7

LTC1732-8.4

17328f

OPERATIO

The LTC1732-8.4 is a linear battery charger controller. The

charge current is programmed by the combination of a

trimmed 800

to the input of the current amplifier (CA). The current

amplifier servos the gate of the external P-channel MOSFET

the charge current. When the voltage at the BAT pin

approaches the preset float voltage, the voltage amplifier

(VA) will start sinking current which shrinks the voltage

above the UVLO level and a program resistor is connected

from the PROG pin to ground. At the beginning of the

charge cycle, if the battery voltage is below 4.9V, the

charger goes into trickle charge mode. The trickle charge

current is 10% of the full-scale current. If the battery

voltage stays low for one quarter of the total charge time,

the charge sequence will terminate.

The charger goes into the fast charge constant-current

mode after the voltage on the BAT pin rises above 4.9V. In

constant-current mode, the charge current is set by the

When the battery approaches the final float voltage, the

charge current will begin to decrease. When the current

drops to 10% of the full-scale charge current, an internal

comparator will turn off the pull-down N-channel MOSFET

at the CHRG pin and connect a weak current source to

ground to indicate an end-of-charge (C/10) condition.

An external capacitor on the TIMER pin sets the total

charge time. After a time-out occurs, the charge cycle is

terminated and the CHRG pin is forced to a high imped-

ance state. To restart the charge cycle, remove the input

voltage and reapply it, or float the PROG pin momentarily.

Replacing the battery while in the charge mode will cause

the timer to be reset if the voltage of the new battery is

below 8.05V. If the voltage is above 8.05V, the timer will

continue for the remaining charge time. In the case when

a time out has occurred, a new battery with a voltage of

less than 8.05V can be inserted and charged automatically

with the full programmed charge time.

For batteries like lithium-ion that require accurate final

float voltage, the internal 2.457V reference, voltage ampli-

fier and the resistor divider provide regulation with

±1%

(max) accuracy. For NiMH and NiCd batteries, the

LTC1732-8.4 can be used as a current source by pulling

mode, the voltage amplifier, timer, C/10 comparator and

the trickle charge function are all disabled.

The charger can be shut down by floating the PROG

high and clamp it at 3.5V.

When the input voltage is not present, the charger goes

reduces the current drain on the battery and increases the

standby time.