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Stepping a voltage up or down from a positive input to a negative output with high efficiency
There is a version of a buck-
This is simply a circuit which can be physically switched between classic buck or boost. It is more expensive because it has redundant switches and is more lossy as it has an extra switch element in the output path as a buck converter and in the input path as a boost converter. There are some control ICs like LM5175 that can do the switch selection automatically.
The inductor can’t be optimum for both modes though. Why not just use a SEPIC?
Can be useful at low power when the transistors are integral to the control IC