A Li-ion battery charger with dual input source capability and high efficiency is proposed in this work. An internal source determination circuit could choose the sufficient energy source for charging system. Also, a new approach of smooth transition to avoid oscillation between charging mode switching is used through current-mode-like smooth transition circuit. Pulse frequency modulation with adaptive off-time control is applied to the charger to reduce the requirement of compensation design. The charging current is controlled not only by the current loop but also through the ripple reduction circuit to suppress the increasing charging current ripple as the battery voltage rises. The chip was fabricated in TSMC 0.35-μm CMOS process.