LTC1734
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For more information www.linear.com/LTC1734
Typical applicaTion
FeaTures DescripTion
Lithium-Ion Linear Battery
Charger in ThinSOT
The LT C
®
1734 is a low cost, single cell, constant-current/
constant-voltage Li-Ion battery charger controller. When
combined with a few external components, the TSOT-23
package forms a very small, low cost charger for single
cell lithium-ion batteries.
The LTC1734 is available in 4.1V and 4.2V versions with
1% accuracy. Constant current is programmed using a
single external resistor between the PROG pin and ground.
Manual shutdown is accomplished by floating the program
resistor while removing input power automatically puts
the LTC1734 into a sleep mode. Both the shutdown and
sleep modes drain near zero current from the battery.
Charge current can be monitored via the voltage on the
PROG pin allowing a microcontroller or ADC to read the
current and determine when to terminate the charge cycle.
The output driver is both current limited and thermally
protected to prevent the LTC1734 from operating outside
of safe limits. No external blocking diode is required.
The LTC1734 can also function as a general purpose current
source or as a current source for charging nickel-cadmium
(NiCd) and nickel-metal-hydride (NiMH) batteries using
external termination.
300mA Li-Ion Battery Charger
applicaTions
n Low Profile (1mm) ThinSOT™ Package
n No Blocking Diode Required
n No Sense Resistor Required
n 1% Accurate Preset Voltages: 4.1V or 4.2V
n Charge Current Monitor Output
for Charge Termination
n Programmable Charge Current: 200mA to 700mA
n Automatic Sleep Mode with Input Supply Removal
n Manual Shutdown
n Negligible Battery Drain Current in Shutdown
n Undervoltage Lockout
n Self Protection for Overcurrent/Overtemperature
n Cellular Telephones
n Handheld Computers
n Digital Cameras
n Charging Docks and Cradles
n Low Cost and Small Size Chargers
n Programmable Current Sources
L, LT , LT C , LT M , Linear Technology and the Linear logo are registered trademarks and
ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
PROG Pin Indicates Charge Status
VIN
5V
IBAT = 300mA
1734 TA01
SINGLE
Li-Ion
BATTERY
10µF +
GND
PROG
DRIVE
BAT
2
4
6
5
LTC1734
VCC ISENSE
3 1
1µF
RPROG
5k
FMMT549
CHARGING
BEGINS
CHARGING
COMPLETE
1734 TA01b
5V
4V
3V
2V
1V
0V
1.5V
VBAT
CONSTANT
CURRENT
VBAT (V)VPROG (V)
CONSTANT
VOLTAGE
VPROG
LTC1734
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pin conFiguraTionabsoluTe MaxiMuM raTings
Supply Voltage (VCC) .................................–0.3V to 9V
Input Voltage (BAT, PROG) ........... –0.3V to (VCC + 0.3V)
Output Voltage (DRIVE) ............... –0.3V to (VCC + 0.3V)
Output Current (ISENSE).................................... –900mA
Short-Circuit Duration (DRIVE) ....................... Indefinite
Junction Temperature .......................................... 125°C
Operating Ambient Temperature Range
(Note 2) ...............................................–40°C to 85°C
Operating Junction Temperature (Note 2) ............. 100°C
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec) ................... 300°C
(Note 1)
ISENSE 1
GND 2
VCC 3
6 DRIVE
5 BAT
4 PROG
TOP VIEW
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
TJMAX = 125°C, θJA = 230°C/W
orDer inForMaTion
LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE
LTC1734ES6-4.1#PBF LTC1734ES6-4.1#TRPBF LTHD 6-Lead Plastic SOT-23 –40°C to 85°C
LTC1734ES6-4.2#PBF LTC1734ES6-4.2#TRPBF LTRG 6-Lead Plastic SOT-23 –40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on nonstandard lead based finish parts.
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/
elecTrical characTerisTics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, GND = 0V and VBAT is equal to the float voltage unless
otherwise noted. All current into a pin is positive and current out of a pin is negative. All voltages are referenced to GND, unless
otherwise specified.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
VCC Supply
VCC Operating Supply Range (Note 5) l4.55 8 V
ICC Quiescent VCC Pin Supply Current VBAT = 5V, (Forces IDRIVE = IBAT = 0),
IPROG = 200µA,(7500Ω from PROG to GND)
l670 1150 µA
ISHDN VCC Pin Supply Current in Manual Shutdown PROG Pin Open l450 900 µA
IBMS Battery Drain Current in Manual Shutdown PROG Pin Open (Note 3) l–1 0 1 µA
IBSL Battery Drain Current in Sleep Mode (Note 4) VCC = 0V l–1 0 1 µA
VUVLOI Undervoltage Lockout Exit Threshold VCC Increasing l4.45 4.56 4.68 V
VUVLOD Undervoltage Lockout Entry Threshold VCC Decreasing l4.30 4.41 4.53 V
VUVHYS Undervoltage Lockout Hysteresis VCC Decreasing 150 mV
LTC1734
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elecTrical characTerisTics
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 LTC1734E is guaranteed to meet performance specifications
from 0°C to 70°C ambient temperature range and 0°C to 100°C junction
temperature range. Specifications over the –40°C to 85°C operating
ambient temperature range are assured by design, characterization and
correlation with statistical process controls.
