Disclosed are a power supply circuit and method for mobile communication terminals, such as PDAs and portable phones. When a main power source such as a battery is abruptly detached from a system, the abrupt detachment of the main power source is detected and the system is stably turned off only with a backup power source. The power supply circuit includes a first switch for providing a driving voltage from the main power source to the system, a second switch for providing a driving voltage from the backup power source to the system when the main power source is detached from the system, a controller for generating a control signal for opening the first switch and closing the second switch when the main power source is detached from the system, and a memory for storing user data.