 3 4.3 A
Supercapacitor Balancer
VBAL Supercapacitor Balance Point As a Percentage of VSCAP, VSCAP = 5V 49 50 51 %
ISOURCE Balancer Source Current VSCAP = 5V, VBAL = 2.4V 50 mA
ISINK Balancer Sink Current VSCAP = 5V, VBAL = 2.6V 50 mA
Top/Bottom Supercapacitor Overvoltage
Threshold
(VSCAP – VBAL) and/or VBAL Rising, CAPSEL = 1 l2.7 2.8 V
Hysteresis 55 mV
Top/Bottom Supercapacitor Undervoltage
Threshold
(VSCAP – VBAL) and/or VBAL Falling, CAPSEL = 1 l–50 –20 mV
Hysteresis 30 mV
ELECTRICAL CHARACTERISTICS
LTC4041
4
Rev A
For more information www.analog.com
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Backup Boost Switching Regulator
VBSTFB BSTFB Pin Servo Voltage l0.78 0.8 0.82 V
IBSTFB BSTFB Pin Input Leakage Current VBSTFB = 0.9V –20 20 nA
VSYS-BACKUP Programmed Boost Output Voltage Range 2.7 5 V
fOSC(BST) Step-Up Converter Switching Frequency 1.0 1.125 1.25 MHz
ILIM(BST) NMOS Switch Current Limit 5.5 6.5 7.5 A
RP(BST) High Side Switch On-Resistance 75 mΩ
RN(BST) Low Side Switch On-Resistance 70 mΩ
VSYS Overvoltage Shutdown Threshold VSYS Rising 5.3 5.5 5.7 V
Hysteresis 100 mV
Boost Undervoltage Lockout Max(VSYS, VSCAP) Falling 2.5 V
Hysteresis 150 mV
DMAX Maximum Boost Duty Cycle 88 %
NMOS Switch Leakage Current BSTEN = 1, CHGEN = 1 0 1 µA
PMOS Switch Leakage Current BSTEN = 1, CHGEN = 1 0 1 µA
tMIN-BACKUP Minimum Backup Time CCPF = 1nF 2.2 ms
SYSGD Comparator
RSTFB Threshold VRSTFB Falling l0.72 0.74 0.76 V
Hysteresis 20 mV
IRSTFB RSTFB Pin Input Leakage Current VRSTFB = 0.9V –50 0 50 nA
SYSGD Delay VRSTFB Rising & Falling 100 µs
Power-Fail Comparator
PFI Input Threshold VPFI Falling
l
1.17
1.16
1.19
1.19
1.21
1.22
V
V
Hysteresis 40 mV
PFI Pin Leakage Current VPFI = 1.3V –100 0 100 nA
PFI Delay to PFO VPFI Falling 0.5 µs
PFO Pin Leakage Current VPFO = 5V 0 1 µA
PFO Pin Output Low Voltage IPFO = 5mA 65 200 mV
Logic Input (BSTEN, CHGEN, CAPSEL, CAPFLT)
VIL Logic Low Input Voltage l0.4 V
VIH Logic High Input Voltage l1.2 V
IIL Logic Low Input Leakage Current BSTEN, CHGEN 0 1 µA
IIH Logic High Input Leakage Current BSTEN, CHGEN 0 1 µA
CAPSEL Pin Leakage Current CAPSEL = 1 10 µA
The l denotes the specifications which apply over the specified operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3) VIN = VSYS = 5V, VSCAP = 2.5V, RPROG = 2k, unless otherwise noted.
ELECTRICAL CHARACTERISTICS
LTC4041
5
Rev A
For more information www.analog.com
The l denotes the specifications which apply over the specified operating
temperature range, otherwise specifications are at TA = 25°C. (Note 3) VIN = VSYS = 5V, VSCAP = 2.5V, RPROG = 2k, unless otherwise noted.
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: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. 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: The LTC4041E is tested under pulsed load conditions such that
TJ ≈ TA. The LTC4041E is guaranteed to meet performance 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 control. The
LTC4041I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The junction temperature (TJ in °C) is calculated from
the ambient temperature (TA, in °C) and power dissipation (PD, in watts)
according to the formula:
TJ = TA + (PD • θJA)
where the package thermal impedance θJA = 43°C/ W.
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.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Open-Drain Output (SYSGD, CAPGD)
Pin Leakage Current 5V at Pin 0 1 µA
Pin Output Low Voltage 5mA Into Pin 65 200 mV
CAPFLT Status Pin
CAPFLT Pin Pull-Down Current VCAPFLT = 200mV 10 µA
Pin Leakage Current 5V at Pin 0 1 µA
Overvoltage Protection
VOV(CUTOFF) Overvoltage Protection Threshold VOVSNS Rising, ROVSNS = 6.2k 6.0 6.4 6.8 V
VOVGT IGATE Output Voltage Active VIN = VOVSNS = 5V 9.4 12 V
VOVGT(LOAD) IGATE Voltage Under Load 5V Through 6.2k Into OVSNS, IIGATE = 1μA 8 8.6 V
IOVSNSQ OVSNS Quiescent Current VOVSNS = 5V 40 µA
OVSNS Quiescent Current in Shutdown BSTEN = 1, CHGEN = 1 25 µA
IGATE Time to Reach Regulation CIGATE = 2.2nF 3.5 ms
Overtemperature (OT) Protection
Overtemperature Shutdown Temperature Rising 160 °C
Hysteresis 15 °C
LTC4041
6
Rev A
For more information www.analog.com
TYPICAL PERFORMANCE CHARACTERISTICS
ISCAP vs VSCAP with Different
PROG Resistor Values
Step-Down Charger Efficiency
vs VSCAP VCAPFB vs Temperature
Step-Down Charger Oscillator
Frequency vs Temperature
Step-Down Charger PMOS
On-Resistance vs VSYS
Step-Down Charger NMOS
On-Resistance vs VSYS
Charging Profile: Two 10F
Supercapacitors In Series
TA = 25°C, unless otherwise noted.
Normal to Backup Mode
Transition Waveform
Backup to Normal Mode
Transition Waveform
V
CHG
= 4.5V
V
SYS
= 5V
R
PROG
= 806
R
PROG
= 1.33k
R
PROG
= 2k
R
PROG
= 4.02k
V
SCAP
(V)
0
0.8
1.6
2.4
3.2
4.0
4.8
0
0.5
1.0
1.5
2.0
2.5
3.0
I
SCAP
(A)
4041 G01
V
SYS
= 5V
R
PROG
= 806
R
PROG
= 1.33k
R
PROG
= 2k
R
PROG
= 4.02k
V
SCAP
(V)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
4041 G02
V
SYS
= 5V
TEMPERATURE (°C)
–45
–10
25
60
95
130
780
785
790
795
800
805
810
815
820
VOLTAGE (mV)
4041 G03
V
SYS
= 5.5V
V
SYS
= 5.0V
V
SYS
= 2.9V
TEMPERATURE (°C)
–45
–10
25
60
95
130
1.80
1.90
2.00
2.10
2.20
2.30
2.40
2.50
FREQUENCY (MHz)
4041 G04
130°C
25°C
–45°C
V
SYS
(V)
2.9
3.2
3.5
3.8
4.1
4.4
4.7
5.0
50
75
100
125
150
175
200
225
250
RESISTANCE (mΩ)
4041 G05
130°C
25°C
–45°C
V
SYS
(V)
2.9
3.2
3.5
3.8
4.1
4.4
4.7
5.0
50
75
100
125
150
175
200
225
250
RESISTANCE (mΩ)
4041 G06
V
SYS
= 5V
R
PROG
V
CHG
= 4.5V
2s/DIV
V
SCAP
2V/DIV
0V
I
SCAP
1A/DIV
0A
4041 G07
V
SCAP
= 3.5V
I
SYS
= 1A
R
PROG
C
SYS
= 100µF
200µs/DIV
V
IN
1V/DIV
V
SYS
1V/DIV
I
SCAP
2A/DIV
0V
0A
4041 G09
V
SCAP
= 3.5V
R
PROG
= 2k
I
SYS
= 1A
2ms/DIV
V
IN
1V/DIV
V
SYS
1V/DIV
I
SCAP
1A/DIV
0A
0V
4041 G08
LTC4041
7
Rev A
For more information www.analog.com
Charge Current Reduction Due to
Input Current Limit
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, unless otherwise noted.