Note 3: Assumes that the external PNP pass transistor has negligible B-C
reverse-leakage current when the collector is biased at 4.2V (VBAT) and the
base is biased at 5V (VCC).
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VCC = 5V, GND = 0V and VBAT is equal to the float voltage unless
otherwise noted. All current into a pin is positive and current out of a pin is negative. All voltages are referenced to GND, unless
otherwise specified.
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Charging Performance
VBAT Output Float Voltage in Constant Voltage Mode 4.1V Version, IBAT = 10mA, 4.55V ≤ VCC ≤ 8V
4.2V Version, IBAT = 10mA, 4.55V ≤ VCC ≤ 8V
l
l
4.059
4.158
4.10
4.20
4.141
4.242
V
V
IBAT1Output Full-Scale Current When Programmed
for 200mA in Constant Current Mode
RPROG = 7500Ω, 4.55V ≤ VCC ≤ 8V,
Pass PNP Beta > 50
l155 200 240 mA
IBAT2 Output Full-Scale Current When Programmed
for 700mA in Constant Current Mode
RPROG = 2143Ω, 4.55V ≤ VCC ≤ 8V,
Pass PNP Beta > 50
l620 700 770 mA
VCM1 Current Monitor Voltage on PROG Pin IBAT = 10% of IBAT1, RPROG = 7500Ω,
4.55V ≤ VCC ≤ 8V, Pass PNP Beta > 50,
0°C ≤ TA ≤ 85°C
0.045 0.15 0.28 V
VCM2 Current Monitor Voltage on PROG Pin IBAT = 10% of IBAT2, RPROG = 2143Ω,
4.55V ≤ VCC ≤ 8V, Pass PNP Beta > 50,
0°C ≤ TA ≤ 85°C
0.10 0.15 0.20 V
IDSINK Drive Output Current VDRIVE = 3.5V l30 mA
Charger Manual Control
VMSDT Manual Shutdown Threshold VPROG Increasing ●2.05 2.15 2.25 V
VMSHYS Manual Shutdown Hysteresis VPROG Decreasing from VMSDT 90 mV
IPROGPU Programming Pin Pull-Up Current VPROG = 2.5V –6 –3 –1.5 µA
Protection
IDSHRT Drive Output Short-Circuit Current Limit VDRIVE = VCC ●35 65 130 mA
Note 4: Assumes that the external PNP pass transistor has negligible B-E
reverse-leakage current when the emitter is biased at 0V (VCC) and the
base is biased at 4.2V (VBAT).
Note 5: The 4.68V maximum undervoltage lockout (UVLO) exit threshold
must first be exceeded before the minimum VCC specification applies.
Short duration drops below the minimum VCC specification of several
microseconds or less are ignored by the UVLO. If manual shutdown
is entered, then VCC must be higher than the 4.68V maximum UVLO
threshold before manual shutdown can be exited. When operating near
the minimum VCC, a suitable PNP transistor with a low saturation voltage
must be used.
