8

LTC1731-8.2/LTC1731-8.4

sn1731 17318fs

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 8.2V

is applied and the program resistor is connected to ground.

After a time-out occurs, the CHRG output will become high

impedance indicating that the charge cycle has ended.

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 to

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

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

scale current (C/10), the N-channel MOSFET turns off and

a weak 100

µAcurrentsourcetogroundisconnectedtothe

CHRG pin. After a time-out occurs, the CHRG pin goes

high impedance indicating that the charge cycle has ended.

By using two different value pull-up resistors, a micropro-

cessor can detect three states from this pin (charging,

C/10 and stop charging). See Figure 1.

Charger Conditions

The charger is off when any of the following conditions

internal resistor divider is disconnected to reduce the

current drain on the battery.

Undervoltage Lockout (UVLO)

An internal undervoltage lockout circuit monitors the input

rises above 8.2V. To prevent oscillation around

Trickle Charge and Defective Battery Detection

At the beginning of the charging sequence, if the battery

voltage is less than 4.95V the charger goes into trickle

charge mode. The charge current is reduced to 10% of the

full-scale current. If the low battery voltage persists for

one quarter of the total charging time, the battery is

considered defective, charging will be terminated and the

CHRG pin output is forced to a high impedance state.

Shutdown

The LTC1731-8.2/LTC1731-8.4 can be forced into shut-

down by floating the PROG pin and allowing the internal

2.5

µA current source to pull the pin above the 2.457V

shutdown threshold voltage. The DRV pin is then pulled

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

2k

100k

CHRG

1731 F01

LTC1731-8.4

OUT

µPROCESSOR

IN

2

7

Figure 1. Microprocessor Interface