1 Abstract— Although microsystems can replenish batteries and energize modules with ambient energy without having to store much energy on board, on-chip photovoltaic cells and thermoelectric generators generate 50–400 mV, which is usually not sufficiently high to operate transistors. Even though stacking cells is possible, the tradeoff in power is ultimately unfavorable because small transducers output little power. Thankfully, transformers can boost millivolt voltages, but not without a significant toll on space. Motion-propelled MEMS switches can also start a harvester, but only in the presence of vibrations. And although 50–300-mV ring- and LC-oscillating networks can charge batteries, initialization requires 1–15 ms. The prototyped 0.18-µm CMOS oscillating starter presented here draws power from 250–450 mV to charge 100 pF to 0.32–1.55 V in 44–92 µs. In steady state, the cost of the starter to the dc-sourced harvester it supports is only a 1.8% drop in power-conversion efficiency. Index Terms—Energy harvester, thermoelectric, photovoltaic, dc- sourced, low-voltage starter, switched-inductor dc–dc converter. I. ENERGIZING WIRELESS MICROSYSTEMS IRELESS microsensors network together to add performance-enhancing and energy-saving intelligence to large, remote, and inaccessible places like factories, hospitals, etc. [1]–[2]. Tiny batteries, however, store insufficient energy to sustain over years the sensor, processor, and transmitter that these devices normally incorporate. This is why research is resorting to ambient sources for help. But since small transducers generate little power intermittently, the role of the harvesting source is to replenish the small on-board battery that powers the system, as Fig. 1 illustrates. Of readily available sources like light, heat, motion, and electromagnetic radiation, sunlight generates the most power at 10–15 mW/cm 3 [3]. And even though artificial lighting and heat output considerably less power at 5–100 µW/cm 3 [3], they are pervasive in consumer applications and mechanical systems. At the millimeter scale, however, photovoltaic (PV) cells produce 300–400 mV and thermoelectric generators (TEGs) output 50–150 mV [4], which are hardly sufficient to operate CMOS transistors. Stacking PV cells is possible, but in the case of CMOS cells and artificial lighting not without a Manuscript received May X, 2014; revised Month X, 2014; and accepted Month X, 2014. Texas Instruments funded this research. The authors are with the School of Electrical and Computer Engineering at the Georgia Institute of Technology, Atlanta, GA 30332-0250 U.S.A. (e-mail: ablanco@gatech.edu, Rincon-Mora@gatech.edu). Copyright © 2014 IEEE. substantial loss in output power [3]. And since microchips only drop 5° to 10° C, on-chip TEGs output less than 150 mV. Fig. 1. Wireless microsystem. With only 50–400 mV at the input and no initial charge in the battery, the conventional charger–supply in Fig. 1 can neither charge the battery nor power the system. This is why Section II of this paper proposes and Section III shows how a prototyped CMOS starter charges a capacitor that supplies start-up energy to the harvester. Section IV then assesses the performance of this technology in light of the applications it supports and the state of the art already reported in literature. Section V ends with a summary of relevant conclusions. II. PROPOSED SWITCHED-INDUCTOR HARVESTER The self-starting harvesting system first proposed in [5] and now prototyped and shown in Fig. 2 uses a slightly modified starter circuit and a 100-pF capacitor C ST to start the system from no-charge conditions. For this, the starter energizes and drains an inductor L X in alternating cycles from the input v H into C ST . When C ST holds enough energy to operate the boost dc–dc converter that the controller and switches S E and S B realize, the controller shuts the starter and commands S E and S B to transfer input power from v H to the battery C BAT . Fig. 2. Proposed self-starting harvester. A. Oscillating CMOS Starter L X and the starter in Fig. 3 comprise an LC oscillator. M SEN is a JFET in [5] and a low-threshold (200 mV) NFET here to remove the need for a JFET. To understand the circuit, consider that, without a harvesting source, all node voltages are 0 V. When v H first rises above M SEN 's threshold voltage, M SEN conducts and v H energizes L X and capacitor C S across t E A 44–93-µs 250–400-mV 0.18-µm CMOS Starter for DC-Sourced Switched-Inductor Energy Harvesters Andrés A. Blanco, Graduate Student Member, IEEE, Gabriel A. Rincón-Mora, Fellow, IEEE W