DRV8825EVM: Current regulation questions

Hi Jeffrey,

Thank you for posting in this forum.

"1. Overcurrent protection trip level is 3A, minimum, but output current is 4A under VREF 2.73V and 1000 PPS conditions. Please let me know why OCP is not working under the above conditions."

The OCP MIN is 3 A. The TYP or MAX could be higher. There will be a device to device variation for this parameter, some devices may be closer or equal to 3 A. We don't have a specification for that. If you further increase the current like a short circuit there will be an OCP. 

That said, you must not set the output current to 4 A for this device. The maximum suggested current for this device is 2.5 A. Do not use a setting to exceed 2.5 A coil current.

"2. There is a difference between calculated chopping current and measured current. For example, 4A (measured current) and 2.73V / (5 x 0.1ohm) = 5.46A (calculated current). Please let me know the reason why. ". 

First of all calculated current of 5.46 A is not a legitimate current setting for this driver. If you need higher current you must consider the higher current stepper driver such as the DRV8462 in DDV package for up to 10 A and DRV8462 in DDW package for up to 5 A coil current. 

The peak current achievable will be limited to VM supply voltage, Stepper coil L and R values and the speed of the stepper motor. Higher speeds will have higher back EMF and oppose the VM in which case VM must be increased to push the desired current. The waveform for 4200 PPS suggests the motor BEMF is high and VM was insufficient. This is typical behavior of stepper current waveforms at higher PPS and lower VM regardless of the driver used. You can search this forum and see similar waveform related questions have been addressed. 

What was the VM voltage used and what was the stepper motor specifications - datasheet - L & R values and speed vs. torque plot? A stepper gate driver DRV8711 may also be considered for higher current steppers.

Also, the DRV8825 is a old generation stepper driver. For newer modern design we suggest the DRV8434 for 2.5 A max. coil current and similar family of devices all the way up to the DRV8462. 

Regards, Murugavel