This paper presents a monolithic reconfigurable step-down switched-capacitor (SC) power converter for self-powered microsystems. The design features an efficient step-down SC power stage with adaptive gain control and on-chip capacitor sizing. The adaptive gain control helps the converter maintain high efficiency continuously. Meanwhile, the size-adjustable pumping capacitors allow the output voltage to be regulated at different desired levels, even with a constant by 50% duty ratio. The monolithic implementation effectively suppresses the switching noises and glitches caused by parasitic components resulting from traditional bonding, packaging, and PCB wiring. The design has been sent for fabrication with a 180-nm CMOS process. Post-layout fully-transistor based simulations show that the converter is capable of precisely providing a variable power supply V ou t from 0.9 to 1.65 V. It achieves a maximum efficiency of 89%, when V out is regulated at 1.65 V and at a switching frequency of 4 MHz.