LM25116: switching frequency noise

Ron Vis

Part Number: LM25116

Dear friend,

Attached my circuit for LM25116 and output noise.

As you can see, measuring the output voltage, i get a noise that is coming from the internal switching frequency - 210Khz. 1V pk-pk

i tried to add snubber but without success.

Please your help with reducing this noise.

Thank you,

Ron.

over 7 years ago

0 David Baba over 7 years ago

TI__Mastermind 26590 points

Hello Ron,

I noticed you do not have any decoupling capacitors on the output. Suggest adding a 1uF ceramic Cout in parallel with existing Cout.

Also, how are you measuring the output voltage? Are you sure this is not noise pick up from using a long ground lead on your probe? Suggests using a spring clip with very short ground and measure across the 1uF you installed.

Hope this helps?

0 Ron Vis over 7 years ago in reply to David Baba

Prodigy 100 points

Hi David,

it helps alot!

it seems the probe gains the output voltage noise from 60-70mV pk-pk to sometime over 1A pk-pk!!

in this case adding ceramic capacitor of 1uF in parallel to Cout - will not make any different.

thank you,

Ron.

0 Ron Vis over 7 years ago in reply to Ron Vis

Prodigy 100 points

Hi David,
When I'm connecting the load, the ripple is getting bigger.
I'm measuring the signal on SW pin, there is ring waveform of 135Mhz.
Should I use RC snubber on lower MOSFET to reduce noises?
Thank you,
Ron.

0 David Baba over 7 years ago in reply to Ron Vis

TI__Mastermind 26590 points

Hello Ron,

Yes an RC snubber will help greatly, because the 135MHz of ringing will no long be present once snubbed. The ringing typically ends up on the output due to the self-resonance of the inductor used (parasitic Capacitance across the inductor). The Resistor of the RC snubber provides damping and will reduce/eliminate ringing.

0 JohnTucker over 7 years ago in reply to David Baba

TI__Guru**** 191445 points

Did this ever get resolved?

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LM25116: switching frequency noise