LTC1734
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Typical perForMance characTerisTics
Float Voltage vs Temperature
and Supply Voltage
IBAT1 vs Temperature
and Supply Voltage
Float Voltage vs IBAT
IBAT2 vs Temperature
and Supply Voltage IBAT1 vs VB AT IB AT 2 vs VBAT
Program Pin Pull-Up Current vs
Temperature and Supply Voltage
Program Pin Pull-Up Current
vs VPROG
Program Pin Voltage
vs Charge Current (200mA)
TEMPERATURE (°C)
–50
4.19
FLOAT VOLTAGE (V)
4.20
4.21
050 75
1734 G01
–25 25 100 125
IBAT = 10mA
PNP = FCX589
4.2V OPTION
VCC = 8V
VCC = 4.55V
IBAT (mA)
0
4.199
FLOAT VOLTAGE (V)
4.200
4.201
200 400 500
1734 G02
100 300 600 700
VCC = 5V
TA = 25°C
PNP = FCX589
4.2V OPTION
RPROG = 2150
TEMPERATURE (°C)
–50
190
IBAT1 (mA)
200
210
050 75
1734 G03
–25 25 100 125
RPROG = 7.5k
PNP = FCX589
VCC = 4.55V AND 8V
TEMPERATURE (°C)
–50
660
IBAT2 (mA)
700
740
050 75
1734 G04
–25 25 100 125
RPROG = 2.15k
PNP = FCX589
VCC = 4.55V AND 8V
VBAT (V)
0
IBAT1 (mA)
210
4
1734 G05
200
190 1235
VCC = 5V
TA = 25°C
RPROG = 7.5k
PNP = FCX589
BAT PIN MUST BE DISCONNECTED
AND GROUNDED TO FORCE
CC MODE IN THIS REGION
VBAT (V)
0
IBAT2 (mA)
750
4
1734 G06
700
650 1235
VCC = 5V
TA = 25°C
RPROG = 2.15k
PNP = FCX589
BAT PIN MUST BE DISCONNECTED
AND GROUNDED TO FORCE
CC MODE IN THIS REGION
TEMPERATURE (°C)
–50
IPROGPU (A)
3.4
3.5
3.6
25 75
1734 G07
3.3
3.2
–25 0 50 100 125
3.1
3.0
VPROG = 2.5V
VCC = 8V
VCC = 4.55V
VPROG (V)
2
2.6
IPROGPU (A)
2.8
3.0
3.2
3.4
3.6
34 5 6
1635 G08
7 8
VCC = 8V
TA = 25°C
IBAT1 (mA)
0
VPROG (V)
0.8
1.0
1.2
200
1734 F09
0.6
0.4
050 100 150
0.2
1.6
1.4
VCC = 5V
TA = 25°C
RPROG = 7.5k
PNP = FCX589
LIMITS AT 25mV DUE TO
PROGRAMMING PIN PULL-UP
CURRENT (IPROGPU)
LTC1734
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Typical perForMance characTerisTics
Program Pin Voltage
vs Charge Current (700mA)
Program Pin Voltage for IBAT1/10
vs Temperature and Supply Voltage
Program Pin Voltage for IBAT2/10
vs Temperature and Supply Voltage
IBAT2 (mA)
0
VPROG (V)
1.4
300
1734 G10
0.8
0.4
100 200 400
0.2
0
1.6
1.2
1.0
0.6
500 600 700
VCC = 5V
TA = 25°C
RPROG = 2.15k
PNP = FCX589
LIMITS AT 6mV DUE TO
PROGRAMMING PIN PULL-UP
CURRENT (IPROGPU)
TEMPERATURE (°C)
–50
140
VPROG (mV)
150
160
050 75
1734 G11
–25 25 100 125
RPROG = 7.5k
PNP = FCX589
VCC = 8V
VCC = 4.55V
TEMPERATURE (°C)
–50
140
VPROG (mV)
150
160
050 75
1734 G12
–25 25 100 125
RPROG = 2.15k
PNP = FCX589
VCC = 8V
VCC = 4.55V
ISENSE (Pin 1): Sense Node for Charge Current. Current
from VCC passes through the internal current sense resistor
and reappears at ISENSE to supply current to the external
PNP emitter. The PNP collector provides charge current
to the battery.
GND (Pin 2): Ground. Provides a reference for the internal
voltage regulator and a return for all internal circuits. When
in the constant voltage mode, the LTC1734 will precisely
regulate the voltage between the BAT and GND pins. The
battery ground should connect close to the GND pin to
avoid voltage drop errors.
VCC (Pin 3): Positive Input Supply Voltage. This pin sup-
plies power to the internal control circuitry and external
PNP transistor through the internal current sense resistor.
This pin should be bypassed to ground with a capacitor
in the range of 1µF to 10µF.
PROG (Pin 4): Charge Current Programming, Charge Cur-
rent Monitor and Manual Shutdown Pin. Provides a virtual
reference voltage of 1.5V for an external resistor (RPROG)
tied between this pin and ground that programs the bat-
tery charge current when the charger is in the constant
current mode. The typical charge current will be 1000
times greater than the current through this resistor (IBAT
= 1500/RPROG). This pin also allows for the charge current
to be monitored. The voltage on this pin is proportional
to the charge current where 1.5V corresponds to the full
programmed current. Floating this pin allows an internal
current source to pull the pin voltage above the shutdown
threshold voltage. Because this pin is in a signal path,
excessive capacitive loading can cause AC instability.
See the Applications Information section for more details.
BAT (Pin 5): Battery Voltage Sense Input. A precision
internal resistor divider sets the final float voltage on this
pin. This divider is disconnected in the manual shutdown
or sleep mode. When charging, approximately 34µA
flows into the BAT pin. To minimize float voltage errors,
avoid excessive resistance between the battery and the
BAT pin. For dynamically stable operation, this pin usu-
ally requires a minimum bypass capacitance to ground
of 5µF to frequency compensate for the high frequency
inductive effects of the battery and wiring.