PROG Voltage Transient
Response To System Step Load
Backup Boost Output Voltage
(VSYS) vs Temperature
Backup Boost Oscillator
Frequency vs Temperature
Backup Boost Maximum
Duty Cycle vs Temperature
Backup Boost Efficiency
vs Load Current for VSYS = 5V
Backup Boost Efficiency
vs Load Current for VSYS = 3.3V
Backup Boost NMOS
On-Resistance vs VSYS
Backup Boost PMOS
On-Resistance vs VSYS
V
SYS
= 5V
R
PROG
R
S
= 10mΩ
V
SCAP
= 2.5V
TOTAL INPUT CURRENT
I
SCAP
SYSTEM LOAD CURRENT (mA)
0
500
1000
1500
2000
2500
3000
0
500
1000
1500
2000
2500
3000
CURRENT (mA)
4041 G10
V
SCAP
= 3.5V
R
PROG
R
S
= 10mΩ
I
SCAP
V
PROG
I
SYS
TIME (ms)
–0.3
0
0.3
0.6
0.9
1.2
1.5
–1.2
–0.8
–0.4
0.0
0.4
0.8
1.2
–1.0
0
1.0
2.0
3.0
4.0
5.0
VOLTAGE (V)
LOAD CURRENT (A)
4041 G11
V
SYS
= SET TO 5V
V
SCAP
= 3.5V
I
SYS
= 1mA
TEMPERATURE (°C)
–45
–10
25
60
95
130
4.6
4.7
4.8
4.9
5.0
5.1
5.2
5.3
5.4
VOLTAGE (V)
4041 G12
V
SCAP
= 2.5V
V
SYS
= 5V
V
SYS
= 2.7V
TEMPERATURE (°C)
–45
–10
25
60
95
130
0.80
0.90
1.00
1.10
1.20
1.30
FREQUENCY (MHz)
4041 G13
V
SYS
= 5V
V
SYS
= 2.7V
TEMPERATURE (°C)
–45
–10
25
60
95
130
82
83
84
85
86
87
88
89
90
MAXIMUM DUTY CYCLE (%)
4041 G14
V
SYS
= 5V
V
SCAP
= 2.5V
V
SCAP
= 3.5V
V
SCAP
= 4.5V
LOAD CURRENT (A)
1m
10m
100m
1
3
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
4041 G15
V
SYS
= 3.3V
V
SCAP
= 1.5V
V
SCAP
= 2.0V
V
SCAP
= 2.5V
LOAD CURRENT (A)
1m
10m
100m
1
0
10
20
30
40
50
60
70
80
90
100
EFFICIENCY (%)
4041 G16
130°C
25°C
–45°C
V
SYS
(V)
2.65
3.05
3.45
3.85
4.25
4.65
5.05
20
40
60
80
100
120
140
160
RESISTANCE (mΩ)
4041 G17
130°C
25°C
–45°C
V
SYS
(V)
2.65
3.05
3.45
3.85
4.25
4.65
5.05
20
40
60
80
100
120
140
160
RESISTANCE (mΩ)
4041 G18
LTC4041
8
Rev A
For more information www.analog.com
TA = 25°C, unless otherwise noted.
TYPICAL PERFORMANCE CHARACTERISTICS
Boost Sleep Mode ISYSQ and
ISCAPQ vs Temperature
Backup Boost Transient
Response to Load Step
Burst Mode to Constant
Frequency Mode Transition
Waveform
OVP Module Shutdown Voltage
(Through 6.2k) vs Temperature
OVSNS Pin Quiescent Current
vs Temperature
Supercapacitor Balancer
Source/Sink Current
Minimum VSCAP to Maintain
Boost Regulation vs ISYS
V
SYS
= 5V
CAPSEL = 0V
I
SCAPQ
, V
SCAP
= 2.5V
I
SYSQ
, V
SCAP
= 2.5V
I
SCAPQ
, V
SCAP
= 4.5V
I
SYSQ
, V
SCAP
= 4.5V
TEMPERATURE (°C)
–45
–10
25
60
95
130
20
30
40
50
60
70
0
2
4
6
8
10
I
SYSQ
(µA)
I
SCAPQ
(µA)
4041 G19
V
SCAP
= 3.5V
C
SYS
= 100µF
L = 2.2µH
V
SYS
I
SYS
TIME (ms)
–0.4
0.0
0.4
0.8
1.2
1.6
4.5
4.6
4.7
4.8
4.9
5.0
5.1
5.2
5.3
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
VOLTAGE (V)
LOAD CURRENT (A)
4041 G20
V
SCAP
= 3.5V
C
SYS
= 100µF
L = 2.2µH
V
SYS
I
SYS
TIME (ms)
–0.4
0.0
0.4
0.8
1.2
1.6
4.5
4.6
4.7
4.8
4.9
5.0
5.1
5.2
5.3
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
VOLTAGE (V)
LOAD CURRENT (A)
4041 G21
TEMPERATURE (°C)
–45
–10
25
60
95
130
6.20
6.25
6.30
6.35
6.40
6.45
6.50
VOLTAGE (V)
4041 G22
V
OVSNS
= 5V
TEMPERATURE (°C)
–45
–10
25
60
95
130
20
25
30
35
40
45
50
55
60
CURRENT (µA)
4041 G23
CAPSEL = 5V
V
SCAP
= 5V
V
SCAP
= 2.5V
V
BAL
/V
SCAP
(%)
0
10
20
30
40
50
60
70
80
90
100
–140
–120
–100
–80
–60
–40
–20
0
20
40
60
I
BAL
(mA)
4041 G24
SYSGD FALLING
PROGRAMMED V
SYS
= 5V
PROGRAMMED V
SYS
= 3.3V
I
SYS
(A)
1m
10m
100m
1
3
0
0.5
1.0
1.5
2.0
2.5
3.0
V
SCAP
(V)
4041 G25
LTC4041
9
Rev A
For more information www.analog.com
PIN FUNCTIONS
VSYS (Pins 1, 24): System Voltage Output Pin. This pin is
used to provide power to an external load from either the
primary input supply or from the backup supercapacitor
if the primary input supply is not available. In addition to
supplying power to the load, this pin provides power to
charge the supercapacitor when input power is available.
VSYS should be bypassed with a low ESR ceramic capaci-
tor of at least 100μF to GND.
PROG (Pin 2): Charge Current Program Pin. An external
resistor from the PROG pin to GND programs the full-
scale charge current. At full scale, the PROG pin servos
to 0.8V. The ratio of the SCAP pin current to the PROG
pin current is internally set to 2500.
IMON (Pin 3): VSYS Current Monitoring Pin. The ratio
between the IMON pin voltage and the differential voltage
between VIN and CLN is internally set to 32. Charge cur-
rent is reduced when the IMON pin voltage reaches 0.8V.
CHGEN (Pin 4): Disable Pin for the Supercapacitor
Charger. Tie this pin to GND to enable the charger or to
a voltage above 1.2V to disable it. Do not leave this pin
unconnected.
BSTEN (Pin 5): Disable Pin for the Backup Boost Converter.
Tie this pin to GND to enable the boost backup or to a volt-
age above 1.2V to disable backup. Do not leave this pin
unconnected.
VIN (Pin 6): Input Pin. Power can be applied directly to
this pin if the optional overvoltage protection (OVP) fea-
ture is not used. For applications where the OVP feature
is required, connect an external N-channel FET between
the input supply VPWR and this pin.
CLN (Pin 7): Negative terminal pin for an external cur-
rent limit sense resistor connected between VIN and this
pin. This resistor is used to monitor the current from VIN
to VSYS. The LTC4041 reduces charge current in order
to maintain 25mV across this sense resistor. However,
it does not limit the system current if the drop exceeds
25mV.
