5

LTC1731-4.1/LTC1731-4.2

BAT (Pin 1): Battery Sense Input. A bypass capacitor of at

least 10

µF is required to keep the loop stable when the

battery is not connected. A precision internal resistor

divider sets the final float potential on this pin. The resistor

divider is disconnected in sleep mode.

CHRG (Pin 2): Open-Drain Charge Status Output. When

the battery is being charged, the CHRG pin is pulled low by

an internal N-channel MOSFET. When the charge current

drops to 10% of the full-scale current for at least 0.32

seconds, the N-channel MOSFET turns off and a 100

µA

current source is connected from the CHRG pin to GND.

When the timer runs out or the input supply is removed,

the current source will be disconnected and the CHRG pin

is forced into a high impedance state.

TIMER (Pin 3): Timer Capacitor and Constant-Voltage

Mode Disable Input Pin. The timer period is set by placing

timer are disabled, the chip will operate in constant-

current mode only. Short the TIMER pin to GND to disable

the internal timer function and the C/10 function.

GND (Pin 4): Ground Connection.

PROG (Pin 5): Charge Current Program and Shutdown

Input Pin. The charge current is programmed by connect-

into shutdown by floating the PROG pin. An internal 2.5

µA

current source will pull the pin above the shutdown

disconnected.

DRV (Pin 6): Drive Output Pin for the P-Channel MOSFET

or PNP Transistor. The impedance is high at this pin,

therefore, a high beta PNP pass transistor should be used.

Bypass this pin with a 1

µF capacitor.

SENSE (Pin 8): Current Sense Input. A sense resistor,

This resistor is chosen using the following equation: