Abstract: The present invention discloses a power supply system for a portable data processor and a method for controlling the system. The power supply system includes a main DC/DC converter and an auxiliary DC/DC converter for operating in a low power consumption mode, a suspended mode, and a normal mode. Low voltage cut off circuits are provided to disable the main and auxiliary converters if the main and backup batteries are unable to provide a sufficient voltage level. This system offers increased efficiency and prevents the batteries from being overdischarged. Further, the function of turning power on is performed by the user's operation of a suspend/resume switch, and the function of turning power off may be performed by software to reduce the number of power switches.