Datasheet

Cell A
Cell B
V
diffa
V
diffb
ta
tb
tc
td
te
tf
V
OV
V – V
OV OVH
D
T0375-01
Thermistor Drive Circuit (TOUT), Thermistor Input (TIN)
3.3V
S0352-01
TOUT
C
THERM
R
THERM
VREF
TIN
General-Purpose Open-Drain Drive (GPOD)
Alerting the Host (XALERT)
bq77PL900
SLUS844B – JUNE 2008 – REVISED JANUARY 2009 ...................................................................................................................................................... www.ti.com
Figure 19. Cell Balancing Timing Chart (Automatic)
The TOUT pin is powered by REG2, can be enabled via FUNCTION_CONTROL [TOUT] to drive an external
thermistor, and is OFF by default. A 10-k Ω , 25 ° C NTC (e.g., Semitec 103AT) thermistor is typical. The maximum
output impedance is 100 Ω .
The bq77PL900 monitors the battery temperature as shown in Figure 20 . A voltage divided by the NTC
thermistor and reference resistor is connected to TIN. The bq77PL900 compares the TIN voltage with the internal
reference voltage (0.975V), and when V
TIN
< V
REF
the bq77PL900 turns OFF the CHG and DSG FETs and sets
STATUS [OVTEMP].
In host-control mode, the host should enable and disable TOUT.
Figure 20. Temperature Monitoring Circuit
The GPOD output is enabled or disabled by OUTPUT_CONTROL [GPOD] and has a default state of OFF.
In stand-alone mode, this pin is used for driving the 0-V/precharge FET for zero-voltage battery charging by
OCD_CFG [ZVC] = 1.
In both modes, the XALERT pin is available and is driven low when faults are detected. The method to clear the
XALERT pin is different in stand-alone mode than in host-control mode. In stand-alone mode, XLAERT is cleared
when all of the faults are cleared. In host-control mode, the host must toggle (from 0, set to 1, then reset to 0)
OUTPUT_CONTROL [LTCLR] and then read the STATUS register.
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