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BQ76PL536A: Overoltage/Undervoltage Fault Delay Time Filter

Nicholas Yanchuk
Prodigy 30 points
Part Number: BQ76PL536A

Hi,

What kind of filter is used for the overvoltage & undervoltage delay time (time set by the CONFIG_COVT[] and CONFIG_CUVT[] registers)? Is it analog or a sampled digital filter? Could you give me any details on how this filter works? I’m concerned about noise/ringing tripping this fault if it’s sampled and gets aliased. In my current application rapid current switching can result in 1Vpk ringing/noise (~200kHz) getting past the input anti-aliasing filter.

Thanks,

Nicholas

  • 0
    TI__Genius 10785 points

    YOu don't need to worry about that. You can have any noise and won't trigger within delay time. It's not sampling so no need to worry about it to. It's like simple comparator with enable function with delay time.

  • 0 in reply to Roger Hwang
    Prodigy 30 points
    Ok so it's not sampled. Perfect!

    Follow up question: Is there any analog filtering along the way at all? If so, do you know the filter cut-off?

    I'm also concerned about noise PREVENTING a fault: i.e. there actually is an overvoltage, but noise/ringing causes the input voltage to dip down below the hysteresis enough to keep the fault from triggering.

    Thanks again,

    Nicholas
  • 0 in reply to Nicholas Yanchuk
    TI__Genius 10785 points
    Use the reference schematic.
    External RC filter is on every cell and it should filter out noise.
    If we have deglitch time then you should be okay. Max delay time is 3.1S and that should be enough.
  • 0 in reply to Roger Hwang
    Prodigy 30 points
    1) Yes we use the external RC filter exactly as specified in the datasheet. In my application, under worst case conditions, 1Vpk ringing/noise gets PAST this filter, directly into the inputs, hence my concern. I have hesitated to add an additional filter beyond some beads, as the datasheet is quite clear that the part is calibrated for this exact filter.

    2) I thought the deglitch was only on startup? How does this help in this situation? Perhaps I'm reading it wrong. The datasheet does use the term "start-up deglitch" which I'm reading to mean only startup, not during normal operation.

    I believe we set our delay time to several seconds. I was hoping to better understand how this delay time / filter works, so I can argue against additional testing required for our product.

    I'm still not understanding exactly how the filter/delay timer works, however I'll try to focus my question again:

    Question:
    If the voltage dips below the overvoltage threshold - hysteresis (likewise above undervoltage + hysteresis) briefly, before the set time is reached, does this reset the delay timer? Or does this just cause it take a small amount longer for the fault to trip (due to analog filter)?

    Thanks,

    Nicholas
  • 0 in reply to Nicholas Yanchuk
    TI__Genius 10785 points
    Think this way. Let say you set the CUVT to 3Secs as example and your UV is 3V. If voltage dip below 3V for 2.9secs then it won't trigger. it has to be below for whole 3secs. duration has to be 3 secs. Does it make sense?