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This is a conceptual question about the best way to filter a dc-dc switching supply with low frequency noise passing through the input to the output.

I think the most effective solution would be to stagger LDO's. Is there an alternative approach in using a combination of RLC circuitry?

Also considering both cases that the switcher is and isn't sensitive to ESR for error feedback.

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    \$\begingroup\$ Your scenario isn't clear -- feel free to edit your question with more information. What do you mean by "passing through the input to the output"? I assume you mean that when there's noise on the input, a (hopefully!) attenuated form of that noise appears on the output -- is that the case? There are so many different ways to build a DC-DC supply, I'm not sure there's a generic answer, but I might start with making a better control loop. \$\endgroup\$
    – TimWescott
    Feb 1 at 3:19
  • \$\begingroup\$ You can get LDOs with 60+ dB of rejection at "low" frequencies. If it really is low and you don't need much power, a carefully chosen LDO is probably the way to go. It really depends on the specifics though. \$\endgroup\$
    – user1850479
    Feb 1 at 3:21
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    \$\begingroup\$ "Low frequency" - how low? Normally an SMPS is regulated, attenuating or eliminating LF noise (such as rectified mains ripple). The first (lowest) strongest spectral peak is usually the switching ripple itself, and at 100s kHz or more, it's easily filtered with a few LCs. An LCLC with judicious layout easily gets total noise into the 100µV RMS range. In contrast, LDOs are almost useless at these frequencies: most roll off heavily by 10kHz or so. There are exceptions (and quite exceptional ones at that!) as @user1850479 said. \$\endgroup\$
    – Tim Williams
    Feb 1 at 5:52
  • \$\begingroup\$ ...Or is the goal to transmit "low frequency" content intentionally? Hm no, "noise" probably wouldn't be called that. If it were, that could be like a ratiometric rather than regulated sort of application: a "DC transformer", or a "transformerless" mains-frequency "variac" (you can use bidirectional switches to build a buck converter that works regardless of input/output polarity, so can operate on AC directly). \$\endgroup\$
    – Tim Williams
    Feb 1 at 5:54

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LC post-filtering the output of a SMPS is practical. Depending on current draw and voltage level (and thus feasible sizes for L and C), it is realistic to achieve a roll-off as low as 100 or 1000 Hz without going to extremes.

This is usually absolutely sufficient because the voltage regulation of the SMPS itself provides very low supply impedance and strong input rejection below approximately said frequency range.

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