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LTC1732-4

Lithium-Ion Linear Battery Charger Controller

Linear Technology 

LTC1732-4/LTC1732-4.2

APPLICATIONS INFORMATION

After a complete charge cycle has occured (VBAT > 4.1V),

and the battery remains connected to the charger, a new

charge cycle will begin if the battery voltage drops below

4.05V because of a load on the battery or self discharge.

For either version, to force a new charge cycle, regardless

of previous conditions, momentarily lift the program

resistor or remove and reapply the input power.

CHRG Status Output Pin

When the charge cycle starts, the CHRG pin is pulled down

to ground by an internal N-channel MOSFET that can drive

an LED. When the battery current drops to 10% of the full-

scale current (C/10), the N-channel MOSFET is turned off

and a weak 35µA current source to ground is connected

to the CHRG pin. A 15ms time delay is included to help

prevent false triggering due to transient currents. The

end-of-charge comparator is disabled in trickle charge

mode. After the timer expires, the charge cycle ends, and

the pin goes into a high impedance state. The timer is used

to terminate the charge cycle. By using two different value

pull-up resistors, a microprocessor can detect three states

from this pin (charging, C/10 and stop charging). See

Figure 1.

V+

VDD

8

VCC

LTC1732

3

CHRG

400k

2k

µPROCESSOR

OUT

IN

1732 F01

Figure 1. Microprocessor Interface

When the LTC1732 is in charge mode, the CHRG pin is

pulled low by the internal N-channel MOSFET. To detect

this mode, force the digital output pin, OUT, high and

measure the voltage at the CHRG pin. The N-channel

MOSFET will pull the pin low even with a 2k pull-up

resistor. Once the charge current drops to 10% of the full-

scale current (C/10), the N-channel MOSFET is turned off

and a 35µA current source is connected to the CHRG pin.

The IN pin will then be pulled high by the 2k pull-up. By

forcing the OUT pin into a high impedance state, the

current source will pull the pin low through the 400k

resistor. When the internal timer has expired, the CHRG

pin will change to high impedance state and the 400k

resistor will then pull the pin high to indicate the charging

has stopped.

ACPR Output Pin

The LTC1732 has an ACPR output pin to indicate that the

input supply (wall adapter) is higher than 4.1V and 54mV

above the voltage at the BAT pin. When both conditions are

met, the ACPR pin is pulled down to ground by an

N-channel MOSFET that is capable of driving an LED.

Otherwise, this pin is in a high impedance state.

End of Charge (C/10) Output

The LTC1732 includes a comparator to monitor the charge

current to detect an end-of-charge condition. This com-

parator does not terminate the charge cycle, but provides

an output signal to indicate a near full charge condition.

The timer is used to terminate the charge cycle. When the

battery current falls below 10% of full scale, the compara-

tor trips and turns off the N-channel MOSFET at the CHRG

pin and switches in a 35µA current source to ground. A

15ms time delay is included to help prevent false trigger-

ing due to transient currents. The end-of-charge compara-

tor is disabled in trickle charge mode.

Output Voltage Selection

The float voltage at the BAT pin can be selected by the SEL

pin. Shorting the SEL pin to ground will set the float

voltage to 4.1V, while connecting it to VCC sets it to 4.2V.

This feature allows the charger to be used with different

types of Li-Ion cells. When charging 4.1V cells, use the

LTC1732-4 with the SEL pin grounded. The LTC1732-4

can also be used for charging 4.2V cells by connecting

the SEL pin to VCC. The recharge threshold level is preset

to 3.8V, thus allowing either 4.1V or 4.2V cells to be

charged. Because the recharge threshold voltage level of

the LTC1732-4.2 is preset for 4.05V, this version is not

recommended for 4.1V cells.

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