DRIVE (Pin 6): Base Drive Output for the External PNP
Pass Transistor. Provides a controlled sink current that
drives the base of the PNP. This pin has current limiting
protection for the LTC1734.
pin FuncTions
LTC1734
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operaTion
The LTC1734 is a linear battery charger controller. Op-
eration can best be understood by referring to the Block
Diagram. Charging begins when VCC rises above the UVLO
(Undervoltage Lockout) threshold VUVLOI and an external
current programming resistor is connected between the
PROG pin and ground. When charging, the collector of
the external PNP provides the charge current. The PNP’s
emitter current flows through the ISENSE pin and through
the internal 0.06Ω current sense resistor. This current is
close in magnitude, but slightly more than the collector
current since it includes the base current. Amplifier A3,
along with the P-channel FET, will force the same voltage
that appears across the 0.06Ω resistor to appear across
the internal 60Ω resistor. The scale factor of 1000:1 in
resistor values will cause the FET’s drain current to be
1/1000 of the charge current and it is this current that
flows through the PROG pin. In the constant current
mode, amplifier A2 is used to limit the charge current to
the maximum that is programmed by RPROG.
The PROG pin current, which is 1/1000 of the charge
current, develops a voltage across the program resistor.
When this voltage reaches 1.5V, amplifier A2 begins di-
verting current away from the output driver, thus limiting
the charge current. This is the constant current mode. The
constant charge current is 1000 • (1.5V/RPROG).
As the battery accepts charge, its voltage rises. When it
reaches the preset float voltage of 4.2V (LTC1734-4.2
version), a precisely divided down version of this voltage
(2.5V) is compared to the 2.5V internal reference voltage
by amplifier A1. If the battery voltage attempts to exceed
4.2V (2.5V at amplifier A1’s input) the amplifier will divert
current away from the output driver thus limiting charge
current to that which will maintain 4.2V on the battery.
This is the constant voltage mode.
When in the constant voltage mode, the 1000:1 current ratio
is still valid and the voltage on the PROG pin will indicate
the charge current as a proportion of the maximum cur-
rent set by the current programming resistor. The battery
charge current is 1000 • (VPROG/RPROG) amps. This feature
allows a microcontroller with an ADC to easily monitor
charge current and if desired, manually shut down the
charger at the appropriate time.
When VCC is applied, the charger can be manually shut
down by floating the otherwise grounded end of RPROG.
An internal 3µA current source pulls the PROG pin above
the 2.15V threshold of voltage comparator C1 initiating
shutdown.
For charging NiMH or NiCd batteries, the LTC1734 can
function as a constant current source by grounding the
BAT pin. This will prevent amplifier A1 from trying to limit
charging current and only A2 will control the current.
Fault conditions such as overheating of the die or excessive
DRIVE pin current are monitored and limited.
When input power is removed or manual shutdown is
entered, the charger will drain only tiny leakage currents
from the battery, thus maximizing battery standby time.
With VCC removed the external PNP’s base is connected
to the battery by the charger. In manual shutdown the
base is connected to VCC by the charger.
LTC1734
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applicaTions inForMaTion
Charging Operation
Charging begins when an input voltage is present that
exceeds the undervoltage lockout threshold (VUVLOI),
a Li-Ion battery is connected to the charger output and
a program resistor is connected from the PROG pin to
ground. During the first portion of the charge cycle, when
the battery voltage is below the preset float voltage, the
charger is in the constant current mode. As the battery
voltage rises and reaches the preset float voltage, the
charge current begins to decrease and the constant voltage
portion of the charge cycle begins. The charge current
will continue to decrease exponentially as the battery
approaches a fully charged condition.
Should the battery be removed during charging, a fast
built-in protection circuit will prevent the BAT pin from
rising above 5V, allowing the precision constant voltage
circuit time to respond.
Manual Shutdown
Floating the program resistor allows an internal 3µA
current source (IPROGPU) to pull the PROG pin above the
2.15V shutdown threshold (VMSDT), thus shutting down
the charger. In this mode, the LTC1734 continues to draw
some current from the supply (ISHDN), but only a negligible
leakage current is delivered to the battery (IBMS).
Shutdown can also be accomplished by pulling the oth-
erwise grounded end of the program resistor to a voltage
greater than 2.25V (VMSDTMax). Charging will cease above
1.5V, but the internal battery voltage resistor divider will
draw about 34µA from the battery until shutdown is en-
tered. Figure 1 illustrates a microcontroller configuration
that can either float the resistor or force it to a voltage. The
voltage should be no more than 8V when high and have
an impedance to ground of less than 10% of the program
resistor value when low to prevent excessive charge current
errors. To reduce errors the program resistor value may
be adjusted to account for the impedance to ground. The
programming resistor will prevent potentially damaging
currents if the PROG pin is forced above VCC. Under this
condition VCC may float, be loaded down by other circuitry
or be shorted to ground. If VCC is not shorted to ground
the current through the resistor will pull VCC up somewhat.
