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BQ25720: Vsys start from 5V

Fred Cheng
Expert 5810 points
Part Number: BQ25720

Hi Tiger,

my customer wonder why did the Vsys start from 5V just like Vin(type-C) instead 12.3V under 4S cell pin setting?

they didn't write any 0x3E or 0x15 to 5V either , is there any other reason regarding this cause that you can think of ??

BTW, they didn't see this when they use barrel jack input (20V),  only the type0C port input.

  • 0
    TI__Intellectual 1805 points

    Hi Fred,

    This is expected behavior. The charger initially assumes the adapter is a 5 V adapter, the charger then assesses the adapter's capability and adjusts the voltage and current accordingly. 

    Regards,

    Jacob

  • 0 in reply to Jacob Rook
    TI__Expert 5810 points

    Hi Jacob,

    so you mean there's actually a PTM during very beginning , so IC pass the input 5V directly to output?

    is that what you saying?

  • 0 in reply to Fred Cheng
    TI__Expert 5810 points

    Hi Jacob,

    update more info,

    you can see when Vsys rise up to 5V, actually  the Q1 and Q4 gate voltage both rise up to 4.xV which can pass this Vbus voltage directly to Vsys,

    but I wonder why BQ25720 behave this way without being mentioned by datasheet.

    so base on the power sequence , we already have voltage on the gate drive even we still at phase 2 ,

    gate drive is supposed to switch at the end of phase 4, right?

  • 0 in reply to Fred Cheng
    TI__Mastermind 33635 points

    Hi Fred,

    I think this is normal startup sequence. As you said, it starts up normal with traditional travel adapter. With Type-C input, we did observe similar two-steps startup. I think this is mainly related to type-C negotiation and detection. 

    The customer can further look into the detailed startup waveform, by observing SW1, SW2, each gate signal and inductor current waveform. 

    I did not see any issue though. 

    Regards,

    Tiger