CAPFLT (Pin 8): Open-Drain Supercapacitor Fault Status
Output. In charger mode, if the voltage of any single
supercapacitor exceeds 2.7V, this pin is pulled low and
charging is disabled. In backup mode, if the voltage of any
single supercapacitor falls below –20mV, the CAPFLT pin
is pulled low and the backup boost is disabled. To keep
charging or backup enabled under any supercapacitor
fault condition, tie this pin high. The current pull-down
capability of the CAPFLT is 10µA.
BAL (Pin 9): Supercapacitor Balance Point. Connect the
common node of a stack of two supercapacitors to this
pin. An internal supercapacitor balancer drives this node
to a voltage that is half of VSCAP. Leave this pin open if
only one supercapacitor is used.
RSTFB (Pin 10): SYSGD Comparator Input. High imped-
ance input to an accurate comparator with a 0.74V falling
threshold and 20mV hysteresis. This pin controls the state
of the SYSGD output pin. An external resistor divider is
used between VSYS, RSTFB and GND. It can be the same
resistor divider as the BSTFB divider to monitor the sys-
tem output voltage V
SYS
. See the Applications Information
section.
SYSGD (Pin 11): Open-Drain Status Output of the SYSGD
Comparator. This pin is pulled to GND by an internal
N-channel MOSFET whenever the RSTFB pin falls below
0.74V.
CAPFB (Pin 12): Supercapacitor (Single or a Stack of
Two) Feedback Pin. An external divider between the
SCAP pin and GND with the center tap connected to the
CAPFB pin programs the final supercapacitor (or stack)
voltage(VCHG). The voltage on this pin nominally servos
to 0.8V.
CAPGD (Pin 13): Supercapacitor Power Good Indicator
Pin. The open-drain output is pulled low until CAPFB rises
to 92.5% of its regulation point.
PFO (Pin 14): Open-Drain Power-Fail Status Output. This
pin is pulled to GND by an internal N-channel MOSFET
when the PFI input is below the falling threshold of the
power-fail comparator. Once the PFI input rises above the
rising threshold, this pin becomes high impedance.
LTC4041
10
Rev A
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PIN FUNCTIONS
IGATE (Pin 15): Gate Pin for the External N-Channel
FET(s). This pin is driven by an internal charge pump
to develop sufficient overdrive to fully enhance the pass
transistors. The first pass transistor, connected between
the input power supply and VIN, is a part of the optional
overvoltage protection module. The second pass transis-
tor, connected between VIN and VSYS, is mandatory and
is used to disconnect the system from the input supply
during backup mode.
OVSNS (Pin 16): Overvoltage Protection Sense Input. If
the overvoltage feature is used, the OVSNS pin should
be connected through a 6.2k resistor to the input power
supply and the drain of an N-channel MOS pass transistor.
If not, this pin should be shorted to VIN. When voltage is
detected on OVSNS, it draws a small amount of current
to power a charge pump which then provides gate drive
to IGATE to energize the external transistor(s). When the
voltage on this pin exceeds 6V (typical), IGATE is pulled
to GND to disable the pass transistor and protect the
LTC4041 from high voltage.
CPF (Pin 17): Minimum Backup Time (tMIN-BACKUP)
Program Pin. Connect a capacitor to this pin to set
tMIN-BACKUP. When backup mode is initiated, the LTC4041’s
backup boost converter stays on for at least tMIN-BACKUP
to prevent any unwanted mode switching. The output of
the power-fail comparator is ignored during this time. Do
not tie this pin to GND or leave it unconnected.
BSTFB (Pin 18): Feedback Input for the Backup Boost
Regulator. During backup operation, the voltage on this
pin servos to 0.8V.
PFI (Pin 19): Power-Fail Input. High impedance input to
an accurate comparator (power-fail) with a 1.19V falling
threshold and 40mV hysteresis. PFI controls the state of
the PFO output pin and sets the input voltage threshold
below which the boost backup is initiated. This thresh-
old voltage also represents the minimum voltage above
which the step-down supercapacitor charger is enabled
and power is allowed to flow from the input to the output
through the external pass transistor(s).
CAPSEL (Pin 20): Supercapacitor Stack Selector Pin. Tie
this pin to a voltage higher than 1.2V if a stack of two
supercapacitors is connected to the SCAP pin or to GND
if a single supercapacitor is connected to the SCAP pin.
Do not leave this pin unconnected.
SW (Pins 21, 22): Switch Pins for the Buck Charger and
the Boost Backup Converter. A 1μH to 2.2μH inductor
should be connected from SW to SCAP.
SCAP (Pin 23): Supercapacitor Pin. Connect a single
supercapacitor or the top of a two-supercapacitor stack
to this pin. Depending on the availability of input power,
the supercapacitor (or the stack) will either deliver power
to VSYS via the boost converter or be charged from VSYS
via the buck charger.
GND (Exposed Pad Pin 25): The exposed pad must
be soldered to the PCB to provide a low electrical and
thermal impedance connection to the printed circuit
board’s ground. A continuous ground plane on the sec-
ond layer of a multilayer printed circuit board is strongly
recommended.
LTC4041
11
Rev A
For more information www.analog.com
BLOCK DIAGRAM
CHARGER
CTRL
ENABLE
BAL
BUCK/BOOST
SUPERCAP CHARGER
BOOST BACKUP
BOOST
CTRL
+
–
+
–
+
–
+
–
+
–
+
–
+
–
14
8
13
20
25
12
23
18
21
4
5
22
10
11
L1
2
9
16
15
+
–
+
–
+
–
+
–
+
–
6 7
17 CP1
6V
OVERVOLTAGE PROTECTION
PWM
OT
6.2k
INPUT
CCPF
CPF
OVSNS
PFI
PFO
CAPFLT
CAPGD
CAPSEL
GND
CAPFB
4041 BD
PROG
RPROG
RFB1
RFB2
BAL
SCAP
CHGEN
BSTEN
SW
SW
BSTFB
RSTFB
RBFB1
RBFB2
COUT
CSCAP
CSCAP
SYSGD
SYSTEM
VSYS VSYS
MN2
CLN
IMON AMPLIFIER
AV = 32
IGATE VIN
MN1
1.19V
6.4k
–20mV
BAL
(SCAP-BAL)
0.74V
CAPFB
10µA
SUPERCAP
FAULT
LOGIC
POWER-FAIL
COMPARARTOR
RPF1
RPF2
19
2.7V
BAL
(SCAP-BAL) OR SCAP
0.8V 0.8V 0.8V
SUPERCAP
BALANCER
0.8V
0.74V
RS
1 243
IMON
IMON
2M
2M
+
–
+
–
+
–
LTC4041
12
Rev A
For more information www.analog.com
OPERATION
The LTC4041 is a complete supercapacitor backup system
manager for a 2.9V to 5.5V supply rail. The system has
three principal circuit components: a full-featured step-
down (buck) supercapacitor charger, a step-up (boost)
backup converter to deliver power to the system load
when external input power is lost, and a power-fail com-
parator to decide which one to activate. The LTC4041 has
several other auxiliary components: an input current limit
(IMON) amplifier, an optional input overvoltage protec-
tion (OVP) circuit, and a system power good (SYSGD)
comparator.
The LTC4041 has three modes of operation: normal,
backup and shutdown. If the input supply is above an
externally programmable PFI threshold voltage, the
LTC4041 is considered to be in normal mode. In this
normal mode power flows from input to output (V
SYS
)
while the step-down switching regulator charges a super-
capacitor or a stack of supercapacitors to a charge voltage
programmed by an external resistor divider connected at
the CAPFB pin. Refer to the Block Diagram.
The total system load is monitored by the IMON amplifier
via an external series resistor, RS, connected between the
VIN and CLN pins. This amplifier can reduce the charge
current from its programmed value (set by the PROG
pin external resistor RPROG) if the external load demand
increases beyond the level set by RS.
When the input supply falls below the PFI threshold,
backup mode disconnects the switches (MN1 and MN2)
to isolate the system (VSYS) from the input, and the boost
converter powers the system load from the supercapacitor
using the external inductor, L1.
THE SUPERCAPACITOR CHARGER
The LTC4041 includes a full-featured constant-current
(CC)/ constant-voltage (CV) supercapacitor charger with
programmable charge current and charge voltage, auto-
matic recharge, supercapacitor good indicator, superca-
pacitor overvoltage detection, and an internal balancer.
