DRV8842 Overcurrent while Stopping

Hi Igor,

Does the fault occur as the motor is stopped, or some time after the motor has stopped?
Do you have to issue a reset or recycle power to run the motor again?
Can you provide a scope capture of the motor current when stopping until the overcurrent fault occurs?
Is it possible there is a short in the motor causing this?

Hey Rick, thanks for the quick reply!

The fault condition happens while stopping. If no fault happens, the motor stops quite fast. If the OC happens while stopping, the motor takes longer to stop, so I can hear the difference in the stopping process.

I connected a 1R resistor in series to the motor to measure the current, but this resistor makes the current decrease and I don't get the fault anymore :s ... I have no other small value high current resistor to measure the current. The back emf must be a little over the limit so the resistor makes the current decrease enough to not trigger OCP.

Do you have any suggestion how I can make the chip less sensible to the back emf? Maybe decay mode (now using slow decay)? Or current limiting on the chip?

I defently don't have a short in the motor.

Thank you,

Igor

Hi Igor,

Can you confirm:
Do you have to issue a reset or recycle power to run the motor again? <-- this confirms it is overcurrent.
What is the motor resistance and inductance, and VM power?
Are you using any of the current regulation features, VREF, I4:0?
What value is your sense resistor?
How do you intend to run the motor (variable speed, or off/on)?

With the above information, I should be able to provide a suggestion.

Hello Rick,

If I issue a reset, I can run the motor again. Overcurrent confirmed then :)
I measured the motor resistance with an cheap multimeter: 1R9 (Sonds little, appliying 24V to the motor it consumes running <2A.)
Sorry, I don't have tools to measure inductance of the motor.. Can't tell you that.
I4:0 is HIGH, Sense resistor 0R15.
I intend to run the motor on/off, no pwm used yet.

Thank you very much,

Igor

Hi Igor,

Thanks for the info. I forgot to ask how is the motor stopped.

Are you placing the outputs in slow decay (brake mode)? If not how do you stop the motor?

Based on your description of the motor stopping quite fast, I am assuming you are.

If using slow decay to stop the motor, please try:
1) Setting I4 low prior to stopping the motor -- this may be the easiest solution
2) PWM'ing the drive input from 100% to 30% prior to stopping

I am still guessing without the current waveforms. If you still have the problem after trying these, please capture the output voltages and the voltage across the sense resistor. These voltages can provide current information, although it is more difficult to interpret.

Hi Rick, thanks for the suggestions!

I captured the current over the sense resistor: I measured 104mV while the motor is running (@ 0.15R) ~= 0,7A.

The back emf produced a spike of 720mV ~= 4.8A at 0.15R. (Maybe to near to the 5A limit)

I'll change the software to decrease current at the I4:0 pins before stopping to see if this reduces my back emf. I'll post results.

Thank you, have a nice weekend!

Igor

Hi Rick,

I got rid of the OCP event!

I stopped the motor with a 255-Step 31 KHz PWM, ramping it down from Full High (255) to 0 in 1ms steps, and the error does not happen anymore. I measured the current over the sense resistor (0.15 ohm) and got ~2.5A when stopping, which is OK for me.

I'll try to reduce the Current with the I pins before stopping to see if this helps too and post my results.

Here is the voltage over sense while stopping the motor with the PWM:

Thanks a lot,

Igor

Hi Igor,

Glad to hear you have removed the OCP. We will await your next post.