The challenges I’ve had with buck or boost conversion is on mixed signal boards where I have very sensitive analog circuits. The ripple and switching noise on the output can make you lose bits on ADCs, or show up on a DAC quite easily. It’s easy to get a buck converter doing the right thing without horrible EMC/EMI if you’re careful and follow the manual, but it’s a lot harder to optimize for something like low noise without utilizing LDOs
For one: most LDOs don’t have a high enough PSRR bandwidth to completely eliminate buck switching noise. Most LDO PSRRs roll off sharply in the low hundreds or high tens of kHz. That’s generally below the switching frequency (and noise frequency) of most buck regulators. If you’re dealing with audio that’s generally fine. RF, however, is a separate problem.
Second: there are more cost effective and wide bandwidth solutions for noise reduction. Capacitance multipliers are one that spring to mind. Ferrite beads are another great means for tamping down high frequency noise.
Third: layout and current return paths are often just as much of a problem as the buck itself. Couple a high current return path into an audio chain with a shared return, and you’re gonna have a problem no matter which way you slice it.
Yeah the PSRR of a typical LDO doesn’t to as well up in the high frequencies, but it generally still does some attenuation.
I’m not an expert myself, but I’ve read that RF noise in audio applications isn’t always okay because it can demodulate into your amplifiers.
Re. Capacitance multipliers, I may be mistaken but I think they typically introduce significant positive gain at high frequencies due to the transistor parasitics. They make sense for very low frequency cut off (AC mains transformer ripple), but I’m not so sure about typical SMPS output. I would bet other filter topologies (LC, etc), would generally do better in practice.
And definitely 100% on the layout. Return currents and common mode noise will get ya.
