I have a question about switching supplies, regulators and filtering. I am working on a device powered by a +12V switching supply that has an analog circuit to process decent-quality (objective, I know) line-level audio. The analog section is powered by +10V, and there is another circuit section (also-analog but just LEDs, switches) powered by +5V. I was thinking of using a linear LDO from +12V -> +10V for analog section power, then a switching regulator for +12V -> +5V for efficiency purposes, since noise on that circuit isn’t as critical. The circuits will be separated on the board, but I’m wondering what the best way is to minimize the influence of the switching supply and regulator on the analog section.
My idea was to run the switched supply +12V line directly to the +5V switching regulator with no filter other than the recommended filter caps for the regulator. For the analog audio section, the +12V switched supply would run through an LC or PI filter, then into the LDO regulator down to +10V. In other words there are two separate branches directly from the +12V supply. Is this a reasonable approach? Or does it make sense to filter with an LC directly from the main +12V supply, then branch off into the two separate regulators? I thought maybe some of the +5V switching noise might carry over if I didn’t use LC to isolate the audio section. To be honest I don’t even know if using an LC filter is necessary, but I figure with the right damping ratio it can only help.
Also, any recommendations on corner frequency for the LC filter? Not sure what frequency the switcher is running and the likelihood of harmonics in the audio range, it’ll most likely be a +12V wall wart from CUI or Phihong. Thanks for any input.
EDIT: BTW for anyone coming across this question looking for information on input filters for switching supplies, I'd recommend this 5-part video series on youtube from Kainkalabs. It's the best explanation of switching noise and filtering I've come across yet.