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DRV10970EVM: Not able to exit trapezoidal operation and enter sinusoidal

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Replies: 2

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Part Number: DRV10970EVM

I am running the DRV10970 with a Maxon 339267 BLDC, with an input voltage of 12V. I am able to spin the motor without issue in trapezoidal mode, however in the desired sinusoidal mode I am encountering two issues. I have the Rcs resistor set to 33k, which should keep the current limit to 900ma

1) At low PWM the motor is "skipping", successfully making 3/4 of a rotation before slowing and then resuming.

Blue = Current through single winding
Green = FG pin

2) At higher PWM the motor spins continuously, however the waveform, which starts out as the third-order harmonic shown in the datasheet then turns into something much more trapezoidal.

Blue = Current through single winding
Green = FG pin

Is it possible for the output to be sinusoidal for the entire range? I understand that trapesoidal could be needed to at higher RPM's however, this should not be an issue at low RPM. I also increased the current limit, by reducing the value of Rcs, and did not see any improvement.

The DRV10975 was also tested and it was able to maintain sinusoidal current draw for the entire RPM range tested. 

  • Hey Chris,

    The team has been notified of your question and we should get back to you very soon.

    Best,
    -Cole
  • Hello Chris,

    You are seeing the current limit being triggered in the DRV10970. As shown in the datasheet, as soon as the peak hits 1.5 times the threshold for current limit (900mA*1.5 = 1.35A), the device will change from sinusoidal mode to trapezoidal mode. Then, if the current waveform stays below the limit for 2 electrical cycles, then sinusoidal will start up again (this is your first set of waveforms). Otherwise, the device will stay in trapezoidal mode (this is your second set of waveforms). This is shown below.


    Since current limit is triggered at a low duty cycle so I am not confident the DRV10970 will be able to drive the motor sinusoidally over this whole range of RPM (since current will increase with increasing RPM).

    If sinusoidal mode is the desired mode (because of lower audible noise, smoother startup, higher efficiency, etc.) then I would highly recommend using the DRV10975 (if you are okay with using a highly configurable device) or the DRV10974 (if you'd like something that is more "plug-and-play"). Note the deciding the DRV10970's trapezoidal mode is "good enough" is an option as well.

    Best,

    -Cole

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