Another method is to directly switch the PROG pin to
a voltage source when shutdown is desired (Caution:
pulling the PROG below 1.5V with VCC applied will cause
excessive and uncontrolled charge currents). The voltage
source must be capable of sourcing the resulting current
through the program resistor. This has the advantage
of not adding any error to the program resistor during
normal operation. The voltage on the PROG pin must
be greater than 2.25V (VMSDT(MAX)) to ensure entering
shutdown, but no more than 0.3V above VCC to prevent
damaging the LTC1734 from excessive PROG pin current.
An exception is if VCC is allowed to float with no other
circuitry loading VCC down. Then, because the current will
be low, it is allowable to have the PROG pin shutdown
voltage applied. A three-state logic driver with sufficient
pull-up current can be used to perform this function by
enabling the high impedance state to charge or enabling
the pull-up device to enter shutdown.
An NPN transistor or a diode can also be utilized to imple-
ment shutdown from a voltage source. These have the
advantage of blocking current when the voltage source
goes low, thus automatically disconnecting the voltage
source for normal charging operation. The use of an NPN
allows for use of a weak voltage source due to the current
gain of the transistor. For an NPN connect the collector to
VCC, the base to the voltage source and the emitter to the
PROG pin. For a diode, connect the anode to the voltage
source and cathode to the PROG pin. An input high level
ranging from 3.3V to VCC should be adequate to enter
shutdown while a low level of 0.5V or less should allow
for normal charging operation. Use of inexpensive small
signal devices such as the 2N3904 or 1N914 is recom-
mended to prevent excessive capacitive loading on the
PROG pin (see Stability section).
LTC1734
PROG
C
RPROG
ADC INPUT
1734 F01
OPEN DRAIN
OR TOTEM
POLE OUTPUT
Figure 1. Interfacing with a Microcontroller
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applicaTions inForMaTion
Sleep Mode
When the input supply is disconnected, the IC enters the
sleep mode. In this mode, the battery drain current (IBSL)
is a negligible leakage current, allowing the battery to re-
main connected to the charger for an extended period of
time without discharging the battery. The leakage current
is due to the reverse-biased B-E junction of the external
PNP transistor.
Undervoltage Lockout
Undervoltage lockout (UVLO) keeps the charger off until
the input voltage exceeds a predetermined threshold level
(VUVLOI) that is typically 4.56V. Approximately 150mV
of hysteresis is built in to prevent oscillation around the
threshold level. In undervoltage lockout, battery drain
current is very low (< 1µA).
Programming Constant Current
When in the constant current mode, the full-scale charge
current (C) is programmed using a single external resistor
between the PROG pin and ground. This charge current
will be 1000 times greater than the current through the
program resistor. The program resistor value is selected
by dividing the voltage forced across the resistor (1.5V)
by the desired resistor current.
The LTC1734 is designed for a maximum current of ap-
proximately 700mA. This translates to a maximum PROG
pin current of 700µA and a minimum program resistor of
approximately 2.1k. Because the PROG pin is in a closed-
loop signal path, the pole frequency must be kept high
enough to maintain adequate AC stability by avoiding
excessive capacitance on the pin. See the Stability section
for more details.
VIN
5V
CHARGE
CURRENT
MONITOR
(UNFILTERED)
CHARGE
CURRENT
MONITOR
(FILTERED) 7.5k
Q2
2N7002
CONTROL 2
FZT549
IBAT
1734 F02
SINGLE
Li-Ion
BATTERY
10F
GND
PROG
DRIVE
BAT
2
4
6
5
LTC1734
VCC ISENSE
3 1
1F
0.1F TO
0.5F
1k
OPTIONAL FILTER
3k
PIN 4
Q1
2N7002
CONTROL 1
CHARGE CURRENT
0
200mA
500mA
700mA
CONTROL 1
LOW
LOW
HIGH
HIGH
CONTROL 2
LOW
HIGH
LOW
HIGH
VIN
5V
7.5k
Q2
2N7002
CONTROL 2
FZT549*
ILOAD
*OBSERVE MAXIMUM TEMPERATURE
1734 F03
GND
PROG
DRIVE
BAT
2
4
6
5
LTC1734
VCC ISENSE
3 1
1F
3k
Q1
2N7002
CONTROL 1
CURRENT
0
200mA
500mA
700mA
CONTROL 1
LOW
LOW
HIGH
HIGH
CONTROL 2
LOW
HIGH
LOW
HIGH
LOAD
Figure 2. Logic Control Programming of Output Current to 0mA, 200mA, 500mA or 700mA
Figure 3. Programmable Current Source with Output Current of 0mA, 200mA, 500mA or 700mA
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applicaTions inForMaTion
The minimum full-scale current that can be reliably
programmed is approximately 50mA, which requires a
program resistor of 30k. Limiting capacitive loading on
the program pin becomes more important when high
value program resistors are used. In addition, the cur-
rent monitoring accuracy can degrade considerably at
very low current levels. If current monitoring is desired,
a minimum full-scale current of 200mA is recommended.
