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DRV8873-Q1: the output seems lost control

Cherry Zhou
Mastermind 22235 points
Part Number: DRV8873-Q1
Other Parts Discussed in Thread: DRV8873, LM2902

Hi team,

A. The DRV8873 output is shorted to ground and goes into protection, but the output still has a residual voltage.

B. Output disconnected from ground, device recovered and voltage is OK.

After the above steps are complete, give a PWM duty cycle to 8873, for example, a 50% duty cycle input, and the output is a 95% duty cycle. The output seems to have lost control: 

The residual voltage after the output is turned off is as follows: 

The voltage after recovery is as follows: 

The PWM input duty cycle is as follows: 
The output voltage waveform is as follows: 
And please see the schematics:
Could you help check this case? Thanks.
Best Regards,
Cherry
  • 0
    TI__Guru 55075 points

    Cherry,

    Is this repeatable on more than one device?  Output PWM is normal BEFORE the short?  Can they scope nFAULT pin and see how the device recovers?

    After an OCP event, the output will go HIZ.  Output will essentially float up to some unknown voltage until the output recovers.  The output with PWM input is what I don't understand.  

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    TI__Mastermind 22235 points

    Hi Ryan,

    Thanks for your support.

    Ryan Kehr said:
    Is this repeatable on more than one device?

    Yes.

    Ryan Kehr said:
    Output PWM is normal BEFORE the short? 

    Correct.

    Thanks and regards,

    Cherry

  • 0 in reply to Cherry Zhou
    TI__Mastermind 33545 points

    Hi Cherry,

    Thank you for your questions. Let me jump in since Ryan is OOO this week.

    First important feedback is VCP connection looks different from datasheet. R1082 is connected to VCP? Please follow to datasheet way. VCP pin should have 1uF to VM and should not connect others. I suggest to try this first.

    After above, if DRV still cause the issue after short test, here is suggestion.

    Looks like DRV is trying to drive every 2.5msec, but something happens. I am wondering some protection circuit is happening.

    My suggestion is

    1) Measure nFAULT signal Togher with above OUTPUT pin.

       -->If external circuit on OUTPUT pin (e.g LM2902) is damaged, output impedance could be the short condition.

       ->If nFAULT =L is observed, recommend to check FAULT status register. Then check which fault is happening.

      ->Recommend to check VM pin voltage together with OUTPUT pin. I could not find BULK cap like ~100uF on VM pin. 

      ->Recommend to remove he motor load and check OUTPUT pin voltage. 

    regards

    Shinya Morita