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LTC4085

4085fc

Programming Charge Current

The formula for the battery charge current is:

CHG

PROG

()

•,

50 000

where VPROG is the PROG pin voltage and RPROG is the

total resistance from the PROG pin to ground. Keep in

mind that when the LTC4085 is powered from the IN pin,

the programmed input current limit takes precedent over

the charge current. In such a scenario, the charge current

cannot exceed the programmed input current limit.

For example, if typical 500mA charge current is required,

calculate:

PROG

= ⎛

⎝⎜

⎞

⎠⎟

500

50 000

100

•,

For best stability over temperature and time, 1% metal

ﬁlm resistors are recommended. Under trickle charge

conditions, this current is reduced to 10% of the full-

scale value.

The Charge Timer

The programmable charge timer is used to terminate the

charge cycle. The timer duration is programmed by an

external capacitor at the TIMER pin. The charge time is

typically:

Hours

TIMER

PROG

()

••

.•

0 1

100

The timer starts when an input voltage greater than the

undervoltage lockout threshold level is applied or when

leaving shutdown and the voltage on the battery is less than

the recharge threshold. At power up or exiting shutdown

with the battery voltage less than the recharge threshold

the charge time is a full cycle. If the battery is greater than

the recharge threshold the timer will not start and charging

is prevented. If after power-up the battery voltage drops

below the recharge threshold or if after a charge cycle the

battery voltage is still below the recharge threshold the

charge time is set to one half of a full cycle.

The LTC4085 has a feature that extends charge time au-

tomatically. Charge time is extended if the charge current

in constant-current mode is reduced due to load current

or thermal regulation. This change in charge time is in-

versely proportional to the change in charge current. As

the LTC4085 approaches constant-voltage mode the charge

current begins to drop. This change in charge current is

due to normal charging operation and does not affect the

timer duration.

Once a time-out occurs and the voltage on the battery is

greater than the recharge threshold, the charge current

stops, and the CHRG output assumes a high impedance

state if it has not already done so.

Connecting the TIMER pin to ground disables the battery

charger.

CHRG Status Output Pin

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

ground by an internal N-channel MOSFET capable of driving

an LED. When the charge current drops below 10% of the

programmed full charge current while in constant-voltage

mode, the pin assumes a high impedance state (but charge

current continues to ﬂow until the charge time elapses).

If this state is not reached before the end of the program-

mable charge time, the pin will assume a high impedance

state when a time-out occurs. The CHRG current detection

threshold can be calculated by the following equation:

DETECT

PROG

50 000

5000

•,

OPERATION

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