DRV8316: nFAULT PU

Hi Daniel,

For nFAULT, we generally recommend an external pullup resistor of 1k to 10kohm because nFAULT is an open drain output. Since it is an open drain output, nFAULT will pull back high through the pullup after the fault is cleared and will experience an RC time constant depending on:

- The pullup value of the resistor
- The capacitance of the nFAULT trace on the PCB

Therefore using the MCU pullup of 25-55kohm can increase the time it takes for nFAULT to go back high depending on the capacitance of the nFAULT trace on the PCB. Sometimes this can be an issue with hardware devices because there is a cycle-by-cycle current condition that clears the fault automatically; but with the devices listed above, I don't think a slower increase in nFAULT will cause any issues as long as you are not using cycle-by-cycle current limiting modes.

We suggest measuring the RC time constant of nFAULT going back high with the MCU pullup when the fault condition is cleared to see if it impacts performance on those drivers. 

Thanks,
Aaron

Hi Daniel,

You'll also want to want nFAULT to not exceed the IOD spec of 5mA, so using a 25k-55k internal pullup means it will require 60-133uA for a 3.3V pullup and 90-200uA for a 5V pullup. You'll be well below this spec, so just ensure that the higher resistor values do not cause a slow RC time constant when nFAULT is pulled back high. 

Same specs apply to DRV8874/DRV8876.

Thanks,
Aaron