LMR38010: strange oscillation at the output

José Pablo Crespo

Prodigy 20 points

Part Number: LMR38010

Tool/software:

Hi!

I'm getting this waveform at the output of a LMR38010S converter, from 48 to 12V. Anyone knows what is happening?

I followed the webbench design tips, I also checked with "classic" design equations and datasheet equations.

This kind of oscillation at the output doesn't change with:

- Increased current (from 50mA to 500mA)

- Changing output capacitance (nominal 22uF, try from 18 to 44uF, effective capacitance at 12V)

- Inserting compensation capacitors in the feedback loop (10pF, 47pF, 100pF)

Output, (100uS/div)

Switching node (5uS/div)

This is the simplified schematic:

Does anybody know what can be wrong?
Thank you!

1 month ago

0 Niranjan Nair 1 month ago

TI__Genius 11060 points

Hi,

The device seems to be in PFM operation with reduced frequency. The output ripple is usually higher in PFM and 62 mV doesn't seem unreasonable. This is what you are referring to right? Are you asking about reducing this 62 mV?

Regards,

Niranjan

0 José Pablo Crespo 1 month ago in reply to Niranjan Nair

Prodigy 20 points

Hi Niranjan! Thanks for answering!

The first picture is not only the switching noise, it has a perturbation at 8kHz. This perturbation is my problem.
Yes, it's in PFM with reduced sw frequency, you can see it switching at 860kHz in the second picture, and when you increase the power consumption it goes into CCM, 1.1Mhz, which is fine, but you still have that perturbation at 8kHz.

Any idea what that 8kHz perturbation can be?