**3. Conclusions**

A charge pump circuit system was presented for energy harvesting based on a flexibletype thermoelectric generator with high-output impedance. Even though the charge pump was operated with a highly resistive TEG, the input voltage could be controlled at a voltage higher than *VDDMIN* by modulating the input impedance of the CP using two-phase operation with low- and high-input impedance modes. The average input impedance could be matched with the output impedance of TEG. The design methodology was proposed to determine the *N* and *C* of the main charge pump when *VOC*, *RTEG*, *VPP*, *f*, and *VDD* MIN were given. The system was fabricated in 65 nm CMOS to demonstrate the functionality of the system with the TEG. Using an equivalent circuit for the TEG, the system was validated with a *VOC* of 0.8 V and an *RTEG* of 400 Ω. *VPP* regulation was successfully observed. The circuit system was also measured with a flexible-type TEG and a thermal source. The system converted thermal energy into power to 30 µW at 2.5 V. By adding a full-bridge rectifier between the energy transducer and the proposed converter, the control circuit would be able to work even with other energy transducers such as piezoelectric or electrostatic vibration energy transducers with an AC equivalent voltage source and high-output impedance.

**Author Contributions:** Conceptualization, T.T.; methodology, K.K. and T.T.; software, K.K.; validation, K.K. and T.T.; formal analysis, K.K. and T.T.; investigation, K.K. and T.T.; writing—original draft preparation, K.K.; writing—review and editing, T.T.; funding acquisition, T.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by Zeon Corp. and Micron Foundation.

**Acknowledgments:** This work was supported by Zeon Corp., VDEC, Synopsys, Inc., Cadence Design Systems, Inc. Rohm Corp., and Micron Foundation. The authors wish to thank M. Futagawa and H. Hirano and S. Ota for technical discussions.

**Conflicts of Interest:** The authors declare no conflict of interest.
