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BQ76920: BAT and REGOUT short circuit

Jeffrey Wright
Expert 2875 points
Part Number: BQ76920

Hello;

I need to understand whether I should change my design to use the circuit shown on the left side of Figure 13 in slua749a  "Figure 13. bq76920 REGSRC Power Option" in the event of a short circuit. I've designed in the circuit on the right side, which separates BAT and REGSRC. Since in this circuit the diode protects REGSRC from a sudden voltage drop due to a short circuit, but BAT is not protected. Does the BAT input also need protection? I had assumed that the 920's HW short circuit comparator would disable the discharge FET within 35-105us, and since these are powered from REGOUT, BAT would not require protection??

Thanks

  • 0
    TI__Guru* 81034 points

    Hi Jeffrey,

    BAT is also protected by the diode since it is a shared path. 

    regards,

    Matt

  • 0 in reply to Matt Sunna
    Expert 2875 points

    Hi Matt;

    ? Actually,  it doesn't appear that the diode is protecting either in this Figure, since Rd provides a path around it ?  I note that the 920-78350 EVM  has Rd = 10K, and so with the 2.2uF filter C, that's a time const. of ~22mS....I'm "assuming" therefore, that this time constant is long enough to allow the 920 to open the discharge FET due to the short circuit (?). So I'm flummoxed as to the purpose of the diode??

  • 0 in reply to Jeffrey Wright
    Expert 2875 points

    After thinking about it, it appears that the only purpose for the diode (that I can divine) is to shunt the 10K Rd when first connecting to the cell stack, to allow the 2.2uF to charge rapidly through the 100 ohmR, rather than taking 50ms through the 10K.

    Correct me if I'm missing something...I'm getting old ;-D