LM5085: Strange waveform at SW node

Yue TANG

Part Number: LM5085

Hi,

my csutomer's schematic is as below [39-58.1V input, 5V/2A output]. , they used the lowest cost configuration according to LM5085EVM user guide. but they found the waveform of SW node is not normal, seems to be two waveforms of different frequencies mixed together. please help to suggest how to further analyze this problem? thanks.

SW node waveform:

zoom in

over 4 years ago

0 Frank De Stasi over 4 years ago

TI__Guru 72845 points

Hello

I will find the responsible engineer and we will get back to you shortly.

Thanks

0 Orlando Murray over 4 years ago in reply to Frank De Stasi

TI__Mastermind 22301 points

Yue,

In your schematic you have no output capacitor ESR or no type 3 ripple network.

With a COT converter, you need to have an ripple generation scheme, either using ESR or artificial ripple generation. Otherwise you will see instabilities as such.

See app note https://www.ti.com/lit/an/snva776a/snva776a.pdf

Hope this helps,

-Orlando

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LM5085: Strange waveform at SW node