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DRV8825 current limiting issue

Other Parts Discussed in Thread: DRV8825

I am using the DRV8825 with a stepper motor, and have experienced an issue when the motor is standing still. Below is the circuit schematic

The schematic does not show a 1.5kohm resistor between the FAULT pin and its connection to the CPU (not shown). RESET was originally tied to FAULT, but is now tied directly to +5V DC.

The supply voltage to the DRV8825 is 36V DC. AVREF and BVREF are driven by the DAC U403. When the motor has stopped moving, the DAC output is set to 0.05V, which should limit the current to 100mA. The DAC is set to  0.6V to provide a current limit of 1.2A when the motor is moving.

When the motor stops moving, I have confirmed that the DAC is outputting the correct voltage. However, one winding of the stepper motor is drawing 2A DC! Looking at the voltage output on the BOUT, I see a PWM signal with 10% duty cycle which corresponds with the amount of power being drawn by the stepper supply. Why is the stepper motor drawing so much DC current when it has stopped?

One other important note - I have a 3rd party development board with the DRV8825 and have hooked it to my PC board in place of the regular part. In this configuration, the system works as expected. I have confirmed that my circuit has the same parts as the development board. The only thing I am concerned over is that the ground pad on the bottom of the part may not be well soldered as we do not have high end equipment for this type of package. We use a hot air rework system to heat up the part with some solder paste on the bottom pad.