An electric device that comprises: a constant voltage output unit including a feedback terminal (F.B.) configured for receiving a feedback voltage and an output terminal configured for outputting an output voltage generated on the basis of the received feedback voltage and a predetermined reference voltage; an adaptor and a battery configured to alternatively supply source power to the constant voltage output unit; a feedback circuit connected between the output terminal and the feedback terminal (F.B.) and configured for receiving a voltage proportional to the output voltage and for adjusting the feedback voltage applied to the feedback terminal; a detector configured for outputting an enable signal when the output voltage is equal to or lower than a predetermined critical value; and a controller configured for receiving the outputted enable signal and further configured for controlling the feedback circuit to adjust the feedback voltage; wherein the constant voltage output unit is further configured for outputting a rated voltage in an adaptor mode, when being supplied with a first power from the adaptor, and for outputting a battery mode voltage lower than the rated voltage in a battery mode, when being supplied with a second power from the battery, said battery mode being set by selection of a user or being automatically set in absence of connection between the adaptor and the electric device; wherein the predetermined critical value is set to a value lower than the rated voltage and the battery mode voltage; wherein when the controller receives the outputted enable signal, the controller controls the feedback circuit to adjust the feedback voltage to cause the constant voltage output unit to output the rated voltage as the output voltage from the output terminal. a constant voltage output unit including a feedback terminal (F.B.) configured for receiving a feedback voltage and an output terminal configured for outputting an output voltage generated on the basis of the received feedback voltage and a predetermined reference voltage; an adaptor and a battery configured to alternatively supply source power to the constant voltage output unit; a feedback circuit connected between the output terminal and the feedback terminal (F.B.) and configured for receiving a voltage proportional to the output voltage and for adjusting the feedback voltage applied to the feedback terminal; a detector configured for outputting an enable signal when the output voltage is equal to or lower than a predetermined critical value; and a controller configured for receiving the outputted enable signal and further configured for controlling the feedback circuit to adjust the feedback voltage; wherein the constant voltage output unit is further configured for outputting a rated voltage in an adaptor mode, when being supplied with a first power from the adaptor, and for outputting a battery mode voltage lower than the rated voltage in a battery mode, when being supplied with a second power from the battery, said battery mode being set by selection of a user or being automatically set in absence of connection between the adaptor and the electric device; wherein the predetermined critical value is set to a value lower than the rated voltage and the battery mode voltage; wherein when the controller receives the outputted enable signal, the controller controls the feedback circuit to adjust the feedback voltage to cause the constant voltage output unit to output the rated voltage as the output voltage from the output terminal.