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DRV8811 not working

Rahul Swaminathan
Prodigy 50 points
Other Parts Discussed in Thread: DRV8811

8030.schematic.pdf

I've just built up a breakout board to test the DRV8811 and It's not working at all! Is there something wrong with my schematic? I've tied SLP to 5V and ENA to GND, and here's what happens (regardless of whether there is a PWM input on the STEP pin or not): the power LED on my board flickers like crazy and I hear some clicks from the motor, but no visible movement. What does this indicate? I'm fairly new to stepper motors so I don't know what's happening!

Thanks in advance for any help.

  • 0
    TI__Mastermind 20985 points

    Hi Rahul,

    A flickering power LED indicates big changes in current through it, and if VCC is tied directly through a series resistor and LED, then the only way current changes is if VCC drops in voltage or GND raises potential.  This could be due to insufficient bypass capacitance, or if your external power supply isn't supplying enough current.

    Btw, we do have an EVM available for the DRV8811: http://www.ti.com/tool/drv8811evm

    Best regards,
    RE

  • 0 in reply to RE
    Prodigy 50 points

    The power supply I'm using is rated for 1A, and my ISENSE resistor is 3.01Ω (so chopping current should be around 0.2A). I've connected the board to a tiny motor, so I'm not sure why so much current is being drawn. Looking at my resistor and capacitor values, is there something wrong there? The motor just clicks a little and doesn't move.

  • 0 in reply to Rahul Swaminathan
    TI__Mastermind 20985 points

    Of course, chopping current is based on VREF, and VREF = VCC.  If VCC has an unstable voltage and causes the LED to flicker, VREF will also be unstable, so I'd focus on fixing that first.  Can you scope VCC?  Consider adding a much larger cap (at least 4uF) on VCC.

    You could also try using a much lower chopping current.  That might give better results.

    Best regards,
    RE

  • 0 in reply to RE
    Prodigy 50 points

    Thanks so much for the help, turns out the 5V regulator I was using couldn't supply enough current. I'll add a larger bypass cap as well on my next board, right now I'm only using a 0.1µF cap for the regulator and the DRV8811.

  • 0 in reply to Rahul Swaminathan
    Prodigy 50 points

    I've fixed the problem with the voltage regulator. However, I have another problem now: the motor jitters and spins randomly, again regardless of the PWM input on the STEP pin. The motor I'm using has a resistance of 23 ohms, so using my 12V power supply, it draws around 0.5A. My chopping current should be around 0.2A, since I'm using 5V as VREF and a 3.01 ohm sense resistor. What could be causing this problem? Also, the motor driver does get quite warm. I've tried adding larger capacitors on both the VIN and the VCC lines, but this doesn't help very much.

  • 0 in reply to Rahul Swaminathan
    TI__Mastermind 21320 points

    Hi Rahul,

    Can you replicate this issue across multiple boards. Sounds like you potentially have a short somewhere.

  • 0 in reply to Nicholas Oborny
    Prodigy 50 points

    Yes, you were right. Turns out, a little excess solder from the power pad found its way to the SLP pin (impossible to see, so it evaded all visual checks). Another identical board works perfectly! Thanks a ton for the help :)