We present a modular power management system that can harvest energy from three sources simultaneously, with available power levels of 25 nW to 100 μW, with one inductor. The DC-DC converter is clocked with energize and dump pulses, and the pulse-widths are generated for constant peak inductor current and for no reversal, without current sensing. We use a comparator to reach the open-circuit-voltage (OCV)-based maximum power point (MPP), and train an oscillator to mimic the comparator output. The oscillator frequency is tuned through a successive-approximation algorithm within 11 comparator cycles. The 180 nm chip has a maximum efficiency of 87% at an input available power of 20 μW (input voltage of 0.6 V), and has an output voltage of 1.5 V. © 2016 IEEE.