Switching Noise Removal @360Khz

Hi

   Check the ESR /ESL of the output capacitors and add more ceramic caps at the output to lower the ESR/ESL. You can try adding a 2nd stage LC filter to minimize the switching noise. Here is blog post that talks about 

https://e2e.ti.com/blogs_/b/powerhouse/posts/design-a-second-stage-filter-for-sensitive-applications

After you make these changes, check the closed loop stability of the system to ensure that the power supply system is stable.

Regards,

Gerold

Sir we can not add a filter now as the schematic design had been completed and testing of the product is going on. I have added Ceramic capacitor at the output but there is an only minor improvement . can u please suggest some alternative solution?

thanks

Hi Balvan

     Can you please share with us oscilloscope shots of how the the output switching ripple looks like? How much reduction are you looking at?

Regards,

Gerold

Hi This is an oscilloscope image .we r looking  50 -55 SFDR for this.

Hi Balvan

Gerold is looking into this and will feedback to you soon.

BR

Ruby

Hi Balvan,

This plot shows the peak in noise at 500 kHz and the LMZ22010 should be running at ~350kHz so I'm not sure this is where the noise is coming from.

If this noise is coming from this module, there isn't much flexibility outside of changing the external filtering. If I am converting the -16dBm correctly, I am estimating 100 mVp-p ripple which seems rather large so I may be interpreting this wrong. This also makes me unsure if this 500 kHz noise is coming from the LMZ22010. Can you post the time domain waveform of the ripple?

How much ceramic capacitance did you add and what is the ESR of the 330 µF caps? If I am interpreting the -16 dBm correctly, I wonder if the high noise could be related to high ESR in the 330µF caps. If they have high ESR, changing to lower ESR caps may help.

Anthony

Hi Anthony

500khz is our message signal . i am getting noise at 350 khz. On LMZ22010 module noise at 350Khz is coming around -40dbm.. in previous image my message signal at -16 dbm and noise at -58dbm . so i am getting SFDR 42 . But we have requirement of  SFDR  around 50-55dbm

Hi Balvan

Anthony is looking into this and will feedback to you soon.

BR

Ruby

Hi Balvan,

Thank you for the clarification. That makes more sense.With -60dBm the peak to peak ripple is only ~0.6 µV assuming 50-Ω impedance. -70dBm should get you to the requirement which is ~0.2 µV peak to peak. It may be possible to do this with the right output capacitor selection, but, adding a second stage filter as Gerold suggested would be easier.

Can you share more details on the output capacitors used? For any non-ceramic capacitors, please share the part number or the ESR.