Different charge currents can be programmed by various
means such as by switching in different program resistors
as shown in Figures 2 and 3. A voltage DAC connected
through a resistor to the PROG pin or a current DAC
connected in parallel with a resistor to the PROG pin can
also be used to program current (the resistor is required
with the IDAC to maintain AC stability as discussed in the
Stability section). Another means is to use a PWM output
from a microcontroller to duty cycle the charger into and
out of shutdown to create an average current (see Manual
Shutdown section for interfacing examples). Because
chargers are generally slow to respond, it can take up to
approximately 300µs for the charger to fully settle after a
shutdown is de-asserted. This delay must be accounted
for unless the minimum PWM low duration is about 3ms
or more. Shutdown occurs within a few microseconds of
a shutdown command. The use of PWM can extend the
average current to less than the normal 200mA minimum
constant current.
Monitoring Charge Current
The voltage on the PROG pin indicates the charge cur-
rent as a proportion of the maximum current set by the
program resistor. The charge current is equal to 1000 •
(VPROG/RPROG) amps. This feature allows a microcontroller
with an ADC to easily monitor charge current and if de-
sired, manually shut down the charger at the appropriate
time. See Figure 1 for an example. The minimum PROG
pin current is about 3µA (IPROGPU).
Errors in the charge current monitor voltage on the PROG
pin are inversely proportional to battery current and can
be statistically approximated as follows:
One Sigma Error(%) ≅ 1 + 0.3/IBAT(A)
Dynamic loads on the battery will cause transients to ap-
pear on the PROG pin. Should they cause excessive errors
in charge current monitoring, a simple RC filter as shown
in Figure 2 can be used to filter the transients. The filter
will also quiet the PROG pin to help prevent inadvertent
momentary entry into the manual shutdown mode.
Because the PROG pin is in a closed-loop signal path the
pole frequency must be kept high enough to maintain
adequate AC stability. This means that the maximum
resistance and capacitance presented to the PROG pin
must be limited. See the Stability section for more details.
Constant Current Source
The LTC1734 can be used as a constant current source
by disabling the voltage control loop as shown in Figure
3. This is done by pulling the BAT pin below the preset
float voltages of 4.1V or 4.2V by grounding the BAT pin.
The program resistor will determine the output current.
The output current range can be between approximately
50mA and 700mA, depending on the maximum power
rating of the external PNP pass transistor.
External PNP Transistor
The external PNP pass transistor must have adequate
beta, low saturation voltage and sufficient power dissipa-
tion capability (including any heat sinking, if required).
To provide 700mA of charge current with the minimum
available base drive of approximately 30mA requires a PNP
beta greater than 23. If lower beta PNP transistors are used,
more base current is required from the LTC1734. This can
result in the output drive current limit being reached, or
thermal shutdown due to excessive power dissipation.
Excessive beta can affect AC stability (see Stability section)
With low supply voltages, the PNP saturation voltage
(VCESAT) becomes important. The VCESAT must be less
than the minimum supply voltage minus the maximum
voltage drop across the internal sense resistor and bond
wires (0.1Ω) and battery float voltage. If the PNP transistor
can not achieve the low saturation voltage required, base
current will dramatically increase. This is to be avoided
for a number of reasons: output drive may reach current
LTC1734
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applicaTions inForMaTion
limit resulting in the charger’s characteristics to go out of
specifications, excessive power dissipation may force the
IC into thermal shutdown, or the battery could become
discharged because some of the current from the DRIVE
pin could be pulled from the battery through the forward
biased collector base junction.
For example, to program a charge current of 500mA
with a minimum supply voltage of 4.75V, the minimum
operating VCE is:
VCE(MIN)(V) = 4.75 – (0.5)(0.1) – 4.2 = 0.5V
The actual battery charge current (IBAT) is slightly smaller
than the expected charge current because the charger
senses the emitter current and the battery charge current
will be reduced by the base current. In terms of β (IC/IB),
IBAT can be calculated as follows:
IBAT(A) = 1000 • IPROG[β/(β + 1)]
If β = 50, then IBAT is 2% low. If desired, the 2% loss can
be compensated for by increasing IPROG by 2%.
Another important factor to consider when choosing the
PNP pass transistor is the power handling capability. The
transistor’s data sheet will usually give the maximum rated
power dissipation at a given ambient temperature with a
power derating for elevated temperature operation. The
maximum power dissipation of the PNP when charging is:
PD(MAX)(W) = IBAT (VDD(MAX) – VBAT(MIN))
VDD(MAX) is the maximum supply voltage and VBAT(MIN) is
the minimum battery voltage when discharged.
