This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DRV8873HEVM: Fault comes on and stays on when PWM goes to zero from 80% or more

Part Number: DRV8873HEVM

When driving a wheel motor for a robot, there is a case when a fault is triggered, and the fault stays in fault mode until power is cycled.

At low motor speeds, say up to 50% PWM, no problems in motion or when PWM goes to zero.

At higher motor speeds, say 80% PWM, no problem in motion, but as soon as PWM goes to zero, a fault is triggered and remains in the triggered state until power to the board is cycled.

If PWM is ramped down, no fault is triggered.

Operating conditions:

24VDC motor voltage

5VDC PWM logic voltage

HW configured for PH/EN mode (for DIR and PWM)

500HZ PWM freq (also tried at 1KHZ, no difference).

Geared wheel motor (wheels do not turn unless turned by the motor)

Slew rates of 53.2V/us, and 13V/uS were tried. No effect on this issue.

Followed layout notes for thermal , and have heatsink on bottom of board (parts do seem to run warmer than expected when idle and not driving a motor). Not a thermal problem since we can load down the motor and no fault is triggered.

  • Hi Chris,

    Thank you for posting to the E2E forum.

    At higher motor speeds, say 80% PWM, no problem in motion, but as soon as PWM goes to zero, a fault is triggered and remains in the triggered state until power to the board is cycled

    In brushed DC motor, when there is a sudden change in the speed (when going to high speed to a complete stop), the back-EMF will decrease rapidly causing the motor current to increase suddenly for a brief moment in the same direction. If this current spikes is above the OCP threshold and last for longer than the OCP deglitch time, it will trigger an OCP fault which is removed once the device is reset. Having a slow ramp down will keep the current below the OCP limit and prevent the fault from triggering. Did you capture any current waveforms? It will be useful to see this along the nFAULT signal to verify if this the reason for the nFAULT latching low.

    500HZ PWM freq (also tried at 1KHZ, no difference).

    This device has a maximum recommended PWM frequency of 100kHz. 500kHz is well above this limit. Is there a reason for wanting a 500kHz PWM frequency? High frequency can cause issues.

    Slew rates of 53.2V/us, and 13V/uS were tried. No effect on this issue.

    Is this the output slew rates or the VM slew rate? Can you provide waveforms of the output voltage as well?