This DC-DC converter is designed to accept a range of input voltages that are present in a car’s electrical system, and provide a regulated 12v output. Nominally, the battery voltage is about 13.8v, but when the engine is being started, the voltage can often droop down to 6 or 7 volts for a short time, and peaks can be present somewhat above the nominal voltage as well. Because the input voltage can be either above or below the desired output of 12v, the converter needed to be a buck-boost configuration.
This project is now in the general electronics section, because I no longer use it in my MP3 Box, so now it’s basically just a fun experiment that I did to explore the world of switching converters. [...]

Source: http://eegeek.net/content/view/20/31/