LTC1734LES6-4.2#TRM - Linear Technology Corporation

LTC1734L

1734lf

Lithium-Ion Linear

Battery Charger in ThinSOT

The LTC

1734L is a low cost, single cell, constant-current/

constant-voltage Li-Ion battery charger controller. When

combined with a few external components, the SOT-23

package forms a very small, low cost charger for single cell

lithium-ion batteries. The LTC1734L is a lower charge

current version of the LTC1734.

The LTC1734L provides a fixed float voltage of 4.2V with

1% accuracy (for 4.1V and 4.15V float voltages, contact

LTC Marketing). Constant current is programmed using a

single external resistor between the PROG pin and ground.

Manual shutdown is accomplished by floating the pro-

gram resistor while removing input power automatically

puts the LTC1734L 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 LTC1734L from operating outside

of safe limits. No external blocking diode is required.

The LTC1734L can also function as a general purpose

current source or as a current source for charging nickel-

cadmium (NiCd) and nickel-metal-hydride (NiMH) batter-

ies using external termination.

■

Low Profile (1mm) ThinSOT

Package

■

Programmable Charge Current: 50mA to 180mA

■

No Blocking Diode Required

■

No Sense Resistor Required

■

1% Accurate Preset Voltage: 4.2V

■

Charge Current Monitor Output

for Charge Termination

■

Automatic Sleep Mode with Input Supply Removal

■

Manual Shutdown

■

Negligible Battery Drain Current in Shutdown

■

Undervoltage Lockout

■

Self Protection for Overcurrent/Overtemperature

■

Cellular Telephones

■

Handheld Computers

■

Digital Cameras

■

Charging Docks and Cradles

■

Low Cost and Small Size Chargers

■

Programmable Current Sources

, LTC and LT are registered trademarks of Linear Technology Corporation.

80mA Li-Ion Battery Charger

PROG Pin Indicates Charge Status

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

BAT

= 80mA

1734 TA01

SINGLE

Li-Ion

BATTERY

10µF

GND

PROG

DRIVE

BAT

LTC1734L

SENSE

1µF

PROG

4.7k

UMT4403

CHARGING

BEGINS CHARGING

COMPLETE

1734 TA01b

1.5V

VBAT

CONSTANT

CURRENT

VBAT (V)VPROG (V)

CONSTANT

VOLTAGE

VPROG

ThinSOT is a trademark of Linear Technology Corporation.

LTC1734L

1734lf

ORDER PART

NUMBER

(Note 1)

Input Supply Voltage (V

) ..........................–0.3V to 9V

Input Voltage (BAT, PROG) ........ –0.3V to (V

+ 0.3V)

Output Voltage (DRIVE) .............. –0.3V to (V

+ 0.3V)

Output Current (I

SENSE

) ................................... –210mA

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

LTC1734LES6-4.2

JMAX

= 125°C, θ

= 230°C/W

ABSOLUTE MAXIMUM RATINGS

PACKAGE/ORDER INFORMATION

S6 PART MARKING

LTE6

The ● denotes the specifications which apply over the full operating

temperature range, unless otherwise noted 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.

ELECTRICAL CHARACTERISTICS

SENSE

GND 2

6 DRIVE

5 BAT

4 PROG

TOP VIEW

S6 PACKAGE

6-LEAD PLASTIC SOT-23

Consult LTC Marketing for parts specified with wider operating temperature ranges.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Supply

Input Supply Voltage (Note 5) ●4.55 8 V

Quiescent V

Pin Supply Current V

BAT

= 5V, (Forces I

DRIVE

= I

BAT

= 0), ●670 1150 µA

PROG

= 200µA,(7500Ω from PROG to GND)

SHDN

Pin Supply Current in Manual Shutdown PROG Pin Open ●450 900 µA

BMS

Battery Drain Current in Manual Shutdown PROG Pin Open ●–1 0 1 µA

(Note 3)

