BQ29400 Datasheet, PDF (4/10 Pages) Texas Instruments – VOLTAGE PROTECTION FOR 2-, 3-, OR 4-CELL Lion BATTERIES

bq29400, bq29400A
bq29401, bq29405
SLUS568B – JULY 2003 – REVISED NOVEMBER 2004
FUNCTIONAL BLOCK DIAGRAM
R_IN
VC1
C_IN
R_IN
VC2
C_IN
R_IN
VC3
C_IN
R_IN
VC4
C_IN
GND
R_VD
VDD
C_VD
I_CD = 0.18 µA (TYP)
1.2 V (TYP)
OUT
CD
C_DELAY
www.ti.com
OVERVOLTAGE PROTECTION
When one of the cell voltages exceeds V_(PROTECT), an internal current source begins to charge the capacitor,
C_(DELAY), connected to the CD pin. If the voltage at the CD pin, V_CD, reaches 1.2 V, the OUT pin is activated and
transitions high. An externally connected NCH FET is activiated and blows the external fuse in the positive
battery rail, see Figure 1.
If all cell voltages fall below V_(PROTECT) before the voltage at pin CD reaches 1.2 V, the delay time does not run
out. An internal switch clamps the CD pin to GND and discharges the capacitor, C_(DELAY), and secures the full
delay time for the next occurring overvoltage event.
Once the pin OUT is activated, it transitions back from high to low after all battery cells reach V_(PROTECT) - V_hys.
DELAY TIME CALCULATION
The delay time is calculated as follows:
ƪ ƫ 1.2 V C(DELAY)
td +
ICD
ƪtd ƫ ICD
C(DELAY) + _{1.2 V}
Where I_(CD) = CD current source = 0.18 µA
4