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DRV8844: nFault triggered when VM powering up

Jayden Li
Intellectual 2925 points
Part Number: DRV8844

Hi team,

My customer is using DRV8844 and facing a nFault triggered when VM powering up.

As shown below, Yellow: nFault Pin voltage, Green: VM voltage

And if we increase the VM rise speed, like 2ms, no nFault is triggered. But the VM slew rate can not be increased because of the inrush current.

Support need

1. Is it normal operation for this device?

2. Why nFault will be triggered with slow VM slew rate?

3. How to work around it?

Sincerely,

Jayden Li

  • 0
    TI__Guru 55565 points

    Jayden,

    Until the device crosses the UVLO threshold, logic in the device is not fully active.  This can cause indeterminate state on nFAULT pin.  It should be ignored on power-up case.

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    TI__Intellectual 2925 points

    Hi Ryan,

    But it is not acceptable in my customer's application. And as I mentioned, they noticed it depends on VM slew rate, and what they want to know is what slew rate will not trigger this weird fault and if this can be avoided by controlling the VM slew rate.

    Thanks!

    Jayden Li

  • 0 in reply to Jayden Li
    TI__Intellectual 2925 points

    Hi Ryan,

    They need to know the specific data, like which slew rate, how much time will cause this unexpected issue? Thanks! 

    Thanks!

    Jayden Li

  • 0 in reply to Jayden Li
    TI__Guru 55565 points

    Jayden,

    You said that the customer can't increase the slew rate, so why does in matter if they can't do anything about slew rate?

    We need to find another way.  What is the length of the low pulse?  I can't tell from scope capture.  Do they monitor it with software?  

    They can make a software change to filter it or it can be filtered in hardware with a capacitor.  

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    TI__Intellectual 2925 points

    Hi Ryan,

    I am trying to convince my customer if they can change the slew rate, but they need to know what slew rate is a guaranteed value for avoiding this nFault trigger. And then they can evaluate if they will control slew rate within this value.

    Thanks!

    Jayden Li

  • 0 in reply to Jayden Li
    TI__Guru 55565 points

    Jayden,

    I am sorry, but I don't think there is a simple answer here.

    Can you share the schematic?  Are LGND/GND connected together and to ground plane?  What is the state of nSLEEP and nRESET?  

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    TI__Intellectual 2925 points

    Hi Ryan,

    Due to the urgency, I reached you through WEBEX to close this asap. You can find my answer in WEBEX window.

    Thanks!

    Jayden Li

  • 0 in reply to Jayden Li
    TI__Guru 55565 points

    Jayden,

    I will wait for your reply here as I requested.  I would like to see nSLEEP/nRESET on same scope capture.

    Regards,

    Ryan