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Original question:

DRV8308: Motor speed drops significantly with small change in load

DRV8308: Closed-Loop Speed Control using Clock PWM Mode

Sadashiva Kamath
Expert 1175 points
Part Number: DRV8308

Hi,

I see in DRV8308EVM's Clock PWM Mode, PWM clock frequency is around 24KHz and duty cycle is around 26.8%. I want to use Clock PWM Mode for Closed-Loop Speed Control.

It is observed that at higher motor torque need, motor speed is reduced from 1500rpm to 1400rpm as it is a open loop control system. 

How to make Closed-Loop Speed Control in Clock PWM Mode? Is this can be achieved by varying duty cycle of 24KHz CLKIN?

  • 0
    TI__Guru 65190 points

    Sadashiva,

    This behavior makes sense as the Clock PWM Mode is open-loop as you mentioned. The DRV8308 does not know that the output torque has changed or that the motor speed has reduced as a result.

    To close the loop you will need to monitor the motor speed via the MCU using either the FGOUT pin, hall sensor output, or an encoder. You can then use the change in speed observed to scale up the duty cycle of the 24kHz input PWM with a PID loop or similar.

    Regards,

    -Adam