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DRV8305: Motor drivers forum

David Gonzalez2
Prodigy 80 points
Part Number: DRV8305
Other Parts Discussed in Thread: DRV8323,

Hello, 

I designed a board based on the DRV8305 and they have been working perfectly. I've been running an algorithm that doesn't need current feedback, but now I would like to implement it. It seems that I did an incredible mistake with the routing of the SP1, SP2 and SP3 pins as I didn't rout a dedicated trace for them and connected directly to the boards ground plane. Measuring the currents with my MCU's ADC I read something like this:

I'm designing a new board based on the DRV8323 and I want to avoid this problem. Does this behaviour can be totally explained by the routing mistake or do you think there might be something else I did wrong?

Here's the schematic and the layout:

Thank you!

David

  • 0
    TI__Mastermind 26046 points

    Hi David,

    Thanks for posting to the MD forum!

    Your layout looks really nice! In terms of current feedback, I do see some noise on your current feedback. Typically we use an RC low pass filter on the SOx signals to cut off any frequencies higher than around 100kHz - 1MHz. This may improve your current feedback readings. 

    In the DRV8305-Q1 EVM, we used a 56-ohm resistor and 2200pF cap to filter noise on the SOx signals. 

  • 0 in reply to Aaron Barrera
    Prodigy 80 points

    Hey Aaron,

    Thanks for the help!

    I didn't do differential routes for the SNx and SPx, I did route the SNx pins but the SPx I connected directly to the ground plane. I believe this should be part of the problem, what do you think?

    I know it's not ideal but I tried adding a RC low pass filter on a protoboard; since this board is a "shield" it's easy for me to add something in between the SOx pins and my mcu's ADCs. I tried to get as close as the values you recommended and ended up with a 47 ohm resistor and 2700pF cap. I only added this filter to one phase and the result was as if I didn't add anything at all. I tried with bigger values for the cap and the resistor and with something around 1kohm and 31 nF you could start to see the signal being filtered, but the reading is getting attenuated. The attached picture shows the response with the big RC filter:

    Having to add such a big filter and having this response makes me think the issue must be caused by something else. What do you think?

  • 0 in reply to David Gonzalez2
    TI__Mastermind 26046 points

    Hey David,

    My apologies, I missed that original part of the question. Yes, you should differentially route the SNx and SPx traces to the sense resistor and have SPx connect to ground as close as possible to the GND at the sense resistor. This ensures the resistor is accurately sensing the motor current at the phase and that differential voltage across SNx-SPx has no ground noise. 

    In your next design with DRV8323, I would recommend adding the RC filter near the SOx pins of the device to eliminate any excess board noise from the traces.