LTC4091 datasheet(13/26 Pages) LINER

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LTC4091

4091fa

For more information www.linear.com/LTC4091

OPERATION

Programming Charge Current

The formula for the battery charge current is:

ICHG =IPROG •52,500 =

VPROG

RPROG

•52,500

where VPROG is the PROG pin voltage and RPROG is the

total resistance from the PROG pin to ground.

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

calculate:

RPROG =

500mA

•52,500 = 105k

For best stability over temperature and time, 1% metal film

resistors are recommended. Under trickle charge condi-

tions, 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:

tTIMER(hours)=CTIMER (F)•RPROG (Ω)•300

The timer starts when an input voltage is applied or when

leavingshutdownandthevoltageonthebatteryislessthan

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

therechargethresholdthetimerwillnotstartandcharging

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 LTC4091 has a feature that extends charge time auto-

matically. Charge time is extended if the charge current in

constantcurrentmodeisreducedduetothermalregulation.

This change in charge time is inversely proportional to the

change in charge current. As the LTC4091 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

groundbyaninternalN-channelMOSFETcapableofdriving

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

programmed full charge current while in constant voltage

mode,thepinassumesahighimpedancestate,butcharge

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:

IDETECT =

0.1V

RPROG

•52,500 =

5250V

RPROG

For example, if the full charge current is programmed

to 500mA with a 105k PROG resistor the CHRG pin will

change state at a battery charge current of 50mA.

Note: The end-of-charge (EOC) comparator that monitors

the charge current latches its decision. Therefore, the

first time the charge current drops below 10% of the

programmed full charge current while in constant voltage

mode, it will toggle CHRG to a high impedance state. If,

for some reason the charge current rises back above the

threshold, the CHRG pin will not resume the strong pull-

down state. The EOC latch can be reset by a recharge cycle

(i.e., VBAT drops below the recharge threshold).

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