Today’s electronic circuits require a number of different voltage potentials. Powering silicon devices like microcontrollers or digital logic require a different voltage than bias supplies or powering strings of LEDs. A system can very easily contain a combination of circuits that require different voltages. With only a single input voltage delivered to the system, a power regulator is needed to produce the necessary voltages.

Read PDF :

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en022891