BSL

Battery Drain Current in Sleep Mode (Note 4) V

= 0V ●–1 0 1 µA

UVLOI

Undervoltage Lockout Exit Threshold V

Increasing ●4.45 4.56 4.68 V

UVLOD

Undervoltage Lockout Entry Threshold V

Decreasing ●4.30 4.41 4.53 V

UVHYS

Undervoltage Lockout Hysteresis V

Decreasing 150 mV

Charging Performance

BAT

Output Float Voltage in Constant Voltage Mode I

BAT

= 10mA, 4.55V ≤ V

≤ 8V ●4.158 4.20 4.242 V

BAT1

Output Full-Scale Current When Programmed R

PROG

= 7500Ω, 4.55V ≤ V

≤ 8V, ●39 50 60 mA

for 50mA in Constant Current Mode Pass PNP Beta > 50

BAT2

Output Full-Scale Current When Programmed R

PROG

= 2100Ω, 4.55V ≤ V

≤ 8V, ●160 180 200 mA

for 180mA in Constant Current Mode Pass PNP Beta > 50

CM1

Current Monitor Voltage on PROG Pin I

BAT

= 10% of I

BAT1

, R

PROG

= 7500Ω, 0.045 0.15 0.28 V

4.55V ≤ V

≤ 8V, Pass PNP Beta > 50,

0°C ≤ T

≤ 85°C

CM2

Current Monitor Voltage on PROG Pin I

BAT

= 10% of I

BAT2

, R

PROG

= 2100Ω, 0.10 0.15 0.20 V

4.55V ≤ V

≤ 8V, Pass PNP Beta > 50,

0°C ≤ T

≤ 85°C

DSINK

Drive Output Current V

DRIVE

= 3.5V ●20 mA

LTC1734L

1734lf

The ● 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.

ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Charger Manual Control

MSDT

Manual Shutdown Threshold V

PROG

Increasing ●2.05 2.15 2.25 V

MSHYS

Manual Shutdown Hysteresis V

PROG

Decreasing from V

MSDT

90 mV

PROGPU

Program Pin Pull-Up Current V

PROG

= 2.5V –6 –3 –1.5 µA

Protection

DSHRT

Drive Output Short-Circuit Current Limit V

DRIVE

= V

●35 65 130 mA

Note 1: Absolute Maximum Ratings are those values beyond which the life

of a device may be impaired.

Note 2: The LTC1734LE 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 (V

BAT

) and the

base is biased at 5V (V

Note 4: Assumes that the external PNP pass transistor has negligible B-E

reverse-leakage current when the emitter is biased at 0V (V

) and the

base is biased at 4.2V (V

BAT

Note 5: The 4.68V maximum undervoltage lockout (UVLO) exit threshold

must first be exceeded before the minimum V

specification applies.

Short duration drops below the minimum V

specification of several

microseconds or less are ignored by the UVLO. If manual shutdown is

entered, then V

must be higher than the 4.68V maximum UVLO

threshold before manual shutdown can be exited. When operating near the

minimum V

, a suitable PNP transistor with a low saturation voltage

must be used.

TYPICAL PERFOR A CE CHARACTERISTICS

Float Voltage vs Temperature

and Supply Voltage

TEMPERATURE (°C)

–50

4.19

FLOAT VOLTAGE (V)

4.20

4.21

050 75

1734 G01

–25 25 100 125

BAT

= 10mA

PNP = FCX589

= 8V

= 4.55V

BAT

(mA)

4.199

FLOAT VOLTAGE (V)

4.200

4.201

50 100 125

1734 G02

25 75 150 175

= 5V

= 25°C

PNP = FCX589

PROG

= 2100Ω

TEMPERATURE (°C)

–50

BAT1

(mA)

050 75

1734 G03

–25 25 100 125

PROG

= 7.5k

PNP = FCX589

= 4.55V AND 8V

Float Voltage vs IBAT

IBAT1 vs Temperature

and Supply Voltage

LTC1734L

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TYPICAL PERFOR A CE CHARACTERISTICS

IBAT2 vs Temperature

and Supply Voltage IBAT1 vs VBAT

TEMPERATURE (°C)

–50

170

BAT2

(mA)

180

190

050 75

1734 G04

–25 25 100 125

PROG

= 2.1k

PNP = FCX589

= 4.55V AND 8V

BAT

(V)

BAT1

(mA)