Once the maximum power dissipation and VCE(MIN) are
known, Table 1 can be used as a guide in selecting some
PNPs to consider. In the table, very low VCESAT is less than
0.25V, low VCESAT is 0.25V to 0.5V and the others are 0.5V
to 0.8V all depending on the current. See the manufac-
turer’s data sheet for details. All of the PNP transistors
are rated to carry at least 1A continuously as long as the
power dissipation is within limits. The Stability section
addresses caution in the use of high beta PNPs.
Should overheating of the PNP transistor be a concern,
protection can be achieved with a positive temperature
coefficient (PTC) thermistor, wired in series with the cur-
rent programming resistor and thermally coupled to the
transistor. The PTH9C chip series from Murata has a steep
resistance increase at temperature thresholds from 85°C to
145°C making it behave somewhat like a thermostat switch.
For example, the model PTH9C16TBA471Q thermistor is
470Ω at 25°C, but abruptly increase its resistance to 4.7k
at 125°C. Below 125°C, the device exhibits a small negative
TC. The 470Ω thermistor can be added in series with a
1.6k resistor to form the current programming resistor for
a 700mA charger. Should the thermistor reach 125°C, the
charge current will drop to 238mA and inhibit any further
increase in temperature.
Table 1. PNP Pass Transistor Selection Guide
MAXIMUM PD (W) MOUNTED
ON BOARD AT TA = 25°C PACKAGE STYLE ZETEX PART NUMBER ROHM PART NUMBER COMMENTS
0.5 SOT-23 FMMT549 Low VCESAT
0.625 SOT-23 FMMT720 Very Low VCESAT, High Beta
1 SOT-89 FCX589 or BCX69
1.1 SOT-23-6 ZXT10P12DE6 Very Low VCESAT, High Beta, Small
1 to 2 SOT-89 FCX717 Very Low VCESAT, High Beta
2 SOT-223 FZT589 Low VCESAT
2 SOT-223 BCP69 or FZT549
0.75 FTR 2SB822 Low VCESAT
1ATV 2SB1443 Low VCESAT
2 SOT-89 2SA1797 Low VCESAT
10 (TC = 25°C) TO-252 2SB1182 Low VCESAT, High Beta
LTC1734
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applicaTions inForMaTion
Stability
The LTC1734 contains two control loops: constant volt-
age and constant current. To maintain good AC stability
in the constant voltage mode, a capacitor of at least 4.7µF
is usually required from BAT to ground. The battery and
interconnecting wires appear inductive at high frequencies,
and since these are in the feedback loop, this capacitance
may be necessary to compensate for the inductance.
This capacitor need not exceed 100µF and its ESR can
range from near zero to several ohms depending on the
inductance to be compensated. In general, compensation
is optimal with a capacitance of 4.7µF to 22µF and an ESR
of 0.5Ω to 1.5Ω.
Using high beta PNP transistors (>300) and very low ESR
output capacitors (especially ceramic) reduces the phase
margin, possibly resulting in oscillation. Also, using high
value capacitors with very low ESRs will reduce the phase
margin. Adding a resistor of 0.5Ω to 1.5Ω in series with
the capacitor will restore the phase margin.
In the constant current mode, the PROG pin is in the feed-
back loop, not the battery. Because of this, capacitance
on this pin must be limited. Locating the program resis-
tor near the PROG pin and isolating the charge current
monitoring circuitry (if used) from the PROG pin with a
1k to 10k resistor may be necessary if the capacitance is
greater than that given by the following equation:
=C
400k
R
MAX(pF)
PROG
Higher charge currents require lower program resistor
values which can tolerate more capacitive loading on the
PROG pin. Maximum capacitance can be as high as 50pF
for a charge current of 200mA (RPROG = 7.5k).
Figure 4 is a simple test circuit for checking stability in
both the constant current and constant voltage modes.
With input power applied and a near fully charged battery
connected to the charger, driving the PROG pin with a
pulse generator will cycle the charger in and out of the
manual shutdown mode. Referring to Figure 5, after a short
delay, the charger will enter the constant current mode
first, then if the battery voltage is near the programmed
voltage of 4.1V or 4.2V, the constant voltage mode will
begin. The resulting waveform on the PROG pin is an
indication of stability.
The double exposure photo in Figure 5 shows the effects
of capacitance on the program pin. The middle waveform
is typical while the lower waveform indicates excessive
program pin capacitance resulting in constant current
mode instability. Although not common, ringing on the
constant voltage portion of the waveform is an indication
of instability due to any combination of extremely low ESR
values, high capacitance values of the output capacitor
or very high PNP transistor beta. To minimize the effect
of the scope probe capacitance, a 10k resistor is used to
isolate the probe from the program pin. Also, an adjustable
load resistor or current sink can be used to quickly alter
the charge current when a fully charged battery is used.
