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BQ77915: Short between VSS and TS (pins 9 and 18)

Ryan Pulsifer
Prodigy 10 points
Part Number: BQ77915
Other Parts Discussed in Thread: STRIKE

Hello,

We have seen a few unexplained failures with the BQ7791500PWT. We're hoping you can shed some light on the situation, or help direct our testing.

Each failure has resulted in a short (54-130ohms) between VSS and TS. The ICs have been removed and we verified that the short is internal for each failure. We have been unable to reproduce these failures by repeating tests of the cell balancer sub circuit, including installing / removing batteries in various configurations. 

Attached below is our cell balancer schematic and a board layout (more photos and details can be provided). We have tested this design fairly rigorously and the balancer works properly prior to failure. 

 Any help is appreciated. Thanks!


  • 0
    TI__Guru 54715 points

    Hi Ryan,

    I'm not sure of the features I'm seeing on the board picture, but it looks like the R50 thermistor may be located near a board edge and connectors (J3, J6). You might check if that area could receive an ESD strike. The trace leads from the R50 pad to the TS pin of the BQ77915.  The unpredictability of ESD may be why it has not reproduced in your controlled test sequences. The ESD protection in the BQ77915 is intended for handling and assembly conditions rather than in-system conditions.  Consider the system design & the board's processing and when the short damage is found.  If it could be ESD damage see if procedural changes may avoid strikes and the damage. 

    If something needs to be designed into the system circuit the TS is powered by pulses from the VTB pin.  Each pulse is about 80 us wide with sampling likely after the first 40 us.  Sampling current is not shown on the data sheet but is likely to be below 1 uA.  Any load on the line may alter the value and temperature protection so be sure to test that any solution provides adequate operation.  Consider a small series resistance in the path to the TS pin, perhaps 100 ohm.  Consider some small capacitance on the board between the thermistor and the pin, 100 pF with 5k equivalent resistance at 25C would give a 500 ns time constant allowing several time constants to settle in the first half of the pulse.  You might also look at an ESD protector designed for communication lines