SLUSCU0I March 2018 – September 2020 BQ77915
As an example, using a 5-Ah battery, with 1C-rate (5 A) charge and 2C-rate (10 A) discharge, the sense resistor is mostly 3 mΩ or less.
The typical current to turn on the FET body diode protection is 625 mA using this example. The typical current to turn off the FET body diode protection with the 3-mΩ sense resistor is 417 mA. Using this example, a > 1 A current, either charge or discharge should provide a solid FET body diode protection detection. A momentary drop through the hysteresis threshold will not cause the body diode protection to drop, but drops of 2 ms or more will cause the FET to toggle.
Observe the device behavior during an OV event (and no other fault is detected). In an OV event, the CHG FET is off and the DSG FET is on. If a discharge of >1 A occurs, the device would turn on the CHG FET to allow the full discharge current to pass through. Once the overcharged cell is discharged to the OV recovery level, the OV fault is recovered and CHG driver turns on (or remains on in this scenario) and the state comparator is turned off.
If the discharge current drops below the V(STATE_D_HYS) threshold for longer than tSTATE when the device is still in an OV fault, the CHG FET may toggle on and off until the overcharged cell voltage is reduced down to the OV recovery level. When the OV fault recovered, the CHG FET will be turned on solidly and the state comparator is off.
Without the FET body diode protection, if a discharge occurs during an OV fault state, the discharge current can only pass through the CHG FET body diode until the OV fault is recovered. This increases the risk of damaging the CHG FET if the MOSFET is not rated to sustain such current through its body diode. It also increases the FET temperature as current is now carried through the body diode.