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Original question:

DRV8251A: Question about OCP and voltage test.

DRV8251: Retry While Using the Demo Board

Saad Habayeb
Prodigy 20 points
Part Number: DRV8251

Tool/software:

Hello,

We are using the DRV8251EVM to test a motor. When I lock the motor, I trigger the OCP but the chip goes into retry mode. 

As far as I know, the board has the DRV8251 and not the DRV8251A.

I modified the board to by-pass the microcontroller and I'm driving IN1 and IN2 with 3.3V and GND, respectively. I still get a retry from the chip.

the board still has the default 0.15 Ohms sense resistor.

When I drive the motor straight from a bench supply, locking the motor triggers the OC on the supply.

Is there a reason why the DRV8251 would behave that way?

  • 0
    TI__Mastermind 34535 points

    Hi, 

    Thank you for your question. Our expert will feedback to you in a day or two.

  • 0
    TI__Genius 12680 points

    Hello Saad,

    It sounds like you have the DRV8251EVM because of its 150mΩ sense resistor (R7), the DRV8251AEVM has a 0Ω sense resistor (R7). 

    Current regulation on the DRV8251 should have a latched disabled response for overcurrent protection, while the DRV8251A should have an automatic retry response. 

    If possible, could you send o-scope captures of this response while triggering on nFAULT? Also, how are you "stalling" the motor (hands, vice, load, etc.)? 

    Best,

    David

  • 0
    Prodigy 20 points

    Hello David,

    I'm holding down the shaft of the motor with pliers.

    the scope shows the 25us restart interval as stated in the datasheet.

    Is it possible that the EVM has the A version?

  • 0 in reply to Saad Habayeb
    TI__Mastermind 22790 points

    What's the current limit on your scope?  

    Can you send a picture of the EVM?  

    Also you should be able to confirm which version of the chip is on it - the DRV8251 should say "DRV8251" in the top row.  DRV8251A will say "DRV8251A" on the chip. See the below pics, you can see the markings on the device

      

    Can you also monitor the VREF voltage and VM voltage?  Make sure VREF stays >0V, and the VM voltage stays above the UVLO threshold.  If your power supply is hitting OCP then its current limit might not be high enough for the stall condition. 

    Best,

    Jacob

  • 0 in reply to Jacob Thompson
    Prodigy 20 points

    I ordered new IC's from Mouser and I still have the same issue.

    when the motor is running without any load, the supply is showing around 1.5A.

    When I hold the shaft down, the supply drops to ~0.44A and the IC get very hot.

    The supply is limited to 3A.

    Are there any conditions where the non A version gets into a restart condition?

  • 0 in reply to Saad Habayeb
    TI__Genius 12680 points

    Hi Saad,

    What's the value of VM? When you say your supply is showing around 1.5A with "no load", does that mean the EVM is pulling 1.5A from VM without any load connected to the outputs? 

    What motor is being used, and what's it's stall current?

    It would be very helpful to see these waveforms. Can you please provide scope shots of VM, INx, nFAULT, and current (if possible)? 

    Best,

    David