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DRV10970: DRV10970 - Troubleshooting output speed variability

Part Number: DRV10970

Hi, We're having an issue where with variable speed (should be fixed) . The drive is driven by a 55 timer and the PWM input looks identical even when the motor is driving at different speeds. The motor tends to operate correctly most of the time, but around 25% of the time drives in a slow mode with no obvious pattern to its occurance. This problem exists over two different boards / motors with the same setup. The output to the motor is visible different (top photo is when operating as expected, and bottom when operating slower.) Blue trace is U, yellow is ahll sensor for U i believe. I would appreciate if anyone has any thoughts on the traces to indicate what may be going on?

Many Thanks

Richard

  • Richard,

    Is it possible that the current limit is being hit? Could you provide the CS voltage during the 2 scenarios?

    Could you also show the PWM duty cycle during the 2 scenarios just to eliminate that?

    Thanks,
    Brian
  • Hi Brian.

    Thanks for you're help and a good question.

    The CS voltage when running smoothly and fast as expected it 800mv but when running slower is around 1.2v which I'm assuming is hitting the limit VILIM_THR? We should have a 23ohm resistor there between cs and gnd.

    To debug the issue we have disconnected and left the PWM floating as per datasheet (100% duty cycle). However the issue also arose when we manually created the pwm at 97% with a 555.

    Thanks

    Richard

  • Richard,

    Yes, clearly the system is hitting current limit when you get lower speed.

    Could you look at the phase current? I suspect that we might be getting bad information from the hall sensors and misdriving the motor and thus hitting the current limit.

    Thanks,

    Brian