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DRV8886AT: Cannot drive above 600mA

Nathan Wier
Prodigy 30 points
Part Number: DRV8886AT

I am using the DRV8886AT to drive a Sure Step STP-MTR-23055D : 2.8A rated Bipolar Stepper.

With other Stepper Controllers I can run this motor at 2.8A with a 24V supply.  With the DRV8886AT, I am only able to get 600mA maximum with a 24V supply.

I am using 15k for RREF, TRQ 100%, 1/8 step size, and DECAY is floating.  I have tried various Decay modes and haven't noticed a significant difference.

My Circuit board has 1.25 mm wide 1 oz copper traces that are 35mm long.

Any thoughts on why I cannot get more current supplied to the motor with this driver?

  • 0
    TI__Guru** 114905 points
    Hi Nathan,

    Where are you measuring the current (power supply or through the windings)?
    How large is your thermal plane?
    Have you monitored the nFAULT pin to determine if there are any problems like overcurrent or overtemperature?
  • 0 in reply to Rick Duncan
    Prodigy 30 points

    Hello Rick,

    Thank you for your prompt response.
    1. I am measuring current at the supply.  Using a Fluke 115 handheld DMM.

    2. 1 oz copper on top and bottom of PCB, 2 layer board.  I have about 110 square mm of copper on the top layer (Red in picture).  I have 6 vias underneath the chip and about 250 square mm on the bottom layer (Blue in picture).

    3. nFAULT pin remains at 5V.  I have a 10k pull-up resistor to DVDD.  I monitored this pin with an oscilloscope to make sure it does not have any faults while running.

    Some additional thoughts I have as I am looking into this.

    1. I have ENABLE and nSLEEP tied together.  I constantly have my clock signal running.
    I see in the datasheet sections 7.4 "The tWAKE time must elapse before the outputs change state after wake-up."
    Also "TI recommends to keep the STEP pin logic low when coming out of nSLEEP or when applying power."

    Just wanted to point this out in case you think it plays a part in my issue.

  • 0 in reply to Nathan Wier
    TI__Guru** 114905 points
    Hi Nathan,

    Measuring the current at the power supply does not indicate the current through the windings. If you are not seeing the nFAULT pin going low, the current should be 2A peak (1.4A RMS) through each winding.

    If you need to confirm current is being regulated properly, a current probe (not a DMM) will provide this information. Another short term method is to add a .1Ohm resistor in series with your winding, and measure the voltage across the resistor.
  • 0 in reply to Rick Duncan
    Prodigy 30 points

    I measured the winding current with an oscilloscope and current probe and saw the current peaking around 10A.  RMS was around 4A.

    This surprises me that I never saw the nFAULT pin go low.  Assuming my current probe is even semi-accurate, I would say I'm driving at max capacity which solves my issue.

    Thank you for your support.

    -Nathan

  • 0 in reply to Nathan Wier
    TI__Guru** 114905 points
    Hi Nathan,

    Glad to hear your issue is solved. It does appear that you are driving the correct current.

    10A can occur in a low inductance stepper motor. If the current remains past the blanking time, an overcurrent event could occur.
    Since you do not see an overcurrent event, the current peak should be short.