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bq76940 EVM: 5 missing schottkys?

Fred P
Expert 2225 points
Other Parts Discussed in Thread: BQ76940

In bq76940 EVM, there are 12 schottky diodes(D16-21,32-37) used to prevent polarity reversal at the AFE inputs upon a batt short circuit (explanation is on p.30).

Why are there no schottky on VC0 to VC5 also?

I think that reversal could come from a PTC kicking in.  Am I right?
That would be some answer to one of my post earlier today: http://e2e.ti.com/support/power_management/battery_management/f/180/t/368369.aspx

If so, VC0 to VC5 schottky would really be missing.


Regards,

Fred

  • 0
    Expert 2225 points

    Hello?  Anybody around for this one?

    Thanks

    Fred

  • 0 in reply to Fred P
    TI__Guru 54715 points

    The schottky patterns on cell inputs VC5B and above were for the short circuit case where the user is uncomfortable with the temporary input current from the input pins during the short circuit condition as described on page 30 of the EVM user guide.  In that condition, the upper inputs will pull toward VSS which is below their local reference of VC5X or VC10X.  The schottly diode would conduct once the external filter capacitor voltage was depleted rather than the current flowing through the IC pin.  The lower 5 cells inputs (VC0 to VC5) do not have these because their local reference is VSS and they will not be pulled below VSS during short circuit.

    If the diodes are not essential to the specific system design, they are an opportunity for leakage which would need to be balanced out of the lower cells,  so they may want to be avoided due to the leakage as well as size and cost.  If they are desired for the upper cells, they may be desired on the lower cells also to provide a similar leakage path.

    The schottky diodes may not provide effective protection for an open PTC or weld strap.  Consider if cell 7 came disconnected below the connection to the BMS board.   The load current would want to flow through Rin(VC6) through the IC and out VC7 through Rin(VC7) reversing the voltage on VC7 with respect to VC6.  Whether a schottky from VC5x to VC7 would conduct may depend on the load current.  This may need a careful circuit analysis or simulation.