DRV8955: nFault problem

tom greive

Part Number: DRV8955

I've designed and built a circuit using the DRV8955. I'm using it in mode 1. As I try to activate an output I get a fault. I don't believe it's over or under voltage, over temp, and the outputs are open circuit. I'm at a loss to figure out why there is no output

.drv9855.pdf

over 2 years ago

0 Keerthivasan Kumanan over 2 years ago

TI__Mastermind 18725 points

Hello Tom,

Can you check the voltage on VCP and make sure it is above the VCPUV. The recommended value for the charge pump capacitor is 0.022 uF rated for VM.

Best,

Keerthi

0 tom greive over 2 years ago in reply to Keerthivasan Kumanan

Prodigy 70 points

Hi Keerthi

My VCP is 17.17v with a supply of 12.17v.

thanks

0 Keerthivasan Kumanan over 2 years ago in reply to tom greive

TI__Mastermind 18725 points

Hi Tom,

Can you try it again with the right value for the charge pump capacitor. Can you give me a capture of the waveforms on in2, in4, Vm, nFault, Vcp, and the outputs.

Best,

Keerthi

0 Ryan Kehr over 2 years ago in reply to Keerthivasan Kumanan

TI__Guru 55615 points

Tom,

Is there any high value (micro-farads) or so capacitance on the outputs? I know you said it is "open load", but maybe there is some capacitance near connector that is not shown in your schematic. This can cause an OCP fault due to inrush current charging the capacitor.

Regards,

Ryan

0 tom greive over 2 years ago in reply to Keerthivasan Kumanan

Prodigy 70 points

We tried jumping IN1 to IN2, & IN3 to IN4. Now I can get Output 1&2 to go high and deliver current. If I try to drive Output 3&4 high I get a fault. Interestingly if I have my optocoupler U6 active when I come out of sleep then Output 3&4 will go high and deliver current. If I switch U6 off and on I get a fault. Sorry for not responding earlier, I had shipped the prototype for some testing, and didn't have access to it. There is no real capacitance on my outputs.

drv8955.pdf

0 tom greive over 2 years ago in reply to Keerthivasan Kumanan

Prodigy 70 points

Is this what you are looking for?

drv8955_traces.pdf

0 Keerthivasan Kumanan over 2 years ago in reply to tom greive

TI__Mastermind 18725 points

Hey Tom,

Where is the switch noise coming form. Is there any way you can get rid of it with a RC filter. Regardless, can you disconnect the optocoupler from the circuit and manually drive the inx pins with a signal generator or an external MCU of some sort?

It seems like an OCP fault as the CP is still operating and it is latched. Can you check the current as well?

Best,

Keerthi

0 tom greive over 2 years ago in reply to Keerthivasan Kumanan

Prodigy 70 points

I have a dry contact switch running the optocoupler. I tried adding a large capacitor to the output of the opto. but it didn't seem to help. I can try a cleaner output, but in the end my application needs to survive a dry contact turning it on. Not shown in my schematic are a 2nd pair of optocouplers, in parallel to the ones shown, that run to a switch.

drv8955_currents.pdf

0 Keerthivasan Kumanan over 2 years ago in reply to tom greive

TI__Mastermind 18725 points

Hey Tom,

Just to clarify the current captures were taken when the opposing two inputs were off. I recommend getting our DRV8955 EVM and also performing tests with that to figure out if there is something wrong with the board and layout itself.

Best regards,

Keerthi Kumanan

0 tom greive over 2 years ago in reply to Keerthivasan Kumanan

Prodigy 70 points

Keerthi,

You are correct about the current captures.

I'm re-designing the circuit to use a pair of relays as an output device.

Thanks for your time.

tom

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DRV8955: nFault problem