LMR23630: Audiable noise heard on inductor when using LMR23630 as inverting buck!

PYTsai

Part Number: LMR23630

We are using LMR23630 as an inverting buck to generate a negative voltage schematics as below. When L6 use 100uH audiable noise heard from inductor L6. If change the L6 to 33uH the auiable noise disappear.

We would like to keep using 100uH for L6, how do i modify the external component to avoid the audiable noise? thanks!

over 6 years ago

0 Timothy Hegarty over 6 years ago

TI__Guru 74342 points

Hi PY,

In general, a robust meachanical design for the inductor is more immune to audible noise. What are the part numbers of the inductors you tested?

Regards,
Tim

0 William Su97 over 6 years ago in reply to Timothy Hegarty

Prodigy 80 points

Please find inductors spec as below and comment.

0 Timothy Hegarty over 6 years ago in reply to William Su97

TI__Guru 74342 points

Hi William, PY,

Looks like the inductor is shielded. Is the converter stable? Maybe you can check the SW node waveform or review the load transient response.

Regards,
Tim

0 Katelyn Wiggenhorn over 6 years ago in reply to Timothy Hegarty

TI__Genius 11040 points

Hello,

In addition to Tim, do you have any switch node and output waveforms? I want to understand if the converter is stable.

Best Regards,
Katelyn

0 William Su97 over 6 years ago in reply to Timothy Hegarty

Prodigy 80 points

SW node waveform as picture, please help to find the noise problem.

0 Katelyn Wiggenhorn over 6 years ago in reply to William Su97

TI__Genius 11040 points

Hello,

Thank you for taking this waveform. The switch node looks unstable. Can you confirm that this image is taken with the 100uH inductor?

Also, can you confirm that your operating conditions are VIN=19V, Vout=-16V, and IOUT=0.3A? If possible, would you be able to take an image of the inductor current? This would be very helpful to see the size of the inductor current ripple.

Thank you,
Katelyn

0 Katelyn Wiggenhorn over 6 years ago in reply to Katelyn Wiggenhorn

TI__Genius 11040 points

Hello,

I want to follow up with you. Would you be help by providing further clarification:

1. Can you confirm that this image is taken with the 100uH inductor?
2. Can you confirm that your operating conditions are VIN=19V, Vout=-16V, and IOUT=0.3A?
3. If possible, would you be able to take an image of the inductor current? This would be very helpful to see the size of the inductor current ripple.

Thank you,
Katelyn

0 William Su97 over 6 years ago in reply to Katelyn Wiggenhorn

Prodigy 80 points

Yes, the operating conditions are VIN=19V, Vout=-16V, and IOUT=0.3A. Purple line is the inductor current waveform.

By the way, we try to change C308 from 27pF to 10pF, The switch node is become stable, waveform as follow.

Can you confirm this modification that C308 is correct?

0 William Su97 over 6 years ago in reply to Katelyn Wiggenhorn

Prodigy 80 points

Hi Katelyn,
1. Yes
2. Yes
3. Please see the next reply

0 Katelyn Wiggenhorn over 6 years ago in reply to William Su97

TI__Genius 11040 points

Hi William,

The waveform with the changed CFF (C308) looks much improved. The CFF will shift the crossover frequency, and in this case it appears to have improved the stability.

If possible, you may want to also take the loop response measurement or test the transient response to verify the stability across corner cases.

Best Regards,
Katelyn

0 William Su97 over 6 years ago in reply to Katelyn Wiggenhorn

Prodigy 80 points

Hi Katelyn,

Due to we don't have any loop response measurement instrument, so can you advise change CFF (C308) from 27pF to 10pF is fine for design ?
We also can not use webench to ensure the design is fine or not?

0 Katelyn Wiggenhorn over 6 years ago in reply to William Su97

TI__Genius 11040 points

Hi William,

Webench is a good simulation tool, and you can verify that there is sufficient margin. Stability can also vary depending on your layout as well. You can also verify stability in your application by monitoring the load transient response.

Best Regards,
Katelyn

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LMR23630: Audiable noise heard on inductor when using LMR23630 as inverting buck!