5
LTC1731-8.2/LTC1731-8.4
sn1731 17318fs
PIN FUNCTIONS
UUU
BAT (Pin 1): Battery Sense Input. A bypass capacitor of at
least 10µF is required to keep the loop stable when the
battery is not connected. A precision internal resistive
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 100µA
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. A capacitor C
TIMER
connected
from this pin to ground sets a 30 hour/µF time period for
charge termination. When the TIMER pin is connected to
V
CC
, the constant-voltage mode and the timer is disabled
and the IC will operate in constant-current mode only.
Short the TIMER pin to GND to disable the internal timer
and the C/10 functions.
GND (Pin 4): Ground.
PROG (Pin 5): Charge Current Program and Shutdown
Input Pin. The charge current is programmed by connect-
ing a resistor, R
PROG
to ground. The charge current is I
BAT
= (V
PROG
• 800Ω)/(R
PROG
• R
SENSE
). The IC can be forced
into shutdown by floating the PROG pin and allowing the
internal 2.5µA current source to pull the pin above the
2.457V shutdown threshold voltage.
DRV (Pin 6): Drive Output Pin for the P-Channel MOSFET
or PNP Transistor. If a PNP transistor is used, it must have
high gain (see Applications Information section). The DRV
pin is internally clamped to 6.5V below V
CC
.
V
CC
(Pin 7): Input Supply Voltage. V
CC
ranges from 8.8V
to 12V when charging. If V
CC
drops below V
BAT
+ 54mV,
for example when the input supply is disconnected, then
the IC enters sleep mode with I
CC
< 30µA. Bypass this pin
with a 1µF capacitor.
SENSE (Pin 8): Current Sense Input. Connect this pin to
the sense resistor. Choose the resistor value using the
following equation:
R
SENSE
= (V
PROG
• 800Ω)/(R
PROG
• I
BAT
)