The charger is a high efficiency, constant frequency
(2.25MHz) synchronous buck converter used to charge
SCAP from VSYS via the SW pin. It is capable of directly
charging the supercapacitor to its charge voltage with an
externally programmable charge current up to 2.5A from
an input supply as high as 5.5V. A zero current com-
parator monitors the inductor current and shuts off the
NMOS synchronous rectifier once the current reduces to
approximately 250mA. This prevents the inductor current
from reversing and improves efficiency for low charg-
ing currents. The charger can be disabled by pulling the
CHGEN pin above 1.2V.
Constant-Current Mode Charging
In constant-current (CC) mode, the average current
delivered to the supercapacitor can reach 2000V/ RPROG.
Depending on the external load condition, the superca-
pacitor charger may or may not be able to charge at the
full programmed rate. The external load will always be
prioritized over the supercapacitor charge current. The
charger will charge at the full programmed rate only if
the sum of the external load and the charger input current
is less than or equal to the input current limit set by RS.
If the buck charger is operating at very low duty cycles
(i.e. if the supercapacitor voltage is very low), the actual
average charge current delivered to the supercapacitor
could vary by as much as 50% of the programmed value.
At low duty cycles, the measurement accuracy of the
inductor current sensing circuitry in the CC servo loop is
low. As a result, the average charge current could over-
shoot or undershoot. When the supercapacitor (or a stack
of supercapacitors) is charged from 0V, the low accuracy
of the inductor current sensing causes the buck to operate
in discontinuous mode. As the SCAP voltage increases
the buck will try to servo the average charge current to
the programmed value. When the SCAP voltage is about
1V, the buck exits discontinuous mode and the average
charge current will be at the programmed level. During
this discontinuous mode of operation, the VSYS voltage
ripple is still well-controlled despite the large inductor cur
-
rent ripple because the buck is running at a low duty cycle.
LTC4041
13
Rev A
For more information www.analog.com
Figure1 shows the buck charger operating in discon-
tinuous mode. The supercapacitor voltage is at 0V and
the charge current is programmed to 500mA. The VSYS
voltage ripple, which is also shown in the same figure, is
about 14mV in this example.
Figure1. Charge Current Waveform for VSCAP <1V
OPERATION
Charge Termination
The charge voltage of the supercapacitor (or the stack) is
set by an external resistor divider connected between the
SCAP pin and ground with its midpoint connected to the
CAPFB pin. As the voltage on the supercapacitor reaches
the pre-set charge voltage, the constant-voltage (CV) loop
of the buck charger starts to regulate the supercapacitor
voltage and the charge current decreases naturally. Once
the charge current drops to 12.5% of the programmed
charge current, the buck charger is disabled and no
charge current will be delivered to the supercapacitor. To
enable the buck charger and resume charging, the super-
capacitor voltage has to fall below the automatic recharge
threshold.
Automatic Recharge
Once the supercapacitor charger terminates, it remains
off drawing only microamperes of current from the super-
capacitor. To ensure that the supercapacitor is always
topped off, a charge cycle automatically begins when
the supercapacitor voltage falls below VRECHRG (typically
97.5%). To prevent brief excursions below VRECHRG from
enabling/disabling the buck charger unnecessarily, the
supercapacitor voltage must be below VRECHRG for at least
5ms (typical) for the charger to be re-enabled.
Supercapacitor Charge Status Indication via the
CAPGD Pin
The CAPGD pin is an open-drain output used to indicate
that the supercapacitor (or the stack) voltage has reached
92.5% of its regulation point. The CAPGD pin is pulled
low until the supercapacitor voltage is above 92.5% of
the final charge voltage at which point the CAPGD pin
becomes high impedance. The supercapacitor voltage has
to fall below 90% of the regulation point to pull the CAPGD
pin low again. The CAPGD pin requires an external pull-up
resistor to either the VSYS pin or to another appropriate
power source. When the charger is disabled, the CAPGD
pin is pulled low.
Supercapacitor Balancer
The LTC4041 has an internal balancer that servos the mid-
point of a stack of two supercapacitors, i.e. the BAL pin
voltage, to half the stack voltage (VSCAP). To activate the
balancer, tie the CAPSEL pin high to indicate that a stack
of two supercapacitors is connected to the SCAP pin with
the midpoint of the stack connected to the BAL pin. The
source/sink capability of the internal balancer is typically
±50mA with VSCAP at 5V. The balancer will try to balance
the stack of supercapacitors even after charging is com-
pleted. The balancer circuitry is disabled if the charger is
disabled. The balancer is also disabled if the CAPSEL pin
is low. When a single supercapacitor is connected to the
SCAP pin, tie the CAPSEL pin low and float the BAL pin.
Differential Undervoltage Lockout
An undervoltage lockout circuit monitors the differential
voltage between VSYS and SCAP and shuts off the char-
ger if the SCAP voltage reaches within 50mV of the VSYS
voltage. Charging does not resume until this difference
increases to 150mV.
C
SYS
= 100µF
12µs/DIV
V
SYS
20mV/DIV
0A
I
SCAP
500mA/DIV
AC–COUPLED
4041 F01
LTC4041
14
Rev A
For more information www.analog.com
OPERATION
Input Current Limit and IMON Monitor
The LTC4041 contains an input current limit circuit which
monitors the total system current (the external load plus
the charger input current) via an external series resis-
tor, R
S
, connected between the V
IN
and CLN pins. The
LTC4041 does not actually limit the external load but as
the external load demand increases, it reduces charge
current, if necessary, in an attempt to maintain a maxi-
mum of 25mV across the VIN and CLN pins. Refer to
Programming the Input Current Limit and IMON Monitor
section in Applications Information. However, if the exter-
nal load demand exceeds the limit set by RS, the LTC4041
does not reduce the load current but the charge current
will drop to zero. In all scenarios, the voltage on the
IMON pin will correctly represent the total system cur-
rent. 800mV on the IMON pin represents the full-scale
current set by the external series resistor, RS.
SUPERCAPACITOR FAULT INDICATION VIA THE
CAPFLT PIN
The LTC4041 is equipped with comparators to detect if
the voltage on the supercapacitor (or either supercapacitor
in the stack) has exceeded the overvoltage (OV) thresh-
old (2.7V typical) or has fallen below the undervoltage
(UV) threshold (–20mV typical). Overvoltage detection is
enabled only during charging and undervoltage detection
is enabled only during backup. Undervoltage detection is
also disabled if a single supercapacitor is used (CAPSEL
pin is set to low).
The CAPFLT pin is an open-drain output pin with a 10µA
(typical) pull-down current source. If the supercapacitor
is not under any fault conditions, the CAPFLT pin is high
impedance. If the supercapacitor is in an OV/UV condi-
tion, the CAPFLT pin is pulled low and charging or backup
is disabled. To ignore the fault condition (and continue
charging or backup), tie the CAPFLT pin high.
BACKUP BOOST CONVERTER
To supply the system load from the supercapacitor
in backup mode, the LTC4041 contains a 1.125MHz
constant-frequency current-mode synchronous boost
switching regulator with output disconnect and auto-
matic Burst Mode features. The regulator can provide a
maximum load of 2.5A from a supercapacitor (or a stack
of two supercapacitors) and the system output voltage
(VSYS) can be programmed up to a maximum of 5V via
the BSTFB pin. See the Applications Information section
for details. The converter can be disabled by pulling the
BSTEN pin high. The boost regulator includes safety fea-
tures like short-circuit current protection, input undervolt-
age lockout, and output overvoltage protection.
Zero Current Comparator
The LTC4041 boost converter includes a zero current
comparator which monitors the inductor current and
shuts off the PMOS synchronous rectifier once the current
drops to approximately 250mA. This prevents the induc-
tor current from reversing in polarity thereby improving
efficiency at light loads.
PMOS Synchronous Rectifier
To prevent the inductor current from running away, the
PMOS synchronous rectifier is only enabled when VSYS >
(VSCAP – 200mV). Additionally, if the current through the
synchronous FET (PMOS) ever exceeds 8A, the converter
skips the next two clock cycles so that the inductor cur-
rent has a chance to discharge safely below this level.
Short-Circuit Protection
The output disconnect feature enables the LTC4041 boost
converter to survive a short circuit at its output. It incor-
porates internal features such as current limit foldback
and thermal shutdown for protection from excessive
power dissipation during short circuit.
Max(VSYS,VSCAP) Undervoltage Lockout
The LTC4041 incorporates an undervoltage lockout circuit
which shuts down the boost regulator when max(VSYS,
VSCAP) drops below 2.5V. This is to ensure that the boost
regulator has enough supply voltage to function properly.
LTC4041
15
Rev A
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OPERATION
Boost Overvoltage Protection
If the BSTFB node were inadvertently shorted to ground,
the boost converter output voltage (V
SYS
) would increase
indefinitely with the maximum current that could be
sourced from the supercapacitor. The LTC4041 protects
against this by shutting off both switches if the output
voltage exceeds 5.5V.
Burst Mode Operation
The LTC4041 boost converter provides automatic Burst
Mode operation which increases the efficiency of power
conversion at very light loads. Burst Mode operation is
initiated if the output load current falls below an internally
set threshold. Once Burst Mode operation is initiated, only
the circuitry required to monitor the output and the super-
capacitor undervoltage comparators (if CAPSEL=H)
are kept alive. This is referred to as the sleep state in
which the backup boost consumes only 75μA (typical,
CAPSEL=H) from the system output and 1μA (typical)
from the supercapacitor(s). When the VSYS pin voltage
drops by about 1% from its nominal value, the boost con-
verter wakes up and commences normal PWM operation.
The output capacitor recharges and causes the LTC4041
to re-enter the sleep state if the output load remains less
than the Burst Mode threshold. The frequency of this
intermittent PWM or Burst Mode operation depends on
the load current. As the load current drops below the
burst threshold, the boost converter turns on less fre-
quently. When the load current increases above the burst
threshold, the converter seamlessly resumes continuous
PWM operation. Thus, Burst Mode operation maximizes
the efficiency at very light loads by minimizing switching
and quiescent losses. However, the output ripple typically
increases to about 2% peak-to-peak. Burst Mode ripple
can be reduced in some circumstances by placing a small
phase-lead capacitor (CPL) between the VSYS and BSTFB
pins. However, this may adversely affect the efficiency and
the quiescent current at light loads. Typical values of CPL
range from 15pF to 100pF.
VSCAP > VSYS Operation
The LTC4041 boost converter will maintain voltage regu-
lation even if its input voltage is above the output voltage.
This is achieved by terminating the switching of the syn-
chronous PMOS and applying VSCAP voltage statically on
its gate. This ensures that the slope of the inductor current
reverses during the time current is flowing to the output.
Since the PMOS no longer acts as a low impedance switch
in this mode, there will be more power dissipation within
the IC. This will cause a sharp drop in the efficiency. The
maximum output current should be limited in order to
maintain an acceptable junction temperature.
SYSGD COMPARATOR
The LTC4041 contains a SYSGD comparator which moni-
tors VSYS under all operating modes via the RSTFB pin
and reports the status via an open-drain NMOS transistor
on the SYSGD pin. At any time, if VSYS falls 7.5% from its
programmed value, the SYSGD pin pulls low after a 100µs
(typical) delay. The comparator also waits 100µs (typical)
after V
SYS
rises above the threshold before making the
SYSGD pin high impedance. Refer to Programming the
SYSGD Comparator section in Applications Information.
POWER-FAIL COMPARATOR AND MODE SWITCHING
The LTC4041 contains a fast power-fail comparator which
switches the LTC4041 from normal to backup mode in the
event the input supply voltage falls below an externally
programmed threshold voltage. This threshold voltage
is programmed by an external resistor divider via the PFI
pin. See the Applications Information section for details
on how to choose values for the resistor divider. The out-
put of the power-fail comparator also directly drives the
gate of an open-drain NMOS to report the status of the
availability of input power via the PFO pin. If input power
is available, the PFO pin is high impedance; otherwise,
the pin is pulled down to ground.
LTC4041
16
Rev A
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OPERATION
At the onset of backup mode, the supercapacitor charger
shuts off and the external NMOS pass transistors (MN1
and MN2 in the Block Diagram) are quickly turned off by
discharging the IGATE pin to ground, thereby disconnect-
ing the system output VSYS from the input and activat-
ing the backup boost converter to promptly deliver load
from the supercapacitor. Although the power-fail com-
parator has a hysteresis of approximately 40mV, it may
not be able to overcome the input voltage spike resulting
from the sudden collapse of the forward current from the
input to VSYS. To prevent repetitive mode switching, the
backup boost stays on for at least the minimum backup
time (tMIN-BACKUP) once activated. The minimum backup
time is programmed by connecting an external capacitor
between the CPF pin and ground. Refer to Programming
the Minimum Backup Time section in the Applications
Information. During this time, the power-fail comparator
output is ignored and an internal switch of approximately
270Ω pulls down the OVSNS pin to help discharge the
input. After the minimum backup time has elapsed, if the
power-fail comparator output indicates that power is still
not available, the backup boost continues to deliver the
load but the pull-down on the OVSNS pin is released.
When the power-fail comparator detects that input power
is available, the OVP charge pump starts to charge up
the IGATE pin but the backup boost converter continues
to deliver system load until IGATE is approximately 8V.
This ensures that the forward conduction path through
the external NMOS pass transistors has been established.
At this point, the backup boost gets deactivated and the
charger turns back on to charge the supercapacitor while
the system load gets delivered directly from the input to
VSYS through the pass transistors.
OPTIONAL INPUT OVERVOLTAGE PROTECTION (OVP)
The LTC4041 can protect itself from the inadvertent appli-
cation of excessive voltage with just two external com-
ponents: an N-channel FET (MN1) and a 6.2k resistor as
shown in the Block Diagram. The maximum safe overvolt-
age magnitude is determined by the choice of external
NMOS and its associated drain breakdown voltage.
The optional overvoltage protection (OVP) module con-
sists of two pins. The first, OVSNS, is used to measure the
applied voltage through an external resistor. The second,
IGATE, is an output used to drive the gate pins of two
external N-channel FETs, MN1 and MN2 (Block Diagram).
The voltage at the OVSNS pin will be lower than the OVP
input voltage by about 250mV due to the OVP circuit’s
quiescent current flowing through the OVSNS resistor.
When OVSNS is below 6V, an internal charge pump drives
IGATE to approximately 1.88 • V
OVSNS
. This enhances the
N-channel FETs providing a low impedance connection to
VSYS and power to the LTC4041. If OVSNS rises above 6V
due to a fault, IGATE is pulled down to ground, disabling
the external FETs to protect downstream circuitry. At the
same time, the backup boost converter activates to sup-
ply the system load from the supercapacitor. When the
voltage drops below 6V again, the external FETs are re-
enabled. If the OVP feature is not desired, remove MN1,
short OVSNS to V
IN
, and apply external power directly
to VIN.
SHUTDOWN MODE OPERATION
The LTC4041 can be shutdown almost entirely by pulling
both CHGEN and BSTEN pin above 1.2V. In this mode,
the internal charge pump is shutdown and IGATE is pulled
to ground disconnecting the forward path from input to
output via the external FETs. Only the internal OVP shunt
regulator remains active to monitor the input supply for
any possible overvoltage condition and consuming about
25μA via the OVSNS pin. Total current draw from the
SCAP pin drops to below 1μA (VSCAP = 2.5V) in shutdown.
Overtemperature (OT) Protection
When the LTC4041 die temperature exceeds 160°C (typi-
cal), the buck charger and backup boost are shut down
to prevent any thermal damage and remain in shutdown
until the die temperature falls to 145°C (typical). In OT,
the forward path from VIN to VSYS is disconnected by
pulling the gate voltage of the external FET(s) to ground.
