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BQ77915: query on input OVP behavior of the device

Mahendra Patel1
Expert 3560 points
Part Number: BQ77915

Hello team

There is a query on OVP behavior of the device BQ7791506

Assuming that a12V battery pack is inserted into a charger with peak voltage of 21V, what is the behavior CHG MOSFET? Is it turned off or put into active region momentarily till the time charger output stabilizes?

In this condition, momentarily high voltage will appear across CHG FET, will it cause any issue to FET drive?

Regards,

Mahendra

  • 0
    TI__Guru 54715 points

    Hi Mahendra,

    If the battery is in UV at 12V with DSG off, CHG is still on, consider the schematic in figure 21 of the BQ77915 data sheet. CHG will pull up close to the 12V battery voltage.  Since the Q2 discharge FET is off, the PACK- will be pulled up by RCHG and RGS_CHG.  The LD resistor will also pull down on PACK-.

    When the 21V charger is connected it will pull PACK- down attempting to go to (12-21) = -9V, but the Q1 FET will turn on as the PACK- drops to about 0V.  As PACK- goes below 0 current will flow in the body diode of Q2 allowing charge, then the body diode protection of BQ77915 turns on the DSG and Q2 is on also.

    If your Q1 turns on slowly it could see a large VGS, consider if you need the zener diode shown in  figure 25.