LMR51635: Switching pin ringing noise

Julia Zhou

Part Number: LMR51635

Tool/software:

Dear technical support:

I'm using LMR51635YDDR, 1.1MHz switching frequency (because I use two DC/DC converter parallel, besides LMR51635, another is LMR22676, 500KHz, from one paper it said when DC/DC converters are parallel, the switching frequency shall be different, so I use 1.1MHz version). the board input 24Vdc, output 5VDC/3A, everything works fine, but it fails EMC radiated emission test, I test pin 5-SW, it has 50MHz ringing noise, I try to add RC snubber circuit to decrease it, it doesn't work, when cap increase to 1300pf/8R ohm, it works, when using 220pFcaps, the output voltage is unstable, from datasheet the SW pin 5, it is push-pull output, could you please tell me how to decrease the ringing noise in pin 5? thank you very much!

Regards

Julia Zhou

1 month ago

0 Richard Fu22 1 month ago

TI__Mastermind 31085 points

Hi Julia,

Failure during radiated emission test usually implies coupling from high dv/dt nodes, like the switch node. Can you confirm that you followed the layout guidelines? Other things you can do (besides the snubber) to improve RE performance is adding shielding on top and around the circuit.

Thanks,

Richard

0 Julia Zhou 1 month ago in reply to Richard Fu22

Prodigy 10 points

Hi Richard:

Thanks your quick reply! for layout I try to follow layout guideline as possible, because of size limit it definitely has some difference. for your suggestion:

1) for snubber circuit, if I increase cap value, the output is unstable, is it correct?

2) Adding shielding on top and around the circuit, I will try them, thank you very much!

Regards

Julia

0 Richard Fu22 1 month ago in reply to Julia Zhou

TI__Mastermind 31085 points

Hi Julia,

Snubber design can be iterative. It looks like your design was stable at 1.3nF?

Try the shielding, I think that would be best.

Thanks,

Richard

0 Julia Zhou 1 month ago in reply to Richard Fu22

Prodigy 10 points

Hi Richard:

I'm not sure which kind of shielding material is better, because this IC LMR51635 is so tiny. if I add ferrite bead which will be serial with SW- pin 5 inductor, does it help? thanks.

Regards

Julia

0 Richard Fu22 1 month ago in reply to Julia Zhou

TI__Mastermind 31085 points

Hi Julia,

I meant like an actual EMI cage. The ferrite bead does not work like that, it helps with conducted EMI. It is typically placed at the input of the converter.

Please see this page for example: RF Shields | Electronic Components Distributor DigiKey

Thanks,

Richard

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LMR51635: Switching pin ringing noise