LTC4041
17
Rev A
For more information www.analog.com
Programming the Supercapacitor Charge Voltage
The charge voltage for a supercapacitor or a stack of
supercapacitors is set by an external resistor divider as
shown in Figure2. The charge voltage is given by the
following equation:
VCHG =0.8V • 1+RFB1
RFB2
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
where 0.8V is the typical CAPFB pin servo voltage
(VCAPFB). Typical values for RFB1 and RFB2 are in the range
of 40k to 2MΩ. Small resistor values result in higher leak-
age current that will discharge the supercapacitor. If the
resistor values are too large, the parasitic capacitance on
the CAPFB pin could create an additional pole and cause
loop instability.
APPLICATIONS INFORMATION
Programming the Input Voltage Threshold for the
Power-Fail Comparator
The input voltage threshold below which the power-fail
status pin PFO indicates a power-fail condition and the
LTC4041 activates the backup boost operation can be
programmed by using a resistor divider from the supply
to GND via the PFI pin such that:
VIN(PF) =1.19V • 1+RPF1
RPF2
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
where 1.19V is the typical power fail threshold voltage
(VPFI). See Block Diagram. The power fail threshold volt-
age should be set to a level between 200mV to 300mV
Figure2. Programming the Charge Voltage
below the nominal input supply voltage so that supply
transients do not trip the comparator. On the other hand,
it should be set high enough so that the V
SYS
voltage
does not drop enough to trip the SYSGD comparator dur-
ing the transition to backup mode. For applications using
the overvoltage protection (OVP) module, select a value
greater than 35k for RPF1.
Programming the Supercapacitor Charge Current
Supercapacitor charge current is programmed using a
single resistor from the PROG pin to ground. To set a
charge current of ICHG, the PROG pin resistor value can
be determined using the following equation:
RPROG =2500 •
0.8V
ICHG
=
2000V
ICHG
where 0.8V is the typical PROG pin servo voltage (V
PROG
).
For example, to set the charge current to 1A, the value
of the PROG pin resistor should be 2k. The minimum
recommended charge current is 500mA, below which the
accuracy of the charge current suffers. This corresponds
to a maximum R
PROG
resistor of 4k. The maximum charge
current is 2.5A.
Programming the Input Current Limit and IMON
Monitor
The input current limit is programmed by connecting a
series resistor between the VIN and CLN pins. To limit the
total system current to ISYSLIM, the value of the required
resistor can be calculated using the following equation:
RS=25mV
I
SYSLIM
For example, to set the current limit to 2A, the series
resistor should be 12.5mΩ. As discussed in the Operation
section, the LTC4041 does not limit the system current
but reduces the charge current to zero in the event the
system load exceeds this limit.
LTC4041 RFB1
RFB2
4041 F02
SCAP
CAPFB
LTC4041
18
Rev A
For more information www.analog.com
APPLICATIONS INFORMATION
The voltage on the IMON pin always represents the total
system current ISYS through the external series resis-
tance, RS. A voltage of 800mV on IMON represents the
full-scale current set by RS. The system current can be
calculated from the IMON pin voltage by using the fol
-
lowing equation:
ISYS =
V
IMON
32 • RS
For example, if the IMON pin voltage is 600mV and RS is
12.5mΩ, then the total system current is 1.5A. As shown
in the block diagram, the IMON pin is not buffered inter-
nally, so it is important to isolate this pin before connect-
ing to an ADC or any other monitoring device. Failure to
do so can degrade the accuracy of this circuit.
Programming the Boost Output Voltage
The boost converter output voltage in backup mode can
be programmed for any voltage from 2.7V to 5V by using
a resistor divider from the VSYS pin to GND via the BSTFB
pin such that:
VSYS =0.8V • 1+RBFB1
RBFB2
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
where 0.8V is the typical BSTFB pin servo voltage (VBSTFB).
See the Block Diagram. Typical values for RBFB1 and RBFB2
are in the range of 40k to 2M. Too small a resistor results
in a large quiescent current whereas too large a resistor
coupled with any parasitic BSTFB pin capacitance creates
an additional pole and may cause loop instability.
Programming the SYSGD Comparator
The threshold for the SYSGD comparator can be pro-
grammed by using a resistor divider from the VSYS pin
to GND via the RSTFB pin such that:
VSYS(SYSGD) =0.74V • 1+RBFB1
RBFB2
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
where 0.74V is the typical SYSGD pin (falling) threshold
voltage (VRSTFB). See the Block Diagram. Typical value for
RBFB1 and RBFB2 are in the range of 40k to 2M. In most
applications, the BSTFB and RSTFB pins can be shorted
together and only one resistor divider between VSYS and
GND is needed to set the VSYS voltage during backup
mode and the SYSGD threshold 7.5% below the VSYS
programmed voltage.
Programming the Minimum Backup Time
The minimum backup time can be programmed by con-
necting an external capacitor between the CPF pin and
ground. For a given capacitor (CCPF), tMIN-BACKUP can be
calculated by the following equation:
tMIN-BACKUP (ms)=2.2 • CCPF (nF)
It is recommended to set tMIN-BACKUP in the range of 1ms
to 0.5s. If t
MIN-BACKUP
is too short, the LTC4041 could
oscillate between charging and backup unnecessarily.
If the minimum backup time is too long, the amount of
energy drained from the supercapacitor on any single
backup event may be more than necessary.
Note: When the LTC4041 is powered on, the C
CPF
capaci-
tor is pre-charged by the internal circuitry to 1V (typical)
with a 1µA current source. The time taken for the initial
pre-charge is given by:
tPRE-CHARGE (ms) = 1 • CCPF (nF)
If a backup event occurs during this pre-charge time, the
total minimum backup duration will be longer than the
programmed value.
Choosing the External Resistor for the Overvoltage
Protection (OVP) Module
When overvoltage protection is activated, the OVSNS pin
is clamped at 6V. The external 6.2k resistor must be sized
appropriately to dissipate the resultant power. For example,
a 1/8W, 6.2k resistor can have at most √PMAX • 6.2kΩ =
28V applied across its terminals. With 6V at OVSNS, the
maximum overvoltage magnitude that this resistor can with-
stand is 34V. A 0.25W, 6.2k resistor raises the value to 45V.
The OVSNS pin’s absolute maximum current rating of 10mA
imposes an upper limit of 68V protection.
LTC4041
19
Rev A
For more information www.analog.com
APPLICATIONS INFORMATION
Choosing the External Transistors (MN1 and MN2) for
the OVP Module and the Input-to-Output Disconnect
Switch
The LTC4041 uses a weak internal charge pump to pump
IGATE above the input voltage so that the N-channel exter-
nal FETs can be used as pass transistors. However, these
transistors should be carefully chosen so that they are
fully enhanced with a VGS of 3V. Since one of these pass
transistors is the OVP FET, its breakdown voltage (BVDSS)
determines the maximum voltage the LTC4041 can with-
stand at its input. Also, care must be taken to avoid any
leakage on the IGATE pin, as it may adversely affect the
FET operation. See Table1 for a list of recommended
transistors.
Table1. Recommended NMOS FETs for Overvoltage Protection
and Disconnect Switch
NMOS FET BVDSS RON
SIR424DP (Vishay) 20V 7.4mΩ
SiS488DN (Vishay) 40V 7.5mΩ
SiS424DN (Vishay) 20V 8.9mΩ
Choosing the Inductor for the Switching Regulators
Since the same inductor is used to charge the superca-
pacitor in normal mode and to deliver the system load in
backup mode, its inductance should be low enough so
that the inductor current can reverse quickly as soon as
backup mode is initiated. On the other hand, the induc-
tance should not be so low that the inductor current is
discontinuous at the lowest charge current setting since
charge current accuracy suffers greatly if the inductor
current is discontinuous. Inductor current ripple (ΔIL) can
be computed using the following equation:
ΔIL=VSCAP • 1– VSCAP
VSYS
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟•1
L • fOSC
Since the lowest recommended charge current set-
ting is 500mA, inductor current will be discontinuous if
the ripple is more than twice that amount, i.e, 1A. For
VSYS = 5V, VSCAP = 3.2V, fOSC = 2.25MHz (buck mode),
and ΔIL = 1A, the theoretical minimum inductor size to
avoid discontinuous operation can be computed using the
above equation to be 0.5μH. To account for inaccuracies
in the system and component values, the practical lower
limit should be 1μH. Since the backup boost operates
at half the frequency (1.125MHz), the inductor current
ripple with a 1μH inductor using the same equation will
be approximately 1A in backup mode. If this is excessive,
inductors up to 2.2μH can be used to lower the inductor
current ripple.