LTC1734
PROG
Li-Ion* 6 TO
20
*FULLY CHARGED CELL
10k
RPROG
3k
TO SCOPE
1734 F04
BAT
2.5V
f = 1kHz
0V
+
5V
0V
PROG PIN
(20pF ON PIN)
PROG PIN
(200pF ON PIN)
PULSE
GENERATOR
2V
1V
0V
2V
SHUT
DOWN
DELAY CONSTANT
CURRENT
HORIZONTAL SCALE: 100s/DIV
1V
0V
CONSTANT
VOLTAGE
Figure 4. Setup for AC Stability Testing
Figure 5. Stability Waveforms
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applicaTions inForMaTion
VCC
VIN
*
1734 F06
LTC1734
*DRAIN-BULK DIODE OF FET
Figure 6. Low Loss Reverse Voltage Protection
Reverse Input Voltage Protection
In some applications, protection from reverse voltage
on VCC is desired. If the supply voltage is high enough, a
series blocking diode can be used. In other cases, where
the voltage drop must be kept low, a P-channel FET as
shown in Figure 6 can be used.
VCC Bypass Capacitor
Many types of capacitors with values ranging from 1µF to
10µF located close to the LTC1734 will provide adequate
input bypassing. However, caution must be exercised
when using multilayer ceramic capacitors. Because of the
self resonant and high Q characteristics of some types of
ceramic capacitors, high voltage transients can be gener-
ated under some start-up conditions, such as connecting
the charger input to a hot power source. To prevent these
transients from exceeding the absolute maximum voltage
rating, several ohms of resistance can be added in series
with the ceramic input capacitor.
Internal Protection
Internal protection is provided to prevent excessive
DRIVE pin currents (IDSHRT) and excessive self-heating
of the LTC1734 during a fault condition. The faults can
be generated from a shorted DRIVE pin or from exces-
sive DRIVE pin current to the base of the external PNP
transistor when it’s in deep saturation from too low a VCE.
This protection is not designed to prevent overheating
of the external pass transistor. Indirectly though, self-
heating of the PNP thermally conducting to the LTC1734
and
resulting in the IC’s junction temperature to rise
above 150°C, thus cutting off the PNP’s base current.
This action will limit the PNP’s junction temperature to
some temperature well above 150°C. The temperature
depends on how well the IC and PNP are thermally
connected and on the transistor’s θJA. See the External
PNP Transistor section for information on protecting the
transistor from overheating.
LTC1734
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package DescripTion
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
1.50 – 1.75
(NOTE 4)
2.80 BSC
0.30 – 0.45
6 PLCS (NOTE 3)
DATUM ‘A’
0.09 – 0.20
(NOTE 3) S6 TSOT-23 0302
2.90 BSC
(NOTE 4)
0.95 BSC
1.90 BSC
0.80 – 0.90
1.00 MAX 0.01 – 0.10
0.20 BSC
0.30 – 0.50 REF
PIN ONE ID
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
3.85 MAX
0.62
MAX
0.95
REF
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
1.4 MIN
2.62 REF
1.22 REF
S6 Package
6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
LTC1734
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For more information www.linear.com/LTC1734
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 9/15 Revised package drawing and reference. 1, 2, 14
LTC1734
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For more information www.linear.com/LTC1734
LINEAR TECHNOLOGY CORPORATION 2001
LT 0915 REV A • PRINTED IN USA
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 l FAX: (408) 434-0507 l www.linear.com/LTC1734
relaTeD parTs
PART NUMBER DESCRIPTION COMMENTS
LT
®
1510-5 500kHz Constant-Current/Constant-Voltage Battery
Charger
Up to 1A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid Batteries
LT1571-1/LT1571-2
LT1571-5
200kHz/500kHz Constant-Current/Constant-Voltage
Battery Charger Family
Up to 1.5A Charge Current for 1-, 2- or Multiple Cell Li-Ion Batteries,
Preset and Adjustable Battery Voltages, C/10 Charge Detection
LTC1729 Li-Ion Battery Charger Termination Controller Can be Used with LTC Battery Chargers to Provide Charge Termination,
Preset Voltages, C/10 Charge Detection and Timer Functions
LTC1730 Li-Ion Battery Pulse Charger Minimizes Heat Dissipation, No Blocking Diode Required, Limits
Maximum Current for Safety
LTC1731 Linear Constant-Current/Constant-Voltage Charger
Controller
Simple Charger Uses External FET. Features Preset Voltages, C/10 Charge
Detection and Programmable Timer
LTC1732 Linear Constant-Current/Constant-Voltage Charger
Controller
Simple Charger Uses External FET. Input Power Good Indication Features
Preset Voltages, C/10 Charge Detection and Programmable Timer
LT1769 200kHz Constant-Current/Constant-Voltage Battery
Charger
Up to 2A Charge Current for Li-Ion, NiCd, NiMH or Lead-Acid Batteries
with Input Current Limit
