Patent ID: 7759907

Claim:
A battery charger comprising: a first input configured to accept DC power from an AC adapter; a second input configured to accept DC power from a Universal Serial Bus power interface; a first output configured to provide power to a system load; a second output configured for connection to a battery; a first isolation circuit coupled in series between the first input and the first output to prevent current flow in a reverse direction to the first input; a second isolation circuit and a current sensing circuit coupled in series between the second input and the first output, wherein the second isolation circuit is connected in parallel with the first isolation circuit to the first output, and wherein the second isolation circuit prevents current flow in a reverse direction to the second input, and the current sensing circuit measures the current flow from the Universal Serial Bus power interface; a bi-directional pass element coupled between the first output and the second output, wherein the bi-directional pass element conducts a charging current in a first direction to the second output during charging of the battery and conducts a discharging current to a second direction from the second output during discharging of the battery; and a charging controller configured to linearly regulate the charging current of the bi-directional pass element based on the current flow from the Universal Serial Bus power interface measured by the current sensing circuit, wherein the current sensing circuit does not measure current provided by the AC adapter when the AC adapter is connected to the first input terminal.