The other considerations when choosing an inductor are
the maximum DC current (IDC) and the maximum DC
resistance (DCR) rating as shown in Table2. The chosen
inductor should have a max IDC rating which is greater
than the current limit specification of the LTC4041 in
order to prevent an inductor current runaway situation.
For the LTC4041, the maximum current that the inductor
can experience is approximately 8A in backup mode. It is
also important to keep the max DCR as low as possible
in order to minimize conduction loss to and help improve
the converter’s efficiency.
Table2. Recommended Inductors for the LTC4041
PART
NUMBER
L
(μH)
MAX
IDC
(A)
MAX
DCR
(mΩ)
SIZE IN mm
(L × W × H) MANUFACTURER
XAL-5020-122
1.2 8.3 20.5 5.68 × 5.68 × 2 Coilcraft
www.coilcraft.com
XAL-6030-122
1.2 10.8 7.5 6.76 × 6.76 × 3.1 Coilcraft
www.coilcraft.com
XAL-6020-132
1.3 915.4 6.76 × 6.76 × 2.1 Coilcraft
www.coilcraft.com
XAL-6030-182
1.8 14
10.52
6.76 × 6.76 × 3.1 Coilcraft
www.coilcraft.com
XAL-5030-222
2.2 9.2 14.5 5.3 × 5.5 × 3.1 Coilcraft
www.coilcraft.com
XAL-6030-222
2.2 15.9
13.97
6.38 × 6.58 × 3.1 Coilcraft
www.coilcraft.com
831532200 2.2 14 15.3 6.5 × 7 × 3 Wurth Electronics
www.we-online.
com
Choosing the VSYS Capacitor
The worst-case delay for the backup boost converter to
meet the system load demand occurs when the PFI input
falls below the externally set threshold at a time when
the buck charger is charging at the highest setting of
LTC4041
20
Rev A
For more information www.analog.com
APPLICATIONS INFORMATION
2.5A and the system load is also very high, e.g., 2.5A.
Under this scenario, as soon as the LTC4041 initiates
backup mode, the inductor current has to reverse from
2.5A (from SW to SCAP) to as high as the boost current
limit of approximately 6.5A (from SCAP to SW). That is a
9A current change in the inductor with a slope of V
SCAP
/L.
At a low supercapacitor voltage of 3.2V, this would take
almost 3μs even with a 1μH inductor. During this transi-
tion, CSYS, the capacitor on the VSYS pin, has to deliver
the shortfall until the inductor current catches up with
the system load demand, and the capacitor will deplete
according to the following equation:
CSYS =ILOAD •Δt
ΔV
The size of the capacitor should be big enough to hold
the system voltage, VSYS, up above the SYSGD threshold
during this transition. For a system load ILOAD = 2.5A and
transition time Δt = 3μs, if the maximum droop ΔV allowed
in the system output is 100mV, the required capacitance
at the VSYS pin should be at least 75μF. The other consid-
eration for choosing the VSYS capacitor size is the maxi-
mum acceptable output voltage ripple during steady-state
backup boost operation. For a given duty cycle D and
load ILOAD, the output ripple VRIP of a boost converter is
calculated using the following equation:
VRIP =ILOAD
C
SYS
• D • 1
f
OSC
If the maximum allowable ripple is 20mV under 2.5A
steady-state load while boosting from 3.2V to 5V
(D = 36%), the required capacitance at VSYS is calculated to
be at least 40μF using the above equation. Refer to Table3
for recommended ceramic capacitor manufacturers.
Table3. Recommended Ceramic Capacitor Manufacturers
AVX www.avx.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay Siliconix www.vishay.com
TDK www.tdk.com
Choosing a Supercapacitor
The backup energy requirement is the main consider-
ation when selecting a supercapacitor. The capacitance
per cell and the number of cells (maximum of two) needed
depends on the system load (ISYS), system voltage (VSYS),
backup boost efficiency (η), supercapacitor charge volt-
age (VCHG) and the duration of the backup (tBACKUP). The
following equation can be used to estimate the amount
of capacitance required for a given backup application:
CSCAP =
V
SYS
•I
SYS
• t
BACKUP
η• (VCHG )2
Another factor to be considered is the current rating of
the supercapacitor. With the LTC4041, the supercapacitor
could be charged with a current as high as 2.5A. During a
backup event, the supercapacitor could be discharged at a
current level as high as 7.5A. It is also important to select
a supercapacitor with low ESR to minimize power losses
in the supercapacitor during charging or backup. Other
factors to be considered are the lifetime of the superca-
pacitor at the charge voltage, and the capacitance degra-
dation over time.
The internal balancer of the LTC4041 is designed to bal-
ance supercapacitors with capacitances greater than
100mF per cell. For lower capacitances, the balancer
servo loop could be unstable.
A list of supercapacitor suppliers is provided in Table4.
Table4. Supercapacitor Suppliers
AVX www.avx.com
Bussman www.cooperbussman.com
CAP-XX www.cap-xx.com
Illinois Capacitor www.illcap.com
Maxwell www.maxwell.com
Murata www.murata.com
NESS CAP www.nesscap.com
Tecate Group www.tecategroup.com
LTC4041
21
Rev A
For more information www.analog.com
APPLICATIONS INFORMATION
Supercapacitor Charger Stability Considerations
The LTC4041’s switching supercapacitor charger contains
three control loops: constant-voltage, constant-current,
and input current limit loop, all of which are internally
compensated. However, various external variables like
load and component values may interfere with the inter-
nal compensation and cause instability.
In constant-current mode, the PROG pin is in the feedback
loop rather than the SCAP pin. Because of the additional
pole created by any PROG pin capacitance, capacitance on
this pin must be kept to a minimum. For the constant-cur-
rent loop to be stable, the pole frequency at the PROG pin
should be kept above 1MHz. Therefore, if the PROG pin
has a parasitic capacitance, CPROG, the following equa-
tion should be used to calculate the maximum resistance
value for RPROG:
RPROG ≤
1
2π• 1 MHz • CPROG
Alternatively, for RPROG = 4k (500mA setting), the maxi-
mum allowable capacitance on the PROG pin is 40pF. If
any measuring device is attached to the PROG pin for
monitoring the charge current, a 1M isolation resistor
should be inserted between the PROG pin and the device.
Backup Boost Stability Considerations
The LTC4041’s backup boost converter is internally com-
pensated. However, system capacitance less than 100µF
or over 1000μF will adversely affect the phase margin and
hence the stability of the converter. Also, if the right-half-
plane (RHP) zero moves down in frequency due to exter-
nal load conditions or the choice of the inductor value,
the phase margin may be reduced to a point which causes
instability. If the output power is POUT, inductor value is
L, efficiency is η, and the input to the boost converter
is VSCAP, the RHP zero frequency can be expressed as
follows:
fRHP =VSCAP
( )
2
2 • π• L • POUT
•η
For the LTC4041’s backup boost to be able to supply
12.5W of output power (2.5A at 5V) from a stack of
supercapacitors charged to 3.2V, the maximum inductor
size should not exceed 2.2μH because of the RHP zero
consideration. Also, too much resistance between the
supercapacitor and the SCAP pin can lower the effective
input voltage of the boost converter causing the RHP zero
to shift lower in frequency and thus causing instability.
This is why it is important to minimize the lead resistance
and place the supercapacitor as close to the SCAP pin as
possible.
PCB Layout Considerations
Since the LTC4041 includes a high-current high-frequency
switching converter, the following guidelines should be
followed in the printed circuit board (PCB) layout in order
to achieve optimum performance and minimum electro-
magnetic interference (EMI).
