2398 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 51, NO. 10, OCTOBER 2016 Fully-Integrated High-Conversion-Ratio Dual-Output Voltage Boost Converter With MPPT for Low-Voltage Energy Harvesting Toshihiro Ozaki, Student Member, IEEE, Tetsuya Hirose, Senior Member, IEEE, Hiroki Asano, Student Member, IEEE, Nobutaka Kuroki, Member, IEEE, and Masahiro Numa, Member, IEEE Abstract— This paper proposes a fully-integrated high- conversion-ratio dual-output voltage boost converter (VBC) with maximum power point tracking (MPPT) circuits for low-voltage energy harvesting. The VBC consists of two voltage generators that generate V OUT1 and V OUT2 . V OUT1 and V OUT2 are three and nine times higher than the harvester’s output V IN , respec- tively. V OUT1 is used as a supply voltage for on-chip application circuits while V OUT2 is used as the charging voltage for a Li-ion secondary battery. The VBC achieves a high voltage conversion ratio (max. × 9) and a high power conversion efficiency. The MPPT circuits control the operating frequencies of the CPs to extract maximum power at each output. The measurement results demonstrated that the circuit converted a 0.59 V input to a 1.41 V output with 75.8% efficiency when the output powers of V OUT1 and V OUT2 were 396 and 0 μW, respectively, and a 0.62 V input to a 4.54 V output with 49.1% efficiency when the output powers were 0 and 114 μW, respectively. Index Terms—Charge pump, DC-DC converter, energy har- vesting, power management circuit, switched capacitor, voltage boost converter. I. I NTRODUCTION E NERGY harvesting has attracted much attention as an alternative energy source for next generation power-aware small-sized LSI applications [1], [2]. For such applications, energy storage devices are an indispensable element to store harvested energy because the output power of a small harvester is basically weak and easily lost, depending on the power generation environment. A thin film Li-ion secondary battery is a good candidate for such applications [3], [4]. However, the output voltages of the harvesters are too low to charge the battery (e.g., the output voltage of a single photovoltaic (PV) cell around the maximum power point is 0.5–0.6 V while the voltage required for charging is higher than 4.0 V). Therefore, a power management circuit with a voltage boost converter (VBC) is strongly required. Fig. 1 shows the concept Manuscript received February 12, 2016; revised April 20, 2016; accepted June 6, 2016. Date of publication August 1, 2016; date of current version September 30, 2016. This paper was approved by Guest Editor Makoto Ikeda. This work was partially supported by VLSI Design and Education Center (VDEC), the University of Tokyo in collaboration with Cadence Design Systems, Inc. and Mentor Graphics, Inc., KAKENHI, and the New Energy and Industrial Technology Development Organization (NEDO). The authors are with the Department of Electrical and Elec- tronic Engineering, Kobe University, Kobe 657-8501, Japan (e-mail: tosihiro@cas.eedept.kobe-u.ac.jp; hirose@eedept.kobe-u.ac.jp). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSSC.2016.2582857 Fig. 1. Power management system. of our power management circuit with a VBC using a PV cell and a Li-ion secondary battery. The VBC generates two boosted voltages of V OUT1 and V OUT2 from the output voltage of the PV cell (V IN ). V OUT1 is used as a supply voltage for internal application circuits, and V OUT2 is used as a charging voltage for the Li-ion battery. VBCs for low-voltage and low-power energy harvesting have been proposed [5]–[8]. Although they can convert a low-voltage harvester’s output to a higher output voltage with high power conversion efficiency (PCE), the output voltages of these converters are less than 3.3 V and are still low to charge Li-ion batteries. Li-ion battery chargers have been reported [9], [10]. However, the minimum input voltages are higher than 3 V, so they cannot receive the low-voltage harvester’s output voltage. A VBC for low-voltage-input and high-voltage-output energy harvesting has been proposed [11]. However, the maximum load current is less than several μA and it is difficult to provide medium voltage for on-chip appli- cation circuits. Thus, fully-integrated VBCs that can convert low-voltage harvester’s output to both a medium voltage for application circuits and a high voltage for a battery charger have not been investigated enough. In this paper, we present a fully-integrated high- conversion-ratio dual-output VBC with maximum power point tracking (MPPT) circuits for low-voltage energy harvest- ing [12]. The proposed VBC is based on our previous VBC 0018-9200 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.