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DRV8424EVM: questions about DRV8424EVM

Jimmy Lai
Expert 5745 points
Part Number: DRV8424EVM
Other Parts Discussed in Thread: DRV8424

Dear Team:

If the customer increases the operating frequency, the current will become a triangle wave as shown in the figure below

Is this normal? What is the reason?

step frequency is 500K.  If 1/256 microsteps are used, the highest control can only reach 1.95pps

Can we increase it to 1M or 1.5M? Do we have another solution can reach higher step frequency ? 

  • 0
    TI__Guru 56650 points

    Hello, Jimmy,

    When the customer increases the operating frequency, it is normal that the current will become a triangle wave due the motor back EMF.

    A higher STEP pulse frequency gives a higher motor spinning speed which generates a high back EMF to against the input voltage. So, even the high side FET is fully on, the coil current still cannot reach the regulation point. That is why you see the a triangle wave. If you have to run on that speed with that motor, would you increase the input voltage to see if the waveform get better?

    Regards,

    Wang Li

  • 0 in reply to Wang5577
    TI__Expert 5745 points

    Hi Li,

    We appreciated your great support!

    1. But our customers use TMC2100 under the same conditions, the sine waves appear as follows, what is the reason?

    2. Step frequency is 500K.  If 1/256 microsteps are used, the highest control can only reach 1.95pps

    3. Can we increase it to 1M or 1.5M? Do we have another solution can reach higher step frequency ? 

    Many Thanks,

    Jimmy

  • 0 in reply to Jimmy Lai
    TI__Guru 56650 points

    Jimmy,

    1. TMC2100 under the same conditions has different current waveform.

    If the output current doesn't reach the regulation point, the high side FET should be fully on to drive the current up. I have checked DRV8424 before. It can be fully on to drive the current up. In that case, we need check the OUTx waveform to make sure both TMC2100 and DRV8424 output is same as the input voltage when the current cannot reach the regulation point.

    If TMC2100 can reach the regulation point; DRV8424 cannot; but DRV8424's high side FET is fully on, I would say DRV8424's input voltage is lower than TMC2100; or DRV8424's speed is higher than TMC2100; or the motor inductance is different during the test,. This is the basic physic law, Vin-Vbemf= L* di/dt

    2. Step frequency is 500K with 1/256 micro-stepping, it is about 1.95 kilo full step pps. It is thousand time of 1.95pps. Do I miss something? 

    3. 500k is quite high for STEP frequency because in that speed (1.95k full step pps), the back EMF voltage is going to higher than the input voltage in that speed. For example, if the motor has 1.8degree/full step with 1.95k full step pps, the motor would spin about 9.7 rotations per second.

    Regards,

    Wang Li