1. Even though the converter can operate in both step-
down (buck) and step-up (boost) mode, there is only
one hot-loop containing high-frequency switching
currents. The simplified diagram in Figure3 can be
used to explain the hot-loop in the LTC4041 switch-
ing converter. Current follows the blue loop when the
switch S2 (NMOS) is closed and the red loop when
switch S1 (PMOS) is closed. So it is evident that the
current in the CSCAP capacitor is continuous whereas
the CSYS current is discontinuous forming a hot loop
with the VSYS pins and GND as indicated by the green
loop. Since the amount of EMI is directly proportional
to the area of this loop, the VSYS capacitor, prioritized
over all else, should be placed as close to the VSYS
pins as possible and the ground side of the capacitor
should return to the ground plane through an array
of vias.
Figure3. Hot-Loop Illustration for
the LTC4041 Switching Converter
VSCAP HOT LOOP
CSCAP
4041 F03
CSYS
S2
S1
L1
VSYS
LTC4041
22
Rev A
For more information www.analog.com
APPLICATIONS INFORMATION
2. To minimize parasitic inductance, the ground plane
should be as close as possible to the top plane of
the PC board (Layer 2). High frequency currents in
the hot loop tend to flow along a mirror path on the
ground plane which is directly beneath the incident
path on the top plane of the board as illustrated in
Figure4. If there are slits or cuts or drill-holes in this
mirror path on the ground plane due to other traces,
the current will be forced to go around the slits. When
high frequency currents are not allowed to flow back
through their natural least-area path, excessive volt-
age will build up and radiated emissions will occur.
So every effort should be made to keep the hot-loop
current path as unbroken as possible.
3. The other important components that need to be
placed close to the pins are the supercapacitor (con-
nected to the SCAP pin) and the inductor L1. Even
though the current through these components is
continuous, they can change very abruptly due to
a sudden change in load demand. Also, their traces
should be wide enough to handle currents as high as
the NMOS current limit (typical 6.5A) in backup boost
mode.
4. Locate the VSYS dividers for BSTFB and RSTFB near
the IC but away from the switching components.
Kelvin the top of the resistor dividers to the positive
terminal of CSYS. The bottom of the resistor dividers
should return to the ground plane away from the hot-
loop current path. The same is true for the PFI divider
and the CAPFB divider.
5. The exposed pad on the backside of the LTC4041
package must be securely soldered to the PC board
ground and also must have a group of vias con-
necting it to the ground plane for optimum thermal
performance. Also this is the only ground pin in the
package, and it serves as the return path for both the
control circuitry and the switching converter.
6. The IGATE pin for controlling the gates of the external
pass transistors has extremely limited drive current.
Care must be taken to minimize leakage to adjacent
PC board traces. To minimize leakage, the trace can
be guarded on the PC board by surrounding it with
VSYS connected metal.
Figure4. High Frequency Ground Currents Follow
Their Incident Path. Slices in the Ground Plane
Cause High Voltage and Increased EMI
4041 F04
LTC4041
23
Rev A
For more information www.analog.com
TYPICAL APPLICATIONS
3.3V Backup System with 12V Buck for Automotive Application
(Charge Current Setting: 1A, Input Current Limit Setting: 2A)
340k
2k
1nF
2.2µH
VSYS
CAPFB
BAL
BSTEN CAPSEL CPF PROGGNDCHGEN
SYSGD
CAPGD
IMON
IGATE
LTC4041
VIN CLN
1.07M
MN2 3.3V
12mΩ
VOUT
3.3V
SUPERCAP
50F
100µF
47µF
4.7µF 0.1µF
SYSTEM
LOAD
BSTFB
RSTFB
SW
SCAP
4041 TA02
OVSNS
PFI
121k
75k
422k
L1: COILCRAFT XAL-5030-222
MN2: VISHAY/SILICONIX SiS488DN
18.2k
2.2µH
1.02M
10nF
10pF
1µF
BST
VIN
12V VIN
EN/UV
PG
SYNC
TR/SS
INTVCC
PGND
LT8610
GNDRT
SW
BIAS
FB
PFO
CAPFLT
348k
698k
5V Backup Application with Non-Backed Up 3.3V Load Option
(Charge Current Setting: 2.5A, Input Current Limit Setting: 2.5A)
200k
113k
38.3k
806Ω1nF
2.2µH
VSYS
CAPFB
BAL
BSTEN CAPSEL CPF PROGGNDCHGEN
SYSGD
CAPGD
IMON
IGATE
LTC4041
VIN
VIN
VOUT
GND
LDO
EN
CLN
1050k
MN1
V
SYS
4.7V TO 5V
10mΩ
SUPERCAP
10F
SUPERCAP
10F
100µF2.2µF
TO BACKED UP
SYSTEM OUTPUT
TO NON-BACKED UP
3.3V SYSTEM OUTPUT
4.7V TO 5V
INPUT SUPPLY
BSTFB
RSTFB
SW
SCAP
4041 TA02a
OVSNS
PFI
L1: COILCRAFT XAL-5030-222
MN2: VISHAY/SILICONIX SiS488DN
PFO
CAPFLT
340k
2.5V OUTPUT
1070k
VSYS
VSYS
1M
LTC4041
24
Rev A
For more information www.analog.com
PACKAGE DESCRIPTION
4.00 ±0.10
(2 SIDES)
5.00 ±0.10
(2 SIDES)
NOTE:
1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm 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
TOP MARK
(NOTE 6)
0.40 ±0.10
23 24
1
2
BOTTOM VIEW—EXPOSED PAD
0.75 ±0.05 R = 0.115
TYP
R = 0.05 TYP PIN 1 NOTCH
R = 0.20 OR C = 0.35
0.25 ±0.05
0.50 BSC
0.200 REF
0.00 – 0.05
(UFD24) QFN 0506 REV A
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
0.70 ±0.05
0.25 ±0.05
0.50 BSC
2.65 ±0.05
2.00 REF
3.00 REF
4.10 ±0.05
5.50 ±0.05
3.10 ±0.05
4.50 ±0.05
PACKAGE OUTLINE
2.65 ±0.10
2.00 REF
3.00 REF
3.65 ±0.10
3.65 ±0.05
UFD Package
24-Lead Plastic QFN (4mm × 5mm)
(Reference LTC DWG # 05-08-1696 Rev A)
LTC4041
25
Rev A
For more information www.analog.com
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog
Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications
subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
A 01/19 Add Condition to IBSTFB spec
Modified Block Diagram pin numbering
Modified Backup Boost Stability Considerations section
4
11
21
LTC4041
26
Rev A
For more information www.analog.com
www.analog.com
ANALOG DEVICES, INC. 2018 to 2019
D16901-0-2/19(A)
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3.5A Charge Current for Li-Ion/Polymer, LTC4156 for LiFePO4 Batteries, Dual Input
Overvoltage Protection Controller, Instant-On Operation with Low Battery, Priority
Multiplexing for Multiple Outputs, 28-Lead 4mm × 5mm QFN Package
LTC4160 Switching Power Manager with USB On-The-Go
and Overvoltage Protection
USB-OTG 5V Output, Overvoltage Protection, Maximizes Available Power from
USB Port, Bat-Track, Instant-On Operation, 1.2A Max Charge Current with Thermal
Limiting, 1.2A Max Input Current Limit, 20-Lead 3mm × 4mm QFN Package
200k
2.2µH
1.18M
255k
1nF
4041 TA03
VSYS
CAPFB
BSTEN CAPSEL CPF PROGGNDCHGEN
SYSGD
CAPGD
IMON
IGATE
LTC4041
VIN CLN
1.05M
MN2
MN1 VSYS
4.7V TO 5V
6mΩ
SUPERCAP
25F
SUPERCAP
25F
100µF
2.2µF
TO BACKED-UP
SYSTEM OUTPUT
TO NON-BACKED-UP
OUTPUT
BSTFB
RSTFB
4.7V TO 5V
INPUT SUPPLY
(PROTECTED
TO 40V)
BAL
SCAP
SW
VSYS
OVSNS
PFI
38.3k
113k
6.2k 1/4W
OVP OPT
L1: COILCRAFT XAL-5030-222
MN1: VISHAY/SILICONIX SiS488DN
MN2: VISHAY/SILICONIX SiS488DN
VPWR
PFO
CAPFLT
806Ω
5V Backup Application with OVP Protection and Non-Backed Up Load Option
(Charge Current Setting: 2.5A, Input Current Limit Setting: 4A)