1734 G05

48 1235

= 5V

= 25°C

PROG

= 7.5k

PNP = FCX589

BAT PIN MUST BE DISCONNECTED

AND GROUNDED TO FORCE

CC MODE IN THIS REGION

IBAT2 vs VBAT

BAT

(V)

BAT2

(mA)

190

1734 G06

180

170 1235

= 5V

= 25°C

PROG

= 2.1k

PNP = FCX589

BAT PIN MUST BE DISCONNECTED

AND GROUNDED TO FORCE

CC MODE IN THIS REGION

Program Pin Pull-Up Current vs

Temperature and Supply Voltage Program Pin Pull-Up Current

vs VPROG

Program Pin Voltage

vs Charge Current (50mA)

TEMPERATURE (°C)

–50

PROGPU

(µA)

3.4

3.5

3.6

25 75

1734 G07

3.3

3.2

–25 0 50 100 125

3.1

3.0

PROG

= 2.5V

= 8V

= 4.55V

VPROG (V)

2.6

IPROGPU (µA)

2.8

3.0

3.2

3.4

3.6

3456

1635 G08

VCC = 8V

TA = 25°C

BAT1

(mA)

PROG

(V)

0.8

1.0

1.2

1734 F09

0.6

0.4

010 20 4030

0.2

1.6

1.4

= 5V

= 25°C

PROG

= 7.5k

PNP = FCX589

LIMITS AT 25mV DUE TO

PROGRAMMING PIN PULL-UP

CURRENT (I

PROGPU

)

Program Pin Voltage

vs Charge Current (180mA) Program Pin Voltage for IBAT1/10

vs Temperature and Supply Voltage

BAT2

(mA)

PROG

(V)

1.4

1734 G10

0.8

0.4

45 90

0.2

1.6

1.2

1.0

0.6

135 180

= 5V

= 25°C

PROG

= 2.1k

PNP = FCX589

LIMITS AT 6mV DUE TO

PROGRAMMING PIN PULL-UP

CURRENT (I

PROGPU

)

TEMPERATURE (°C)

–50

140

PROG

(mV)

150

160

050 75

1734 G11

–25 25 100 125

PROG

= 7.5k

PNP = FCX589

= 8V

= 4.55V

Program Pin Voltage for IBAT2/10

vs Temperature and Supply Voltage

TEMPERATURE (°C)

–50

140

PROG

(mV)

150

160

050 75

1734 G12

–25 25 100 125

PROG

= 2.1k

PNP = FCX589

= 8V

= 4.55V

LTC1734L

1734lf

BLOCK DIAGRA

–

–+

–

UVLO

VOLTAGE

REFERENCE

2.5V

SHUTDOWN

GND

1734 BD

4PROG

PROG

2.5V

10µF

A1A2

1.5V

–

2.15V

3µA

BAT

SINGLE

Li-Ion

CELL

6DRIVE

SENSE

1µF

BAT

/250

TEMPERATURE AND

CURRENT LIMITING

60Ω0.24Ω

REF OUTPUT

DRIVER

BAT

PIN FUNCTIONS

UUU

SENSE

(Pin 1): Sense Node for Charge Current. Current

from V

passes through the internal current sense resis-

tor and reappears at I

SENSE

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 LTC1734L 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.

(Pin 3): Positive Input Supply Voltage. Supplies

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

Current Monitor and Manual Shutdown Pin. Provides a

virtual reference voltage of 1.5V for an external resistor

PROG

) tied between this pin and ground that programs

the battery charge current when the charger is in the

constant current mode. The typical charge current will be

250 times greater than the current through this resistor

BAT

= 375/R

PROG

). This pin also allows for the charge

current to be monitored. The voltage on this pin is propor-

tional to the charge current where 1.5V corresponds to the

full programmed currrent. 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 instabil-

ity. 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 usually

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 LTC1734L.

LTC1734L

1734lf

A1’s input) the amplifier will divert current away from the

output driver thus maintaining 4.2V on the battery. This is

the constant voltage mode.

