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DRV8870: Abnormal Output Waveform During COAST Status

Part Number: DRV8870

Hi teams

My customer encounters some problems with DRV8870. The application details are listed below:( I didn't obtain the schematic from the customer)

Schematic/Configuration: IN+ connected with MCU GPIO and IN- connected to GND. 30Khz Switching frequency with 12V voltage supply.

It means that only two operation status could be achieved: Forward and coast (As the IN- connected to GND and always be logic low level.)

Issue phenomenon: When the PWM duty cycle in IN+ is 50% the motor spin normally but when the duty cycle decrease to 33% the motor stops with obvious vibration. The output voltage waveform is shown below.

My questions:

  1. Are 30Khz switching frequency is suitable for DRV8870? What factor we should consider during switching frequency selection? (efficiency? motor parameter? )
  2. For such a design that connects IN- to GND is there any potential risk?

  • Hi Gabriel,

    Can you also provide the current waveforms?

    It is probable that the average current (torque) is not enough to move the motor when switching between forward and coast.

    An alternative is to switch between forward and brake. This will allow the current to remain in the motor during the brake time. This is a common drive technique when PWM'ing at high frequencies.