buck-boost converter

A converter that can output either above or below the input, like a thermostat that can both heat and cool. It's essential when the source droops across a range that straddles the target — a lithium battery sags from 4.2 V down to 3.0 V while your device wants a steady 3.3 V, so you must boost when low and buck when high.

The classic topology inverts the polarity and gives V_out = −V_in × D/(1−D), where D below 0.5 bucks and above 0.5 boosts. Cleaner relatives like the SEPIC and Ćuk converters keep the output positive. The cost is more parts and stress on the switch, which sees both input and output voltages stacked together.