DRV8426: Undershoot at AOUT

Hi Higa-san,

What waveform is the yellow trace, current through phase A or voltage on one of the AOUT? If it was voltage I do not see any voltage chopping that corresponds to current regulation. What decay mode was used?

If this is current waveform (I think full-step mode was used) it looks normal. This bump may be due to back EMF which will reduce or disappear if the motor is mechanically loaded. The specification you mentioned was for voltage at the output pins.

Please clarify.

Regards, Murugavel 

Hi Murugavel-san

Thank you for your reply.

The yellow trace is the voltage of AOUT.

There isn't the current trace in waveform.

Best regards,

Higa

Hi Higa-san,

Thank you for the clarification. This decrease below GND or increase above the supply rail happens during the dead-time of the bridge due to the inductive kick back of the motor coils plus the body diode drop. This is normal behavior and the driver is designed to be able to support this. Usually this does not exceed 1V. See below capture of the phase voltages with DRV8426EVM and a NEMA17 stepper motor.

I looked at the customer's voltage waveform capture. There appears to be an offset to the GND potential of negative 875 mV. I expect that to be 0V. The delta to the peak is < 1V, actually - (1.75-0.875) = - 0.875 V. Based on the delta value it looks to be within the -1V continuous absolute maximum specification. I observed close to this delta value with my EVM setup and no offset. Perhaps it was oscilloscope grounding related issue?

Regards, Murugavel