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Original question:

BQ24780S: BQ24780S

BQ24780S: BQ24780S

Liangyu_Ge@asus.com
Prodigy 100 points
Part Number: BQ24780S
Other Parts Discussed in Thread: BQ24780,

Hi,I am from ASUS SZ NB POWER Team.We have some questions about bq24780 now.Our protection use the independent comparator of bq24780(when CMPIN pull high,CMPOUT pin's output is low,our protect will be enable).We find independent comparator doesn't work at the second protection after ACFET is latched off .

we can find COMPOUT pull  low when AC_BAT_SYS  is up to 25v at the first time.however,when AC_BAT_SYS  pull up to 25v again,COMPOUT doesn't pull low.Can you give a reason for this?Our ovp is under 26V,which is charge IC's ovp.In normal operation,our ovp will work before charge IC trigger ovp.

Before ACFET is latched off,our  ovp works normally with bq24780's  independent comparator.There is a picture about it below.

What is the difference between ACFET is latched off or not?Why independent comparator dosen't work persistently? And we want to know how to make sure that the independent comparator will  work normally.

Thanks!

  • 0
    TI__Genius 11850 points

    Hi,

    When the ACFET is latched off due to input OVP, then CMPOUT is fixed at logic high. This is why your independent comparator stops working after ACFET is latched off.

    Please take a look at ChargeOption1 bit [3], which is related to your issue:

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 100 points

    Hi,

    I am afraid your reply is not correct for my question.In fact ,COMOUT can change its level when the ACFET is latched off.It fails to work after several  transformation.Looking forward to your reply.

    Thanks!

  • 0 in reply to Angelo Zhang87
    Prodigy 100 points

    Hi,

    we find that the independent comparator fails to work .COMOUT pull high when COMIN arrive at 2.372v.

    Thanks!

  • 0 in reply to Liangyu_Ge@asus.com
    TI__Genius 11850 points

    Hi,

    Please provide a list of your register settings. Do any register values change after CMPOUT stops switching?

    Can you give us more information about your OVP? You mentioned that it is below 26 V, but what is the exact voltage threshold, and does it work by turning off ACFET?

    Finally, please capture a waveform like the one below, which shows VBUS, VSYS (green), CMPIN, and CMPOUT as VBUS is increased from 20 V to 27 V to enter ACOVP. Initially, the independent comparator works normally, and CMPOUT responds to changes in CMPIN. After the device enters ACOVP, ACFET and RBFET turn off, causing VSYS to decay, and CMPOUT is fixed at logic high even though CMPIN continues to toggle above and below 2.3 V.

  • 0 in reply to Angelo Zhang87
    Prodigy 100 points

    Hi,

    There is a picture about register settings.It's collected before ovp.

    There is an other picture after ovp.And the register values don't change.

    Our ovp works by pulling low ACFET_GATE.When VSYS returns to normal voltage,ACFET_GATE is released from ovp.ACFET is latched off only if ACFET_GATE is pulled low for 9.1s.

    Our ovp works with the independent comparator of charge IC.It works normally before ACFET is latched off.

    It doesn't work after ACFET is latched off.

    What is the difference between the two conditions?And how to make sure that the independent comparator works normally?

    Thanks!

  • 0 in reply to Liangyu_Ge@asus.com
    TI__Genius 11850 points
    Hi,

    Thanks for sharing that information. So your OVP protects against system over-voltage, not input over-voltage, as I initially thought. What is your system over-voltage threshold?

    Can you also share your schematic? In your last waveform, VBUS = 20.1 V, but VSYS ranges from 13.8 V to 35.8 V. It looks like there must be a system load connected. If so, please try disconnecting the system to see if the independent comparator works correctly then.

    Best regards,
    Angelo
  • 0 in reply to Angelo Zhang87
    Prodigy 100 points

    Hi，

    It is different to describe our ovp in a schematic.our problem focus on the independent comparator of bq24780s. The independent comparator doesn't work after ACFET is latched off.There is a picture about latching off ACFET below.When ACFET is latched off，you can try to have an experiment about the independent comparator.And you will find where the question is.If you have a result, please share me a picture.

    Thanks!

  • 0 in reply to Liangyu_Ge@asus.com
    TI__Genius 11850 points

    Hi,

    After ACFET/RBFET are latched off, the independent comparator should continue working, as shown in the waveform below:

    Given that your CMPOUT is fixed at logic high, my guess is that the device's input OVP is kicking in. This would cause the independent comparator to stop working, as you see in your waveforms. However, I will double check this and let you know if there are any other conditions aside from ACOVP that can cause the independent comparator to stop working.

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    TI__Genius 11850 points
    Hi,

    I've confirmed that ACOVP should be the only condition that disables the independent comparator and causes CMPOUT to be fixed at logic high, so I would recommend focusing your debugging effort on ACOVP.

    Best regards,
    Angelo