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DRV8825 PWM frequency

Chaz Miller
Prodigy 30 points
Other Parts Discussed in Thread: DRV8825

From what I understand, microstepping drivers like the DRV8825 output PWM on each output, and the output duty cycles are varied in a sinusoidal-approximating sequence to effect the microstepping drive.

The DRV8825 datasheet mentions PWM several places, but I could not find reference to the actual PWM frequency used on the outputs. The datasheet does reference an input step rate limit of 250kHz. At 32 steps/macrostep that would be a 8kHz pseudo-sine wave output. If these numbers are real I am led to assume the PWM frequency must be greater than 250kHz to reconstruct a 8kHz sine wave with 5-bit precision.

I wonder if the rate is really this high because it seems to me that for motor applications PWM frequencies in the tens of kHz are more common. Perhaps the 250kHz limit is the limit of the step sequencer and only applies to full-step operation?

Thanks

chaz

  • 0
    TI__Expert 7455 points

    Hi Chaz,

    The internal current regulation PWF frequency is stated in the datasheet as 30 KHz under the fPWM field (Internal Current Control PWM Frequency).

    The 250 KHz frequency is for the STEP input and it refers to how fast you can articulate this pin, not necessarily how fast you will want to articulate it in real life. Actually, no stepper that I know of, can move that fast.

    We do have to consider, however, that when using microstepping the STEP frequency is in essence "scaled" down. For example, at 32 degrees of microstepping, a frequency of 250 KHz on the STEP input would translate to the equivalent of 7812.5 full steps. That now looks a little bit more realistic, although even then it is a rather fast speed for a stepper. Will you ever see this in a real life application? Chances are the answer to that question is no, but we never know. At 45V, speeds of this sort are plausible, according to some motor datasheets, so we must entertain the possibility.

    Hope the info helps. Best regards,

    Jose Quinones

  • 0 in reply to Jose Quinones
    TI__Prodigy 30 points

    Thanks for your response. This makes sense.

    chaz