In many medical monitoring systems, each patient wears several sensors. Those sensor nodes transmit specific physiological signals in a group of small monitoring systems. The sensed data is collected wirelessly by the central health server. Because each sensor node works as a micro system, it must be sustained for a long time without changing the battery. To achieve high conversion efficiency over a wide load range, a battery-free, nano-power buck converter with dynamic on/off time (DOOT) control was recently proposed by Ming-Wei Lee from Taiwan’s Industrial Technology Research Institute (ITRI) together with Chen-Chih Huang and Yin-Hsi Lin from Realtek Semiconductor Corp. and Tzu-Chi Huang, Chun-Yu Hsieh, Yao-Yi Yang, Yu-Huei Lee, Yu-Chai Kang, and Ke-Horng Chen from National Chiao Tung University.

Their DOOT control can predict the on/off time at different input voltages without a power-consuming zero-current-detection (ZCD) circuit. To adapt to the fluctuations in a harvesting system, the proposed α-calibration scheme guarantees accurate ZCD over process, voltage variation, and temperature (PVT) in the DOOT.

For its part, the adaptive-phase-lead (APL) mechanism improves the inherent propagation delay that arises due to a low-power and non-ideal comparator. As a result, load regulation is improved by as much as 30 mV. In addition to consuming low static power, the nano-power bias circuit generates a 1-V reference voltage.

The test chip was implemented in 0.25-μm CMOS with a die area of 0.39 mm2. Experimental results showed 95% peak efficiency, static power of 217 nW, and load regulation of 0.1 mV/mA. See "A Battery-Free 217 nW Static Control Power Buck Converter for Wireless RF Energy Harvesting with α-Calibrated Dynamic On/Off Time and Adaptive Phase Lead Control," IEEE Journal Of Solid-State Circuits, April 2012, p. 852.