When in the constant voltage mode, the 250:1 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 250 • (V

PROG

) 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 V

is applied, the charger can be manually shut

down by floating the otherwise grounded end of R

PROG

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 LTC1734L can

function as a constant current source by grounding the

BAT pin. This will prevent amplifier A1 from trying to limit

charge current and only A2 will control the current.

Fault conditions such as overheating of the die or exces-

sive 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 V

removed the external PNP’s base is connected to

the battery by the charger. In manual shutdown the base

is connected to V

by the charger.

OPERATIO

The LTC1734L is a linear battery charger controller.

Operation 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 program 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 0.24Ω 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 0.24Ω resistor to appear across

the internal 60Ω resistor. The scale factor of 250:1 in

resistor values will cause the FET’s drain current to be 1/

250 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 value

that is programmed by RPROG.

The PROG pin current, which is 1/250 of the charge

current, develops a voltage across the program resistor.

When this voltage reaches 1.5V, amplifier A2 begins

diverting current away from the output driver, thus limit-

ing the charge current. This is the constant current mode.

The constant charge current is 250 • (1.5V/R

PROG

As the battery accepts charge, its voltage rises. When it

reaches the preset float voltage of 4.2V, 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

APPLICATIO S I FOR ATIO

WUUU

Charger 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 ris-

ing above 5V, allowing the precision constant voltage

circuit time to respond.

LTC1734L

1734lf

APPLICATIONS INFORMATION

WUUU

Manual Shutdown

Floating the program resistor allows an internal 3µA

current source (I

PROGPU

) to pull the PROG pin above the

2.15V shutdown threshold (V

MSDT

), thus shutting down

the charger. In this mode, the LTC1734L continues to

draw quiescent current from the supply (I

SHDN

), but only

a negligible leakage current is delivered to the battery

BMS

Shutdown can also be accomplished by pulling the other-

wise grounded end of the program resistor to a voltage

greater than 2.25V (V

MSDT

Max). Charging will cease above

1.5V, but the internal battery voltage resistor divider will

draw about 34µA from the battery until shutdown is

entered. Figure 1 illustrates a microcontroller configura-

tion 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 imped-

ance to ground. The programming resistor will prevent

potentially damaging currents if the PROG pin is forced

above V

. Under this condition V

may float, be loaded

down by other circuitry or be shorted to ground. If V

not shorted to ground, the current through the resistor will

pull V

up slightly.

Another method is to directly switch the PROG pin to a

voltage source when shutdown is desired (Caution: pull-

ing the PROG below 1.5V with V

applied will cause

excessive and uncontrolled charge currents). The volt-

age source must be capable of sourcing the resulting

current through the program resistor. This has the ad-

vantage of not adding any error to the program resistor

during normal operation. The voltage on the PROG pin

must be greater than 2.25V (V

MSDT(MAX)

) to ensure

entering shutdown, but no more than 0.3V above V

prevent damaging the LTC1734L from excessive PROG

pin current. An exception is if V

is allowed to float with

no other circuitry loading V

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

implement 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. Using an NPN

allows the use of a weak voltage source due to the current

gain of the transistor. For an NPN, connect the collector to

CC,

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 V

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).

Sleep Mode

When the input supply is disconnected, the IC enters the

sleep mode. In this mode, the battery drain current (I

BSL

)

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

UVLOI

) 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).

Figure 1. Interfacing with a Microcontroller

LTC1734L

PROG

µC

PROG

ADC INPUT

1734 F01

OPEN DRAIN

OR TOTEM

POLE OUTPUT

LTC1734L

1734lf

APPLICATIONS INFORMATION

WUUU

Programming Constant Current

When in the constant current mode, the full-scale charge

current is programmed using a single external resistor

between the PROG pin and ground. This charge current

will be 250 times the current through the program resis-

tor. The program resistor value is selected by dividing the

voltage forced across the resistor (1.5V) by the desired

resistor current:

RPROG = 375/IBAT

The LTC1734L is designed for an absolute maximum

current of 210mA. This translates to a maximum PROG pin

current of 840µA and a minimum program resistor of 1.8k.

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.

