Safety Timer
The charger has a safety timer that controls the maxi-
mum length of time for a charge cycle. It is user selec-
table from 1 to 10 hours. If this timer expires and the
battery has failed to reach the termination current, the
charger enters the fault state and is latched off.
Charging does not continue until either power is cycled
or the ENC pin is strobed low and then high again. The
timer does not start until CC mode is entered and con-
tinues through CV mode. Precharge mode is not includ-
ed in the STMR.
Timing Equation:
0.1647 x R
STMR
= t (in seconds); R
STMR
= R1 + R2
Where R1 is the resistor from MARGIN to ground and
R2 is the resistor from STMR to MARGIN.
Thermistor Input
Battery temperature can be monitored using an exter-
nal thermistor. The THM input goes to a comparator
and is pulled up internally by a 10kΩ resistor. The volt-
age on this pin must be above V
HYS-HOT
and below
V
HYS-COLD
in order to start charging. Thermistor func-
tionality can be disabled by grounding the THM pin
and charging is independent of temperature. During
charging, if the voltage on the THM pin goes below
V
HOT
volts, the charger pauses until the voltage rises
above V
HYS-HOT
volts. Also, if the voltage on the THM
pin goes above V
COLD
volts, the charger pauses until
the voltage falls below V
HYS-COLD
volts. All timers
pause while charging is interrupted and resume when
the temperature returns to the valid range. The charger
enters the fault state when an overtemperature or
undertemperature condition occurs. A 10kΩ NTC ther-
mistor is recommended for this pin.
Charge Status Indicators
The CHARGE, FAULT, and DONE pins can be used as
digital discretes or as LED drivers; they are open drain.
When charging, the CHARGE pin is held low. When the
charger detects a full battery/termination condition,
I
TERMINATE
, the DONE pin goes low and the CHARGE
pin goes high impedance. Any charging fault (overtem-
perature or expiration of the safety timer) causes the
FAULT pin to flash at a 4Hz rate and the CHARGE and
DONE pins go high impedance. The FAULT pin stays
on solid, not flash, if the fault occurs in CV mode.
DS2731
Cache-Memory Battery-Backup Management IC
______________________________________________________________________________________ 13
Table 1. Thermistor Threshold
COLD 0.739 27.040 0 4
