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DRV8870: cannot drive the motor steadily

Part Number: DRV8870

Hi team,

Here's an issue from the customer may need your help:

The schematic design is consistent with the manual requirements, the current limit is 1.3A, and the IN1, in2 low level is driven with 8kHz PWM 85%~100% duty cycle. Drives a brushed DC motor, and it works normally (30 RPM) if the drive line is short (15 cm), while if extend it by a few centimeters, the motor will almost stop. 

Checking the OUT1 waveform, only about 20us is high, but normally 100us should be high.

(75% duty cycle)

(75% duty cycle with normal output)

(75% duty cycle with no load output)

(75% duty cycle, abnormal output with load)

Could you help check this case? Thanks.

Best Regards,