DRV8840: FAULT bit occurs right after motor is running

Hi Joonas,

Brushed DC motors have high inrush currents at startup until back EMF builds up. Please refer to this information. If this current exceeds the IOCP specification 6 A MIN it will result in a fault which will stop the output drive and the motor and nFAULT will be LOW. If current regulation is properly configured to not to exceed 6 A at startup such OCP faults can be avoided.

It is also possible the die temperature exceeded TSD limit during motor startup. In this case the outputs will be disabled and automatically enabled when the temperature drops below hysteresis specification. This would also pull the nFAULT LOW.

The behavior in both the faults are different. Observing this will help differentiate and understand whether it was an OCP or TSD that caused the fault.

In addition to these if the VM voltage falls below UVLO threshold at startup a fault will happen. Can we capture the startup motor current using a current probe and along with it VM voltage. This scope shot will help us diagnose the issue. Thank you.

Regards, Murugavel   

Hi Murugavel,

Actually we have measured the output voltage already earlier and it seems that some times it can drive the motor like 100ms and then it suddenly drops down. Here are the oscilloscope figures from one start-up. I need to reconnect the motor once it comes back from service to be able to repeat this and acquire current output as well.

Hi Joonas,

Joonas Hollo said:

Actually we have measured the output voltage already earlier and it seems that some times it can drive the motor like 100ms and then it suddenly drops down. Here are the oscilloscope figures from one start-up.

This indicates an issue. I think we may be able to narrow down the issue by looking at the start up current waveform and the corresponding VM supply voltage waveform  once you get the motor. Please keep us posted. Thanks.

Regards, Murugavel 

Hi, Here are some more scope readings from the startup. There are three different time DIV to see closer start-up currents measured over 1Ohm resistor.
It seems to be like 4DIV = 4V/1Ohm := 4A peak current. Of course this is now measured without load.

What is the limitation for start-up current?

Hi Joonas,

Last week was a holiday week here in the US. Sorry about the delayed response.

Based on your start-up current measurements it appears that you may have exceeded 6 A the overcurrent limit of the device, hence the fault condition. The reason I'm saying this not only because the motor was with no load when you did this measurement but also because you used a 10 Ω series resistor for current measurement. Most DC motors has very low resistance of just a few ohms. Adding 10 Ω to this would limit the inrush current compared to without having this resistor. Did you observe a fault while running with this series resistor? Because this was < 6 A I'd expect no OCP fault happening. To measure true inrush current you must use a very low series resistance (current shunt) such as 0.1 Ω or use a clamp on current probe. 

Are you using current regulation? Please share the schematic.

Joonas Hollo said:

Actually we have measured the output voltage already earlier and it seems that some times it can drive the motor like 100ms and then it suddenly drops down. Here are the oscilloscope figures from one start-up. I need to reconnect the motor once it comes back from service to be able to repeat this and acquire current output as well.

Going back to these scope captures, were they at the IC output or at the power supply? If at the IC output did the nFAULT line go LOW whenever the output drops out? This may be pointing to a thermal shutdown. OCP shutdown is latching and during this shutdown the outputs will be disabled until the device is reset or power is cycled. 

Regards, Murugavel  

Hi,

And thanks for the reply. As you can see below, there´s no current regulation instead if we don´t drive the controller with PWM. 
Just to note that I was using one (1) Ohm not 10 Ohms in first current measurements. That´s still quite remarkable compared to shunt resistors.

Overall, I think that this OVP might be the case we are facing. So need to consider adding soft start resistor into the application or try to accelerate motor slowly to full speed.

Hi Joonas,

Joonas Hollo said:

Just to note that I was using one (1) Ohm not 10 Ohms in first current measurements.

Sorry about my oversight with this. 

Joonas Hollo said:

Overall, I think that this OVP might be the case we are facing. So need to consider adding soft start resistor into the application or try to accelerate motor slowly to full speed.

I assume you meant OCP. If the outputs were disabled after a brief on time it certainly would be OCP (overcurrent). If the outputs were disabled after a brief on time and then automatically turned after the device cooled off it would be a TSD (thermal shutdown happening). Accelerating and decelerating the motor to the target speed would help mitigate this issue during motor speed ramp up and ramp down. Thanks.

Regards, Murugavel 

Thanks Murugavel,

My mistake the OVP/OCP, definitely I meant OCP not over voltage..

We will make a sequence for accelerating/decelerating and try if that helps. Thank you very much for your help.

Sure, thanks!