Datasheet

t
D
= (1.2V x C
D
) / I
CH1
1.2V
Cell Voltage
(VCn - VC(n-1),
VC6 – GND)
V
PROTECT
OUT
CD
LVO
V
PROTECT
- V
TH
2.4V
t
DA
= (1.2V x C
D
) / I
DS1
bq77PL157A4225
www.ti.com SLUSA00B –MARCH 2010–REVISED APRIL 2012
When the recovery option is selected, if all cell voltages fall below V
PROTECT
– V
TH
(threshold hysteresis), an
internal current source begins to discharge capacitor C
CD
, also acting as a delay timer. If any cell voltage returns
back above V
PROTECT
– V
TH
before V
CD
reaches V
CD,TH1
, the delay timer is reset and the OUT and LVO pins
remains active (i.e. in the fault state). The CD pin is charged back to its maximum value V
CD,TH2
.
If all cell voltages remain below V
PROTECT
– V
TH
long enough for the voltage at the CD pin to reach V
CD,TH1
, then
the OUT and LVO pins are deactivated (i.e. output returns to the no fault state). An internal switch clamps the
CD pin to GND, discharges the capacitor C
CD
, and resets the full delay time for the next occurring overvoltage
event.
The delay time for detecting an overvoltage fault is the time between charging C
CD
from 0 to V
CD,TH1
and can be
calculated as follows:
t
D
= (V
CD,TH1
× C
CD
) / I
CH1
C
CD
= (t
D
× I
CH1
) / V
CD,TH1
where I
CH1
= CD charge current = 0.2 µA (typical)
The recovery delay time is the time between discharging from V
CD,TH2
to V
CD,TH1
. The minimum output active time
(t
OA
) can be calculated as follows:
t
OA
= (V
CD,TH2
– V
CD,TH1
) × C
CD
) / I
DS1
C
CD
= (t
OA
× I
DS1
) / (V
CD,TH2
– V
CD,TH1
)
where I
DS1
= CD discharge current = 0.2 µA (typical)
Figure 5. Timing for Overvoltage Sensing (With Recovery Option)
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