8

LTC1731-4.1/LTC1731-4.2

pull-up resistors, a microprocessor can detect three states

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

Figure 1.

When the LTC1731 is in charge mode, the CHRG pin is

pulled down by an 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 resis-

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

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

100

µA 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 100k resistor.

When the internal timer has expired, the CHRG pin will

change to high impedance state and the 100k resistor will

then pull the pin high to indicate the charging has stopped.

Refer to Table 1 for the summary.

Table 1. CHRG Pin Interface with Microprocessor

IN

OUT

STATUS

Low

High

Charging

Low

Hi-Z

Charging

High

High

C/10

Low

Hi-Z

C/10

High

Hi-Z

Stop Charging

End of Charge (C/10)

The LTC1731 includes a comparator to monitor the charge

current to detect an end-of-charge condition. 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 100

µA current source to ground.

2k

100k

CHRG

1731 F01

LTC1731

OUT

µPROCESSOR

IN

2

7

Figure 1. Microprocessor Interface

turn off the external P-channel MOSFET. The internal

timer is reset in the shutdown mode.

Programming Charge Current

The formula for the battery charge current (see Block

Diagram) is:

ground.

For example, if 0.5A charge current is needed, select a

For best stability over temperature and time, 1% resistors

are recommended. The closest 1% resistor value is 19.6k.

Programming the Timer

The programmable timer is used to terminate the charge.

The length of the timer is programmed by an external

capacitor at the TIMER pin. The total charge time is:

The timer starts when the input voltage greater than 4.1V

is applied and the program resistor is connected to ground.

After a time-out occurs, the CHRG output will turn into a

high impedance state to indicate that the charging has

timer and also puts the charger into a constant-current

mode. To only disable the timer function, short the TIMER

pin to GND.

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 100

µA current source to ground is connected

to the CHRG pin. After a time-out occurs, the pin will go

into a high impedance state. By using two different value