BQ29441 Datasheet, PDF (6/17 Pages) Texas Instruments – Voltage Protection for 2-Series, 3-Series, or 4-Series Cell Li-Ion Batteries

bq29440, bq2944L0
bq29441, bq29442, bq29443, bq29449, bq2944L9
SLUSA15C – JUNE 2010 – REVISED NOVEMBER 2010
APPLICATIONS INFORMATION
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PROTECTION (OUT) TIMING AND DELAY TIME CAPACITOR SIZING
The bq2944x uses an external capacitor to set delay timing during an overvoltage condition. When any of the
cells exceed the overvoltage threshold, the bq2944x activates an internal current source of nominally 140 nA,
which charges the external capacitor. When the external capacitor charges up to a voltage of nominally 1.2 V,
the OUT pin transitions from a low state to a high state, by means of an internal pull-up network, to a regulated
voltage of no more than 9.5 V when I_OH = 0 mA.
Cell Voltage
VC1–VC2
VC2–VC3
VC3–VC4
VC4–GND
V_PROTECT
V_PROTECT – V_HYS
OUT
t_d
L
H
Figure 4. Timing for Overvoltage Sensing
T0461-01
Sizing the external capacitor is based on the desired delay time as follows:
td
C_CD = _XDELAY
Where t_d is the desired delay time and x_DELAY is the overvoltage delay time scale factor, expressed in seconds
per microFarad. x_DELAY is nominally 9.0 s/µF. For example, if a nominal delay of 3 seconds is desired, the
customer should use a C_CD capacitor that is 3 s/9.0 s/µF = 0.33 µF.
The delay time is calculated as follows:
t_d = C_CD ´ X_DELAY
If the cell overvoltage condition is removed before the external capacitor reaches the reference voltage, the
internal current source is disabled and an internal discharge block is employed to discharge the external
capacitor down to 0 V. In this instance, the OUT pin remains in a low state.
For latched versions of the bq2944x, if an overvoltage condition has caused the OUT pin to transition to a high
state, the external capacitor remains charged even after the overvoltage condition has been removed. In this
instance, the OUT pin remains in a high state.
For non-latched versions, the OUT pin is allowed to transition back from a high to low state when the overvoltage
condition is no longer present, and the external capacitor is discharged down to 0 V.
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