DRV8899-Q1: VREF RC time constant limitation

Ann,

Good question. I would think it should be longer than 200ns. Let me confirm it and reply to you ASAP.

Hello Wang,

May I know if we have any update here? Thanks a lot!

Best regards,

Ann Lien

Ann,

I sent 2nd request to system/design team yesterday. But, not get the answer yet.

But, I pretty much sure, the low pass filter time constant should be longer than 200ns. if less than 200ns, we don't need to add that capacitor and cannot call it as a low pass filter.

If we don't have a low pass filter, VREF pin could get a rectangle waveform which could affect the motor driver current regulation.

For the RC time constant, I would think it should be few time longer than the longest PWM cycle time. If we assume PWM frequency is 20kHz. The cycle time is about 50us. I would like customers put the RC filter time constant longer than 200us.

Hello Wang,

May I know if designer team has replied your question? Thanks a lot!

Best regards,

Ann Lien

Ann,

Yes. They did. I put the answer to this post yesterday. But, the system seems messed up and gave two duplicate posts yesterday. I deleted one. Now, both of them were gone. Let me find the delete post.

Hello Wang,

Thanks for your reply!!

Best regards,

Ann Lien

Ann,

You are welcome. I saw the new datasheet update on this issue, it puts the filter time constant longer than 10x PWM cycle time. It gives more margin.