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BQ25720: Vbus drop after 5V start up

Fred Cheng
Expert 5660 points
Part Number: BQ25720

Hi team,

customer find its Vbus (yellow) drop right after 5V (blue) system start up, and then Vsys(purple) drop sequentially,

is this type-C problem or our charger current limit problem?

  • 0
    TI__Expert 7920 points

    Hello Fred,

    This appears to be an input voltage regulation (VINDPM) issue.

    1. What is the input voltage limit (Reg0x3D)? If the input voltage drops below this register setting, the device enters VINDPM and reduces charge current to provide priority to the system load. By default, the input voltage is regulated at 1.28V below its no-load voltage (18.72V in this case). Does the customer have any charge current while this is happening? It is possible that the device is unable to regulate in the input voltage in VINDPM due to an input current spike, causing the adapter to shut down. The customer should lower the input voltage limit (Reg0x3D) to the minimum value (3.2V) to see if the issue persists.

    2. When this event occurs, check STAT_VDPM in Prochot Status (Reg0x21). If the device enters VINDPM, this register will latch until manually cleared. This could further support that the issue is related to VINDPM.

    3. As you mentioned, it's possible that this is an input current limit (IINDPM) issue. The customer should observe the input current to see if there are any spikes that could be triggering IINDPM. However, IINDPM should not cause VBUS to drop to 0V. Still, it is worth measuring the input current as it could be causing VBUS to shut down like I previously mentioned.

    Please help to provide more information so that we can continue to look into this issue.

    Regards,
    Alec

  • 0 in reply to Alec Lehman
    TI__Expert 5660 points

    Hi Alec,

    I know the type-C adapter is not perfect, 

    but according to VINDPM, the VBUS should stick to 18.27V whole process even VSys drop to 0V,

     do you know why?

  • 0 in reply to Fred Cheng
    TI__Expert 7920 points

    Hello Fred,

    If the system is demanding too much current, the device will not be able to regulate at VINDPM. I suggest the customer tries lowering the input voltage limit (Reg0x3D) so more instantaneous current can be provided.

    Regards,
    Alec

  • 0 in reply to Alec Lehman
    TI__Expert 5660 points

    Hi Alec

    even we lower down the threshold ,

    the adapter seems already shut down anyway w/o VINDPM

  • 0 in reply to Fred Cheng
    TI__Expert 7920 points

    Hello Fred,

    How much current is SYS drawing when this occurs?

    Regards,
    Alec

  • 0 in reply to Alec Lehman
    TI__Expert 5660 points

    No much actually,

    I think the adapter board is having thermal shutdown,

    Mao Vbus drop

    since they use our PD so we know the situation,

    but in that case , I don’t think VINDPM would help

  • 0 in reply to Fred Cheng
    TI__Expert 7920 points

    Hello Fred,

    Please help to check Prochot Status (Reg0x21(bit 10)) to see if TSHUT has been triggered. The thermal shutdown rising temperature is 155C. I would not expect this to occur at startup.

    If thermal shutdown is not being triggered, enable LOWER_PROCHOT_VDPM in Prochot Option 0 (Reg0x33(bit 0)). This will ensure the threshold of the PROCHOT_VINDPM comparator follows the input voltage limit set in Reg0x3D. If the issue persists, this will rule out VINDPM as being a problem.

    Further, please check to see if any faults are being triggered in the Charger Status Register.

    Regards,
    Alec