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BQ76200: BQ76200 Brownout Response

Phil Reid
Prodigy 210 points
Part Number: BQ76200
Other Parts Discussed in Thread: BQ78350

G'day All,

The BQ76200 has a minimum operating voltage spec at Vbat of 8V.

Our pack has fairly long connections from the PCB to the cells.

During a short the voltage collapse from a nominal 30V to <10V at the PCB.

What would the BQ76200 do to the CHG / DSG outputs if voltage dropped below the 8V?

Could I expect the chip to disable the FETs (especially the DSG) in a timely fashion?

  • 0
    TI__Mastermind 34610 points

    Hello Phil,

    The BQ76200 is not a protector, it does not know if there is a short condition. The FETs would turn-off if the charge-pump voltage goes below UVLO which would disable the FETs (This likely would happen before BAT goes below the 8-V minimum, where the part would be completely off). Again this is not a protection feature. Ideally you will have a protector that disables the BQ76200 FETs after a SCD event.

    Do you have a protection IC?

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 210 points

    G'day Luis,

    Yes we have a protection IC, the system is designed around the BQ76200, BQ78350 & BQ769300.

    The concern was turn off time, if BAT does drop below the 8V operating voltage.

    And how the internal switch that pulls the gates to their respective off points would behave during this event..

    Regards

    Phil

  • 0 in reply to Phil Reid
    TI__Mastermind 34610 points

    Hello Phil, 

    The charge-pump would likely be depleted before the supply capacitor goes below 8V. As such the FETs would likely turn-off before the supply voltage goes below 8-V.

    As to what the states of the FETs are if the part goes below 8-V. I am not sure what it would be, they may be at a Hi-Z state when the part itself is OFF.

    If you are worried of BAT possibly becoming less than the 8-V minimum during a large transient event, you could potentially just add a diode or Schottky diode in the path for the supply input. This way there will not be any current out of the supply capacitors and ideally the protections will triggers before the supply voltage decreases below the turn-off point.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Prodigy 210 points

    G'day Luis,

    SLUA794 is a little cautious about the BAT / PACK filters.

    As to turn the FET off it pulls the gate to the respective BAT / PACK voltages.

    Haven't wrapped my head around adding the diode based on this circuit.

    Regards

    Phil

  • 0 in reply to Phil Reid
    TI__Mastermind 34610 points

    Hello Phil,

    Yes correct, when the part commands the drivers OFF it will pull CHG to BAT+ and DSG to PACK+. However, when the part is completely OFF, I am not sure the state of these pins. As I mentioned they could be Hi-Z but I cannot say at the moment.

    I was thinking of something like this (You could use a Schottky instead):

    Even if BAT+ were to go very low due to a short-circuit or large transient, the device may still be able to remain powered due to Cb still having some charge. Ideally protections would trigger that ensure the battery voltage comes back up before Cb is drained below the minimum supply voltage.

    Best Regards,

    Luis Hernandez Salomon