Patent ID: 7679343

Claim:
A power supply system comprising: an external power supply unit generating direct-current output voltage; and an electronic device coupled to the external power supply unit and operable on the output voltage of the external power supply unit; the external power supply unit including a voltage control circuit receiving control current and controlling the output voltage of the external power supply unit in accordance with the control current; the electronic device including: a rechargeable battery generating battery voltage; a system circuit that operates on system voltage generated from at least either one of the output voltage of the external power supply unit and the battery voltage of the rechargeable battery; and a detection circuit detecting the difference between a detection subject, which includes at least one of the battery voltage of the rechargeable battery, an output current of the external power supply unit, and a charging current charging the rechargeable battery, and a reference signal, which is in accordance with the detection subject, and generating the control current in accordance with the detected difference; and wherein the voltage control circuit controls the output voltage of the external power supply unit to be equal to the minimum voltage possible for the external power supply unit to generate when the control current is minimum.