LIBRARYBuck regulators (step-down).

A buck regulator steps a voltage down without cooking a regulator. Instead of burning the excess voltage as heat, it switches the input on and off fast and filters the result, so almost all the energy reaches the output. It is how you get 3.3 V from a 12 V input efficiently.

How switching avoids the heat

An LDO wastes the voltage it drops as heat. A buck does not drop voltage across a resistor at all. It chops the input into pulses with a fast switch, then smooths those pulses back into a steady lower voltage with an inductor and a capacitor. Little is wasted, so a buck can be over 90 percent efficient where an LDO doing the same big step-down would be far worse.

The basic buck

A buck is a switch, an inductor, a second switch or a diode, and an output capacitor. The switch feeds the inductor in pulses; the inductor stores energy in its magnetic field and hands it to the output smoothly; the capacitor holds the output steady between pulses. The controller adjusts the timing to keep the output on target.

Duty cycle sets the output

The fraction of each cycle the main switch is on is the duty cycle, and for a buck it sets the output as that fraction of the input. Fifty percent of 12 V gives 6 V; a smaller duty cycle gives a lower output. The controller trims the duty cycle continuously to hold the output fixed as the input and load move.