Excessive RF noise from LM46002

TIMOTHY CRIST

I used the online tools to design my switcher circuit; however, we are failing radiated emissions around 80Mhz. Vin seems to have a large transient when switching at 600mA to 900mA load. Adding capacitance to the input does not reduce emissions noticeably. Adding a ferrite to the output leads, and one to the input seems to work, but it is expensive and difficult to implement at this late stage. Any suggestions based on the supplied schematic?

FYI: Nominal input is 12V. Output is 5V. Load 500mA to 1A. Layout is 4 layer with GND on inner two layers. Spacing is tight (no long traces).

over 9 years ago

0 Peter Meaney over 9 years ago

TI__Mastermind 31580 points

Hi Timothy

This post should be in the non-isolated dc/dc forum, I will move it and one of my colleagues will answer your question in that forum.

Regards

Peter

0 Frank De Stasi over 9 years ago in reply to Peter Meaney

TI__Guru 74055 points

I am passing this to the apps engineers for this device.

0 Akshay Mehta over 9 years ago

TI__Genius 15810 points

Hello,
Radiated emissions are usually related to the layout and chosen BOM. May we see your layout images? Please send PDF copies of individual layers.

Thanks,
Akshay

0 TIMOTHY CRIST over 9 years ago in reply to Akshay Mehta

Prodigy 160 points

Akshay,

I have the PDF layout available, but I am having trouble attaching it to this forum. Do you have an email address I can send it to ?

Thanks,

Tim

0 YangZhang over 9 years ago in reply to TIMOTHY CRIST

TI__Genius 11085 points

Please send it over to simpleswitcherapps@ti.com

0 TIMOTHY CRIST over 9 years ago in reply to YangZhang

Prodigy 160 points

Files were sent Friday. Any updates from your review?

Kind regards,

Tim

PS. I will attach pdf layouts to this post as well.PDF Layout.pdf

0 TIMOTHY CRIST over 9 years ago in reply to TIMOTHY CRIST

Prodigy 160 points

PDF layout was sent Friday and again on this list Monday. Any updates?

0 Akshay Mehta over 9 years ago in reply to TIMOTHY CRIST

TI__Genius 15810 points

Hello Timothy,

I apologize for a delayed response.

I know you mentioned that the changes were going to be expensive and difficult, but I don't see much of an option otherwise. There are two relatively big issues:

1) Your feedback trace is long and, while I am not sure, it seems like passing under the IC to the via on the top feedback resistor. Ideally the FB trace should be very short and the feedback components (two resistors and a capacitor) should be placed right next to the pin. The VOUT sense trace to the top feedback resistor can be long, but again, has to stay away from the SW node and not pass below the IC. This way you can couple a lot of switching noise on FB which passes on to VOUT.

2) Your SW node is long and not so fat. Having a plane direct connect instead of relief connects on all components is also much required. Tiny relief connections add parasitic inductances and can cause noise related issues.

How are you adding capacitors? The general idea is to select a low ESR ceramic capacitor whose SRF is the same as the frequency that you are filtering. It is at this frequency, that the capacitor is mostly resistive and can damp the noise.

I hope this helps.

Regards,
Akshay

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Excessive RF noise from LM46002