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DRV8837 over aggressive OCP?

Guan Yang
Prodigy 60 points
Other Parts Discussed in Thread: DRV8837

I am using a DRV8837 with this DC gearmotor. There are some problems with this and I think this might be related to the OCP.

I have tried various different ways of driving the DRV8837, for example simply setting IN1/IN2 and asserting \SLEEP, or asserting \SLEEP first and then PWM'ing IN2 at both low and 50% duty cycles.

In every case, the motor simply doesn't start if it is connected when the board powers up.

However, if I let the board power up first, then connect the motor, everything works fine.

Has anyone else encountered the same problem, and do you have some kind of trick - I'm guessing some form of PWM, or some order in which I should assert the pins - to make it work reliably?

  • 0
    TI__Mastermind 21305 points

    Hi Guan,

    Can you post your schematic? What are you using for the motor supply?

    So you are saying you have to connect the motor after applying power to get it to spin? I have never heard of this. Do you power both VCC and VM before connecting the motor? OCP should disable if the over current event is removed.

  • 0 in reply to Nicholas Oborny
    Prodigy 60 points

    Yes, that's exactly what is going on. With an earlier motor (that unfortunately is with another team member right now, but I believe it was similar but a 3V version) I managed to get it running with 1 MHz PWM for a few thousand cycles, after which I could just leave it on. No such luck with this one.

    Here's the motor part of the schematic:

    And the power part:

    I have powered it from both the lithium primary battery that's the intended power supply, and also with 3.3V from a bench supply. I get the same results with both supplies. VM and VCC are the same.

    I also get the same results with either a 2 ohm and a 10 ohm resistor in series with the motor.

  • 0 in reply to Guan Yang
    TI__Mastermind 21305 points

    Hi Guan,

    I am unsure what is happening here. Some scope shots may help to find an answer. Can you provide scope shots of the motor driver inputs/outputs in both scenarios? A snap shot of the layout around the DRV8837 may be helpful to look at as well.

  • 0 in reply to Nicholas Oborny
    TI__Mastermind 26065 points

    Hi Guan

    Could you remove the 10uF cap between OUT1 and OUT2 and test again?

    I think maybe the "motor startup current" + "10uF charging current" together caused the OCP event. 

    For spark filter caps, <1uF is recommended and I usually see 0.1uF for such a small DC motor.

    Thanks.

    Wilson Zuo

    Motor Application Team