The minimum full-scale current that can be reliably pro-

grammed is approximately 10mA, which requires a pro-

gram resistor of 37.4k. Limiting capacitive loading on the

program pin becomes more important when high value

program resistors are used. In addition, the current

monitoring accuracy can degrade considerably at very

low current levels. If current monitoring is desired, a

minimum full-scale current of 50mA 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 I

DAC

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 deasserted. 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 50mA minimum

constant current.

CHARGE

CURRENT

MONITOR

(UNFILTERED)

CHARGE

CURRENT

MONITOR

(FILTERED) 7.5k

2N7002

CONTROL 2

FZT549

BAT

1734 F02

SINGLE

Li-Ion

BATTERY

10µF

GND

PROG

DRIVE

BAT

LTC1734L

SENSE

1µF

0.1µF TO

0.5µF

OPTIONAL FILTER

PIN 4

2N7002

CONTROL 1

CHARGE CURRENT

50mA

125mA

175mA

CONTROL 1

LOW

HIGH

CONTROL 2

LOW

HIGH

LOW

HIGH

7.5k

2N7002

CONTROL 2

FZT549*

LOAD

*OBSERVE MAXIMUM TEMPERATURE

1734 F03

GND

PROG

DRIVE

BAT

LTC1734L

SENSE

1µF

2N7002

CONTROL 1

CURRENT

50mA

125mA

175mA

CONTROL 1

LOW

HIGH

CONTROL 2

LOW

HIGH

LOW

HIGH

LOAD

Figure 2. Logic Control Programming of Output Current to 0mA, 50mA, 125mA or 175mA

Figure 3. Programmable Current Source with Output Current of 0mA, 50mA, 125mA or 175mA

LTC1734L

1734lf

Monitoring Charge Current

The voltage on the PROG pin indicates the charge current

as a proportion of the maximum current set by the

program resistor. The charge current is equal to 250 •

PROG

) amps. This feature allows a microcontrol-

ler with an ADC to easily monitor charge current and if

desired, manually shut down the charger at the appropri-

ate time. The minimum PROG pin current is about 3µA

PROGPU

Errors in the charge current monitor voltage on the PROG

pin and in the full-scale charge current are inversely

proportional to battery current and can be statistically

approximated as follows:

One Sigma Error(%) ≅ 1 + 0.08/I

BAT

(A)

Dynamic loads on the battery will cause transients to

appear 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 momen-

tary 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 LTC1734L 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

voltage of 4.2V by grounding the BAT pin. The program

resistor will determine the output current. The output

current range can be between approximately 10mA and

180mA, 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 180mA of charge current with the minimum

available base drive of approximately 20mA requires a

PNP beta greater than 9.

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 (0.3Ω) and battery float voltage. If the PNP transis-

tor 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 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 100mA with

a minimum supply voltage of 4.75V, the minimum operat-

ing V

is:

CE(MIN)

(V) = 4.75 – (0.1)(0.3) – 4.2 = 0.52V

The actual battery charge current (I

BAT

) is slightly less

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 (I

BAT

can be calculated as follows:

BAT

(A) = 250 • I

PROG

[β/(β + 1)]

If β = 50, then I

BAT

is 2% low.

If desired, the 2% loss can

be compensated for by increasing I

PROG

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:

D(MAX)

(W) = I

BAT

DD(MAX)

– V

BAT(MIN)

)

DD(MAX)

is the maximum supply voltage and V

BAT(MIN)

the minimum battery voltage when discharged.

APPLICATIONS INFORMATION

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LTC1734L

1734lf

Once the maximum power dissipation and V

CE(MIN)

are

known, Table 1 can be used as a guide in selecting a

suitable PNP transistor. In the table, very low V

CESAT

less than 0.25V, low V

CESAT

is 0.25V to 0.5V and the others

are 0.5V to 0.8V all depending on the current required. See

the manufacturer’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

current programming resistor and thermally coupled to

the transistor. The PRF chip series from Murata has a

steep resistance increase at temperature thresholds from

85°C to 145°C making it behave somewhat like a thermo-

stat switch. For example, the model PRF18BA471QB1RB

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 180mA charger. Should the

thermistor reach 125°C, the charge current will drop to

60mA and inhibit any further increase in temperature.

