Part Number:bq77915
We've had a couple of incidents of our bq77915-based battery protection boards failing which we've been unable to explain. The circuit we are using is straight out of the datasheet. I've attached a PDF of the schematic (it is missing a resistor between TS and VTB which has since been added).
On the board that failed, the DSG pin of the IC was internally shorted to VSS. Apart from that the IC was still functional. Providing the IC wasn't trying to drive the DSG pin high it would still power up and operate the CHG pin correctly. When it tries to drive the DSG pin then it draws too much current and the voltage at VDD drops to almost zero (voltage gets dropped across R1 - the series resistor on the VDD pin).
The MOSFETs were tested and found to still be working. There was some damage to the current sense resistor (R14), but it is possible that this was damaged mechanically during the disassembly of the battery pack. The current sense resistor is 1/2W part (Susumu KRL1220E-M-R120-F-T5).
The pack is charged at 300mA maximum. Discharge currents are substantially less (less than 100mA). The cells are 4S NCR18500A. We're not sure exactly of what conditions the board failed under. As far as we can tell it was just during normal operation (no short circuits, overloads, etc). There is also secondary protection external to the pack which prevents the cells from being completely discharged or overcharged.
We've tried to try to reproduce the failure on the bench by testing a range of conditions including shorting the output, over-charging and under-discharging. We've tried testing with the real load, as well as dummy loads with exaggerated inductances. Still, the IC has held up.
Any thoughts on what could produce this type of failure?
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