Patent ID: 8350408

Claim:
A power management circuit comprising: a first terminal connected to an external power supply; a second terminal connected to a battery; a selection circuit configured to make a comparison between a voltage supplied from the external power supply via the first terminal and a battery voltage supplied from the battery via the second terminal, and to select one from among the voltage supplied from the external power supply and the battery voltage supplied from the battery; an output terminal configured to output the voltage thus selected by the selection circuit to an external load circuit; and a DC/DC converter charging circuit configured to charge the battery using the voltage supplied from the external power supply, wherein the DC/DC converter charging circuit is a switching regulator which comprises a first transistor provided between the first terminal and a fixed voltage terminal, and wherein the power management circuit further comprises an input current detection unit configured to detect a current that flows through the first transistor, and wherein the input current detection unit comprises: a first resistor one terminal of which is connected to the first terminal, and which is provided in series with the first transistor; a second resistor, a second transistor, and a third resistor arranged in series between the first terminal and the fixed voltage terminal; and an operational amplifier arranged such that the electric potential at the other terminal of the first resistor is input to a non-inverting input terminal thereof, and such that the electric potential at a connection node that connects the second resistor and the second transistor is input to an inverting input terminal thereof, where the operational amplifier outputs a voltage drop that occurs at the third resistor as a signal which corresponds to the current that flows through the first transistor, and wherein the switching operation of the first transistor is controlled according to a signal output from the input current detection unit.