TPS55340 high frequency oscillation (sw node ringing ?)

Pial

Expert 7280 points

Dear team,

After a design based on webench simulations, my customer observes a 1V pk-pk oscillation on Vout (and SW node)

Attached is a document compiling all the information, Vin/Vout/Iout/Schematic/Layout and observation.

Could you please help me answering them ?

Thanks a lot for your help!

Best regards,

PA20161104 - SW and Vout oscillations issues.docx

over 8 years ago

0 JohnTucker over 8 years ago

TI__Guru**** 191445 points

What you observe is not "ripple voltage" but rather it is switching noise. It is normal to have this but you have to be very careful in measuring it. You need to use "tip and ring" method directly across the output capacitor or short (< 1 cm) pigtail ground lead. If it is in fact excessive after careful measurement, it may be layout related. I do not see C59. It needs to be very close to VIN pin of IC. You can also consider an RC snubber from SW to GND. It will help remove some of that.

0 Pial over 8 years ago in reply to JohnTucker

TI__Expert 7280 points

Hello John,
Thank youo for your help.
I'll ask to do some cleaner measurement and add a snubber if necessary.
www.ti.com/.../slpa010.pdf

Good catch for C59. I Guess it could explain a lot.

I'll keep you updated!
Thanks again,
PA

0 Pial over 8 years ago in reply to Pial

TI__Expert 7280 points

Hello John,

Following some additional measurements both on their board (with short pigtail ground lead) and on the EVM, they still find about ~1V pk-pk ringing on the output..

Given the fact that the output is 16V @ 300mA, is this pk-pk voltage to be expected ? (it does't seem very much to me)

Thanks again!

Best regards,

0 JohnTucker over 8 years ago in reply to Pial

TI__Guru**** 191445 points

You will always see some switching noise on the output. 1 V p-p seems high to me. I would probably consider a snubber or additional filtering. However if they can live with that amount of ripple then it is ok with me.

0 Pial over 8 years ago in reply to JohnTucker

TI__Expert 7280 points

maybe a small value cap on the output will do the trick..
e2e.ti.com/.../2243.understanding-measuring-and-reducing-output-voltage-ripple

Thanks again for your comments!

0 JohnTucker over 8 years ago in reply to Pial

TI__Guru**** 191445 points

That will certainly help. "Additional filtering" from my previous post.

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TPS55340 high frequency oscillation (sw node ringing ?)