A method and apparatus to supply a power voltage to a mobile device in a variable manner in order to provide high output power and a high quality sound environment and a mobile device using the same. The apparatus includes a volume control unit to detect installation of an external power supply and to generate a voltage control value corresponding to a current volume level, and a DC-DC converter unit to convert a voltage of an internal power supply in a variable manner on the basis of the voltage control value generated in the volume control unit in order to drive the switching power amplifier.