I have an issue with a board that I designed using the DRV8825 (two per board). We designed the boards, built and tested them, and everything seemed fine. Recently, I got a report about the stepper motors not operating properly. When I investigated, I found that there was a problem with the VREF signal. (One divider sets AVREF and BVREF for both parts).
I am setting VREF with a divider (1.30K over 1.00K and 0.1uF cap to ground) from a 3.3V supply. This gives me a nominal 1.43V reference. Combined with my 0.5 ohm sense resistors, I should be looking at about 0.6A peak current. The circuit does work as designed. However, on this particular board, VREF is sitting at 0.069V and phase current is accordingly lower. Unpowered, the resistance from that node to ground is about 27 ohms instead of about 770 ohms typical for a working board. There is anecdotal evidence that at least one other board in the field has this same issue (out of about 8 currently deployed).
I am using an independent 3.3V supply to generate VREF (instead of V3P3OUT) and divider resistance is quite a bit lower than what is shown in some of the reference schematics. I have an E-stop switch, which cuts the VM power (24V regulated) while leaving 3.3V intact.
Is there anything that I might have done to damage that input? Is it sensitive to backpowering? Is it particularly sensitive to ESD? I cannot find anything in the datasheet that would explain the issue.