DRV8711EVM: Calculating the drive register values.

Hi Nathan,

Thanks for posting in this forum. Pages 21 to 23 of the datasheet provides the guidance for calculation of these parameters. Yes you're on the right track. The rise time will be a function of the EMI performance with your stepper motor, wire harness length, power FET characteristics etc. So if EMI is high you may have to slow down the rise time from 500 ns. And yes based on your calculation the IDRIVEN and IDRIVEP settings are the closest.

The IFS full-scale current setting is done by proper sizing of the Rsense values, ISGAIN and the TORQUE register settings. This would be for the 4.2 A in your application.  

OCPTH is used for short circuit detection. Technically the threshold should be higher than 4.2 A with sufficient head room to avoid false shut down. The lowest available value for this setting is 320 mV worst case. If you're looking at a dead-short at the output this could be rising current all the way to 320/Rdson mΩ = 320/3.5 = 91 A for the HS FET (good idea to use the lowest Rdson value). For LS FET the short circuit current flow to VM should also include the Rsense. Regardless, for such low Rdson values this may be too high to stress the power FET momentarily and perhaps damage it especially at higher supply voltages. For design safety a resettable fuse or power supply current limiting would be better options to consider. If the short happens at the motor side the cable length related resistance would also limit the short current.

You're correct with the OCPDEG setting idea. If the OCPTH value of short circuit current is achieved prior to expiry of this deglitch time due to fast di/dt it won't be detected until the end of the deglitch time. This means current may get higher than calculated before it is detected.

I hope this helps. 

Regards, Murugavel  

Hi Nathan,

"TDRIVEN and TDRIVEP are the estimated rise times as mentioned above? So 500ns." - No, they are gate drive times. They are times during which gate driver can deliver IDRIVEN/IDRIVEP currents to Mosfets gates. The safest settings for TDRIVEN and TDRIVEP would be 2us (especially in case of adding LS Mosfet gate resistors).

With IDRIVEN/IDRIVEP set to 100/50mA you should get rise/fall times somewhere around 150ns.

I have blown a couple of Mosfets and drivers on DRV8711EVM board playing with Nema 34 motor at the very beginning and I will give you a few tips that may help you not to destroy DRV8711EVM board.

- Set IDRIVEN/IDRIVEP to 100/50mA before going out of "SLEEP" mode, do not use higher settings.

- If you want to use VM lets say over 30V you can add additional VM capacitor of around 1000-2200 uF.

- If you want to spin your motor over 1000rpm (but lets say under 2500rpm!) the value of above mentioned VM capacitor should be even higher (4700 - 10000uF) in order to keep VM under 60V in case motor stops suddenly and dumps braking energy into DC bus.

If you damage Mosfets or/and DRV8711 the next step could be adding LS Mosfet gate resistors mentioned in chapter 8.1.2 of DRV8711 datasheet.

Regards.

Grzegorz