This paper presents an energy harvesting system for the indoor light applications. A dual-input dual-output buck-boost converter is adopted as the power stage running in discontinuous conduction mode (DCM) with constant on-time pulse skipping modulation (COT-PSM) control in order to serve a large input voltage range (1.5 to 5.5 V) with minimized power losses. A new maximum power point tracking (MPPT) technique based on the Fractional Open-circuit Voltage (FOCV) method with an adaptive fraction is proposed. The adaptive fraction is obtained through the Perturb & Observe (P&O) method. A fully digitalized MPPT controller is designed without using any current sensor and microcomputer to save power. The proposed MPPT approach has a high adaptability to the different PV cells. The circuits are designed using the TSMC 0.18 μm BCDMOS process