DRV8821: Problem with DRV8821

Hello Martin,

Your input signals from the micro-controller seem correct but I have some questions regarding your schematic.

• Do you have a bulk capacitor on the main power supply?
• Have you bypassed each VM to GND with a capacitor?

Not having those capacitors in place can bring down the power supply voltage which could trigger an undervoltage fault and disable the outputs.

Can you obtain the following information for me?

• Remove the two motors and obtain the voltage waveform for the 4 outputs.

Thank you Pablo for your prompt reply. Capacitors are there on the main power supply. I added capacitors on VM pins but didn't help.

I removed the motors. Voltages when the pulses are applied are 0V on the 4 outputs (open outputs).

Martin

Hello Martin,

You should be seeing the output voltage pulses even without the motors connected. It could be possible that there might be a fault being triggered.

If you are able to, can you obtain the voltage values for pins V3P3, CP1, CP2, and VCP.  These pins should be affected in case of a fault.

Hi Pablo,

Voltages measured:

V3P3 = 3.3V

CP1 = 0V

CP2 = 15V

VCP= 14.5V

Hope these help to understand the problem.

Regards

Martin

Hi Martin,

It seems like the charge pump voltage is too low to actually enable the high side FETs. According to the datasheet (image below), the VCP should be around 23V for VM=15V. 

There could be a board layout or soldering issue causing a short somewhere. Do you have extra boards and have you seen the same problem in all of them?

Pablo,

I have 2 more boards all with the same problem. Can you give me some guidelines where the layout/shorts issues might be hiding?

Martin

Hi Martin,

Thank you for sending the layout. We discovered a short on the layout which is causing the issue. I would like to close his thread and continue discussion in other means of communication.

Pablo,

I tried cutting the conductor and bypassing the short using a wire but the motors still don't spin. I also tried ABRESET=0 and the COUT/DOUT motor doesn't work. I measured again the voltages after fixing the short, same results V3P3=3.3V; CP1=0V; CP2=15V; VCP=14.5V.

Martin

Yes.Thank you.

Martin