Datasheet

Sample-and-HoldCircuit
OV Comparator
VC11
VC10
VC9
VC8
UVComparator
OVth
UVth
Delay
Counter
Delay
Counter
OV/UVComparator
120 s NumberofCells
/MonitorOVandUV
m ´
SamplingSequence:50ms
+
+
+
–
–
–
S0346-01
Cell Overvoltage Detection and Recovery
bq77PL900
SLUS844B – JUNE 2008 – REVISED JANUARY 2009 ...................................................................................................................................................... www.ti.com
Figure 7. Cell Voltage Monitoring Circuit
Cell overvoltage detection is the same as host control mode for the FET OFF state, but the recovery conditions
are different. The CHG FET is turned OFF if any one of the cell voltages remains higher than V
OV
for a period
greater than t
OV
. As a result, the cells are protected from an overcharge condition. Also XLAERT changes from
High to Low. Both V
OV
and t
OV
can be programmed in the internal EEPROM.
Recovery in Host Control Mode
The recovery condition is as follows:
1. All cell voltages become lower than V
OV
( Δ V
OVH
is ignored).
2. Additionally, the host must send a sequence of firmware commands to the bq77PL900 to turn ON the CHG
FET.
The command sequence required is as follows:
1. The host must toggle LTCLR from 0 to 1 and then back to 0.
2. Then set the CHG control bit to 1. To reset XLAERT high, the host must read the status register.
Figure 8 illustrates the circuit schematic in overvoltage protection mode in Host Control Mode. Figure 9 illustrates
the timing of this protection mode.
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Product Folder Link(s): bq77PL900