LM20242: High switching noise at output voltage

Sam mishra

Part Number: LM20242
Other Parts Discussed in Thread: ALLIGATOR

Hello,

I have designed DC to DC converter with IC LM20242 using WEBENCH Design tool. Input is of 24VDC and output is 10VDC, 0.5A at switching frequency 545kHz.

I'm facing problem in output voltage. it has high switching noise at output(nearly 7 volt peak to peak).

below is snap shot of schematic and layout.

please kindly highlight the issue that is causing this problem.

Your early replay most appreciated.

over 4 years ago

0 Samuel Jaffe over 4 years ago

TI__Mastermind 25590 points

Sam,

Thank you for the full schematic and layout.

Can you please share some scope shots of the issue you're seeing. I'd like to see the SW node, VOUT, VIN, and if possible IL during this large ripple.

Thanks,
-Sam

0 Samuel Jaffe over 4 years ago in reply to Samuel Jaffe

TI__Mastermind 25590 points

Sam Mishra,

I have not heard back so I will close this thread. Please reply or start a new post if you have further questions (this thread will lock 30 days from the original post date).

-Sam

0 Sam mishra over 4 years ago in reply to Samuel Jaffe

Prodigy 120 points

Sorry for late reply.

Here is attachment contains waveform of SW node, VIN and VOUT. All waveform are in AC coupling.

Thanks.scope shots.pdf

0 Samuel Jaffe over 4 years ago in reply to Sam mishra

TI__Mastermind 25590 points

Sam,

Are you using the tip and barrel method to measure VOUT across C90? It is important to measure VOUT ripple using the tip and barrel method because the alligator clip loop can pick up lots of noise.

-Sam

0 Sam mishra over 4 years ago in reply to Samuel Jaffe

Prodigy 120 points

Hello Samuel,

Yes, we are using same measurement technique as you mentioned to measure output voltage across output capacitor.

Please suggest me any change if required in my Schematic or in layout.

-Sam

0 Samuel Jaffe over 4 years ago in reply to Sam mishra

TI__Mastermind 25590 points

Sam,

Sorry for my delayed response - I have been out of office. I will take a look at this on Monday.

Thanks for your patience,
-Sam

0 Samuel Jaffe over 4 years ago in reply to Samuel Jaffe

TI__Mastermind 25590 points

Sam,

It looks like there's some ringing on SW. Please correlate the amplitude and frequency of this ringing to VOUT and let me know how these relate.

Try placing a snubber on the switch node. This may help the output voltage ripple by reducing the SW ringing.

You can also try placing another cap (like C84) on the other side of the IC. Scratch off some solder mask and see how much this helps.

-Sam Jaffe

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LM20242: High switching noise at output voltage