This paper presents an SC voltage doubler-based voltage regulator for ultra-low power energy harvesting applications. It produces a stable 1.2-V power supply, using inputs from 0.63 to 1.8 V. External compensation and an on-chip output capacitor ensure good performance even with zero load current and any load capacitance. The regulator tolerates arbitrary input ramp-ups, and is immune to blackout and brownout. A stability analysis for the regulator control loop is presented. The regulator ASIC is implemented in a 180 nm CMOS process. The measured regulator peak power and current efficiency are 63 and 49 %, respectively. The performance has been characterized with load currents from zero to (Formula presented.)A.