DRV8842: Current regulation

Camila,

Do you have share scope captures of the current waveform into the load?  Are you using the EVM of your own board?  If own board, please share schematics.

The nFAULT pin transitions low?  Is it a latched fault where you have to cycle nSLEEP to clear or does the part try to automatically recover?

Regards,

Ryan

Hi Ryan,

Here is the photo of the current going to the motor (1V=1A) when an external current limiting is put on the source supply:

Here without the external current limiting:

Closeup of it:


And here is the schematic of the driver:



We have two motors with the same configuration, but one the I0-I4 is set to 100% of Ichop and the other to 50% (Ichop = 4.3A), neither of them works...

When I connect the motor the nFAULT pin goes low but the nSLEEP stays high, I would need to disconnect it and press reset for the nFAULT to go back to high.

Any idea what can be the problem?

Thank you,

Camila

Camila,

Let me investigate closely and will respond in the morning.

Regards,

Ryan

Camila,

Is it possible to add another channel to your scope and capture nFAULT pin response?  Would also be good to capture current level when nFAULT occurs.  It looks like the driver is hitting OCP which can occur if the current is ramping too quickly.  Is there any capacitance on your motor?

Regards,

Ryan

Hi Ryan,

Yellow is the voltage on Out 1 and Blue the voltage on nFAULTY (both in relation to gnd):

Do you know why is zero even before the sparks?

When the motor is working (with the external current limiting) the nFAULTY is 3.3V as expected.

Thanks,

Camila

Camila,

That is very bizarre behavior on nFAULT.  Do you have an external 10k pull-up resistor soemwhere on the schematic to 3.3V?  You mentioned the nFAULT is latched low before: 

When I connect the motor the nFAULT pin goes low but the nSLEEP stays high, I would need to disconnect it and press reset for the nFAULT to go back to high.

But that is not what I am seeing in your scope capture.  nFAULT is an open-drain output, so should pull hard to GND for a fault and release to 3.3V thru the pull-up resistor when everything is normal.

Regards,

Ryan

Hi Ryan,

I only have the R167 10k ohm going from nFAULT to 3.3V (output of pin 15) as it shows on the schematic.

I only recorded the changing parts with the oscilloscope, but if I measure with the multimeter I can read mV on nFAULT after trying to drive the motor.

Cheers,

Camila

Camila,

Is your 3.3V supply stable?  Is your motor supply stable?  nFAULT should not bounce like this.  Need to start checking all the supplies to make sure nothing is dropping during this condition?

Regards,

Ryan

Hi Ryan,

It should be stable, otherwise I would see weird behavior in other parts of the board, but I'll check it with the oscilloscope next week.

Thanks,

Camila

Camila,

OK, thanks.

Regards,

Ryan

Hi Ryan,

Sorry for the delay, I forgot to get back to you regarding the power supply stability.

I tested it but without the full load and the power supply looked very stable (the board I have now can't drive the motor, I'll get back the one that I was using next week and then I can do this test again).

I bought the DRV8842 evaluation board to try to isolate the problem and be able to debug more efficiently.

And do you know if I can simulate the DRV8842 on LTSpice? I couldn't find a Spice model for it.

Thanks,
Camila

Camila,

Thank you for the update.  Unfortunately, we do not have a model for this device.

Regards,

Ryan