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DRV11873: Motor drive IC does nothing upon power-up

Mark Molnar
Prodigy 30 points
Part Number: DRV11873

We have assembled a large number of BLDC motor driver boards using the DRV11873.

Most of them operate as expected.

About 0.5% of them don't do anything when powered on--Only about 40mA of current is displayed on the power supply ammeter. 

They don't attempt to start the motor spin-up process, no matter how long they are powered on.

Speed PWM input is tied to V5 for 100% duty cycle. 

Has anyone else had this problem with this device?

  • 0
    TI__Expert 6065 points
    Hey Mark,

    Please give a litle more information on the failed units:
    1) Check if the RD pin is 5V to indicate there is a locked condition.
    2) Make sure 5V is actually outputted from V5.

    Sincerely,
    Sanmesh U.
  • 0 in reply to Sanmesh Udhayakumar
    TI__Expert 6065 points
    Hey mark,

    Did you get any opportunity to follow up on these steps or would like any other help from me?
  • 0 in reply to Sanmesh Udhayakumar
    Prodigy 30 points
    I was pulled off this project for the moment.
    1) RD is at 5V, indicating locked condition, checked on 2 boards.
    2) Both boards checked show 5V outputted from V5.
  • 0 in reply to Mark Molnar
    TI__Expert 6065 points

    Hey Mark,

    I did an E2E search and there have been a lot of postings of this issue with the cause possibly being false lock detect as described here (www.ti.com/.../slva778.pdf).
    The description of the problem symptoms and fix can be found below:
    1) e2e.ti.com/.../2649866
    2) e2e.ti.com/.../651600

    Basically the summary is the cause is possibly being false lock detect and the fix is possibly changing the input PWM frequency to something else so fg pin (which corresponds to motor speed) is not near multiples of 101.6 Hz. False lock detection can occur at all multiples of FG frequency near 101.6Hz.

    Let me know if this fixes your issue

  • 0 in reply to Sanmesh Udhayakumar
    Prodigy 30 points

    Hello Sanmesh,

    I found and read SLVA778   AFTER   the board was prototyped and designed, and we experienced this failure mode also.

    Early in my post it states that the input signal PWM is at 100%, i.e., tied to V5. for full speed.

    The issue in this post is that the motor never tries to start. Hence, I don't see how the DRV11783 could detect and indicate a locked rotor.

    Once the board is powered on, the motor just sits there and does nothing. No alignment pulse, no forced commutation, no rotation whatsoever. The DRV11783 does consume some  "quiescent"  power, but beyond that, nothing else happens.

    There are so few adjustments that can be made, leads one to believe the DRV11783 devices on these non-functioning boards are defective--not exactly what management wants to hear.

  • 0 in reply to Mark Molnar
    TI__Expert 6065 points
    Hey Mark, give me a little time to talk to my coworkers and brainstorm what might be going on. Here are some quick thoughts below:

    1) The current of 40mA is way bigger than the 5mA supply current listed in the electrical spec. This shows that either the motor is trying to start the motor or something is damaged in the chip.

    2) I misspoke earlier about false lock detect. It is not caused by input PWM frequency being multiples of 101.6 Hz internal sampling clock. Rather it occurs when fg pin (which corresponds to motor speed) is multiple of 101.6 Hz internal sampling clock. Theoretically, since motor is stopped, fg frequency should be 0. But check what the fg pin in outputting and see what its frequency is.

    3) Make sure alignment isn't happening by using an oscilloscope. dont just use sight. It could be that on alignment, the device is hitting ocp.

    4) In general, compare the signals of the working units with that of the non-working units, and see if you find any differences.
  • 0 in reply to Sanmesh Udhayakumar
    TI__Expert 6065 points

    Hey Mark,

         I talked to my coworkers and got some feedback. We haven't seen this behavior before, but have some suggestions.

    1)  Please double check with oscilloscope on all the phase voltages of the motors that there is no attempted alignment pulse. The current of 40mA is way bigger than the 5mA supply current and signifies either the motor is trying to start the motor or something is damaged in the chip. What could be happening is that the motor tries to align but immediately hits Over Current protection because of the low resistance of the motor, and it happens so fast that it seems the motor doesn't move and align.  It might be unlikely but just double check.

    2) I think it would help a lot if you could share with me the oscilloscope waveform of what happens when starting the motor. Compare it to the typical startup waveform below that I got from the DRV11873 datasheet (www.ti.com/.../drv11873.pdf). I know you said nothing happens, but it's be good to actually see an oscilloscope capture to make sure.

    3) In general, compare the signals of the working units with that of the non-working units, and see if you find any differences.

    Let me know if this helps, if anything is unclear or need other help, or if you already solved the problem.

    Sincerely,

          Sanmesh U.