App note: Minimizing switching regulator residue in linear regulator outputs

Jim Williams from Linear Technology explains how to battle switching regulator residue in linear regulator outputs. If you want to clean up the switching regulator output by using a linear regulator, you’ll run into difficulties.

Linear regulators are not very good at suppressing high frequency noise, and switching regulators output loads of it. The “secret sauce” to suppress the high frequency noise is to add ferrite beads on both the input, and the output of your linear regulators. More about this is covered in the app note.

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    1. @Casey O’Donnell,

      The end concept here is to introduce a switching regulator/converter (buck, boost. buck+boost) to efficiently handle a wide range of input voltages, and then take the switching output which is closer to the linear regulator’s designed input voltage to avoid the losses which will happen with a linear regulator alone (thereby requiring a “hunk of metal” as you said to dissipate the linear regulator’s input vs. output voltage difference). The added advantage is that linear regulators will then reject the noise generated by the switching “pre-regulator” and present a cleaner DC output to the following subsystems. Switching pre-regulation, especially in the case of battery powered devices which are sensitive to power supply noise makes post-regulation linear regulators highly desirable as they reduce the switching noise. The combination allows (especially with buck-boost switching pre-regulators) to get every last drop of energy out of a battery while still providing cleaner DC power to the system.

  1. A good reminder of basic circuit techniques! Sometimes switchers are required, for example right now I’m working on reducing noise on the output of an isolated DC-DC converter. That can’t be done without using some kind of AC signals, as far as I know.

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