Stability

The LTC1734L 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 frequen-

cies, and since these are in the feedback loop, this capaci-

tance 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

feedback loop, not the battery. Because of this, capaci-

tance on this pin must be limited. Locating the program

resistor 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 capaci-

tance is greater than that given by the following equation:

MAX pF PROG

()

=400

APPLICATIONS INFORMATION

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Table 1. PNP Pass Transistor Selection Guide

Maximum P

(W)

Mounted on Board

at T

= 25°C Package Style ZETEX Part Number ROHM Part Number Comments

0.2 SC-70 UMT4403 Smallest Size

0.2 SC-70 UMT2907A Smallest Size

0.5 SOT-23 FMMT549 Low V

CESAT

0.625 SOT-23 FMMT720 Very Low V

CESAT,

High Beta

1 SOT-89 FCX589 or BCX69

1.1 SOT-23-6 ZXT10P12DE6 Very Low V

CESAT,

High Beta, Small

1 to 2 SOT-89 FCX717 Very Low V

CESAT,

High Beta

2 SOT-223 FZT589 Low V

CESAT

2 SOT-223 BCP69 or FZT549

0.75 FTR 2SB822 Low V

CESAT

1 ATV 2SB1443 Low V

CESAT

2 SOT-89 2SA1797 Low V

CESAT

LTC1734L

1734lf

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 50mA (R

PROG

= 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 pro-

grammed voltage of 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

APPLICATIONS INFORMATION

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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 represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

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.

Reverse Input Voltage Protection

In some applications, protection from reverse voltage on

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.

Figure 5. Stability Waveforms

LTC1734L

PROG

Li-Ion* 20Ω TO

200Ω

*FULLY CHARGED CELL

10k

PROG

TO SCOPE

1734 F04

BAT

2.5V

f = 1kHz

Figure 4. Setup for AC Stability Testing

PROG PIN

(20pF ON PIN)

PROG PIN

(200pF ON PIN)

PULSE

GENERATOR

SHUT

DOWN DELAY CONSTANT

CURRENT

HORIZONTAL SCALE: 100µs/DIV

CONSTANT

VOLTAGE

VCC

VIN

1734 F06

LTC1734L

*DRAIN-BULK DIODE OF FET

Figure 6. Low Loss Reverse Voltage Protection

Bypass Capacitor

Many types of capacitors with values ranging from 1µF to

10µF located close to the LTC1734L 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 (I

DSHRT

) and excessive self-heating of the

LTC1734L during a fault condition. The faults can be

generated from a shorted DRIVE pin or from excessive

DRIVE pin current to the base of the external PNP

transistor when it’s in deep saturation from a very low

. This protection is not designed to prevent overheat-

ing of the external pass transistor. Indirectly though, self-

heating of the PNP thermally conducting to the LTC1734L

LTC1734L

1734lf

 LINEAR TECHNOLOGY CORPORATIO N 2001

LT/TP 0802 2K • PRINTED IN THE USA

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 200kHz/500kHz Constant-Current/Constant-Voltage Battery Up to 1.5A Charge Current for 1-, 2- or Multiple Cell Li-Ion Batteries,

LT1571-5 Charger Family 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 Termina-

tion, 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

LTC1733 Standalone, Monolithic Linear Li-Ion Battery Charger C/10 Charge Detection and Programmable Timer, ≤1.25A,

4.2V Single-Cell, Thermal Feedback Loop

LTC1734 200mA to 700mA Li-Ion Linear Charger in ThinSot Pin Compatible with the LTC1734L, Higher Charge Current

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

RELATED PARTS

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900

●

FAX: (408) 434-0507

●

www.linear.com

APPLICATIONS INFORMATION

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PACKAGE DESCRIPTIO

and

resulting in the IC’s junction temperature to rise

above 150°C, thus cutting off the base current to the PNP

transistor. This action will limit the transistor junction

temperature to a temperature well above 150°C. The

temperature depends on how well the IC and PNP are

thermally connected and on the transistor’s θ

. See the

External PNP Transistor section for information on pro-

tecting the transistor from overheating.

S6 Package

6-Lead Plastic TSOT-23

(Reference LTC DWG # 05-08-1636)

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