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DRV8835 failure after some time of operation

Other Parts Discussed in Thread: DRV8835

Hi!

First of all I want to admit that I haven't any experience in motor drivers circuts design. 

I'm using DRV8835 to control 2 brushed DC motors with 1.6A stall current. Motors are used to wirelessly drive toy car so there are possibilities of fast changing of current directions. After some time of intensive operation toy loses its ability to ride and changing DRV8835 chip solves the problem. That happend two times already. Schematic is below. Vbat = 7.5V.

Am I right to think that this happens because reverse motor current even higher then 1.6A and overcurrent protection doesn't save the chip?

Am I right to think that slowing of acceleration will solve the problem? Also how can I design motor driver which can successfully handle this overcurrent whithout slowing an accelaration?

  • Hi Rasul,

    Rasul Kishov43 said:
    Am I right to think that this happens because reverse motor current even higher then 1.6A and overcurrent protection doesn't save the chip?

    The reverse motor current is probably not the problem. The device is designed to handle currents higher than 1.6A for short periods of time. The problem is more like due to a voltage surge on VM when reversing currents.

    Do you have a local 10uF cap on the device VM? If not, please place one close to the DRV8835 as shown in the datasheet. Also, please look at the VM voltage with respect to GND at the pins of the DRV8835 when reversing currents. If you are experiencing large voltage spikes (greater than the abs max voltage), you may need more than the 10uF capacitance.

  • Rick,
    Thank you very much.
    So I'll put 10uF cap tomorrow close to DRV8835.
    My scematics was copied from Pololu zumo bot shield (www.pololu.com/.../resources). I'm really surprised about this quiet obvious design error.
  • Dear Rick.
    I found the way to connect that cap to my board and after full day of work there weren't same issues again. Thank you very much!
    But yet i got another trouble which I belive is not connected to the TI chip.
    Suddenly one of my motors lost half of its speed only in reverse mode while in straight mode it's still work ok. I think that's because of the low reliability of this motors (they have metal brushes). Feels like choosing the right motor for an application is not so simple.
    Just want to know whats your experiense tells you about this.
    Anyway you helped me a lot. Thank you very much.
  • Hi Rasul,

    Glad to hear the issue have been resolved.

    I have not heard of a motor suddenly losing half of its speed due to the brushes.
    If the two motors are the same, can you swap the two motors to determine if the device or motor is the problem? This should help debug.
  • Hi Rick. 

    Thank you very much. I figured out that the motor was broken. I changed it and now all works fine.

    But DRV8835 fault happened again (I made chip swap already). I think that the distance from the capacitor to the chip is still too long and I can't solder it closer in my case. 

    I will check what happens on Vbat pin during operation to see if there too high voltage spikes. Unfortunatly I haven't enough time for that know. 

    Thank you again. 

  • Hi! 

    Just wanted to say that TVS diodes on the outputs solved the problem.