Patent ID: 8008889

Claim:
A charging circuit for charging a secondary battery by using a first direct current power supply that generates and outputs a first voltage, the charging circuit comprising: a charging circuit unit configured to charge the secondary battery with only the first voltage received from the first direct current power supply; and a power supply switching circuit unit configured to receive the first voltage from the first direct current power supply, a second voltage generated from power supplied from outside, and a secondary battery voltage from the secondary battery, and supply as a power supply a highest voltage among the received voltages to the charging circuit unit, wherein the charging circuit unit operates by using, as the power supply, the highest voltage received from the power supply switching circuit unit; and wherein the charging circuit unit uses as the power supply a third voltage generated by boosting the first voltage to charge the secondary battery with a constant current or a constant voltage; wherein the charging circuit unit includes an inductor including a terminal to which the first voltage is input, a switching element configured to perform a first switching operation according to a first control signal input to a first control electrode thereof, the switching element being connected between another terminal of the inductor and ground voltage, a synchronous rectification switching element configured to perform a second switching operation contrary to the first switching operation performed by the switching element according to a second control signal input to a second control electrode thereof to control a charging current supplied to the secondary battery, the synchronous rectification switching element being connected to the another terminal of the inductor, a current detecting circuit unit configured to detect a current level of the charging current supplied to the secondary battery and generate and output a signal indicating the detected current level, and a control circuit unit configured to control the first switching operation performed by the switching element and the second switching operation performed by the synchronous rectification switching element according to the secondary battery voltage and the signal received from the current detecting circuit unit.