coupling a first input terminal for receiving a first external power source to a system power terminal via a first isolation diode, wherein the first external power source is provided by an AC adapter;

coupling a second input terminal for receiving a second external power source to the system power terminal via a second isolation diode connected in series with a current sensing circuit, wherein the second external power source is provided by a USB interface;

coupling a first bypass transistor across the first isolation diode, wherein the first bypass transistor is turned on when the first external power source is selected to provide power to the system power terminal;

coupling a second bypass transistor across the second isolation diode, wherein the second bypass transistor is turned on when the second external power source is selected to provide power to the system power terminal;

coupling an overriding diode between the first input terminal and a control terminal of the second bypass transistor to force the second bypass transistor off and to effectively isolate the USB interface from the system power terminal when the AC adapter is connected to the first input terminal;

coupling an internal battery to the system power terminal via a series-connected regulating transistor, wherein the regulating transistor comprises a first terminal connected to the system power terminal, a second terminal connected to the internal battery, a control terminal, and a configurable body terminal connected to the system power terminal when the system power terminal has a greater voltage than the internal battery and connected to the internal battery when the internal battery has a greater voltage than the system power terminal;

charging the internal battery by linearly regulating the regulating transistor with an adjustable voltage at the control terminal of the regulating transistor to conduct a charging current in a first direction from the system power terminal to a positive terminal of the internal battery during a charging mode; and

using the current sensing circuit to measure current provided by the USB interface and to reduce the charging current in response to an increase in load current by varying the adjustable voltage at the control terminal of the regulating transistor when the measured current from the USB interface exceeds a predefined current threshold, wherein the current sensing circuit does not measure current provided by the AC adapter to reduce the charging current.