Biography
Enrollment Date: 2007
Graduation Date：2010
Degree：M.S.
Defense Date：2010.06.11
Advisors：Zhihua Wang
Department：Institute of Microelectronics，Tsinghua University
Title of Dissertation/Thesis：Research and design of power management system for subthreshold CMOS digital circuit
Abstract:
Aggressive supply voltage scaling to below the device threshold voltage provides significant energy and leakage power reduction in logic circuits. Consequently, it is a compelling strategy for energy-constrained systems, such as wireless sensor networks, where the key metric is energy dissipation rather than high-speed performance. However, the performance of subthreshold digital circuits is very sensitive to temperature and process variations, particularly in deeply scaled technologies. The idea that forms the foundation for this paper is to compensate such sensitivity by providing to the subthreshold circuits a variable supply voltage that is changing dependent on temperature and on the particular occurrence of process parameters. Characteristics of MOS transistors in subthreshold region are analyzed and power management design methods to improve performance of subhreshold logic circuits are summarized in this dissertation. Based on these researches, a DC-DC converter structure with a charge pump and a low drop-out linear regulator is presented, which makes the propagation delay of a subthreshold digital circuit almost insensitive to temperature and process variations with a feed-backward technique. Complete analysis and design steps of the power management system are proposed in this paper, including a low power bandgap reference with high power-supply rejection, a variable step-down conversion ratio Makowski charge pump working in intermittent mode and a LDO regulator using a feedforward ripple-cancellation technique to improve power-supply rejection. In this paper, the DC-DC converter regulated output voltage from 0.3V to 0.6V, in the range from 2.8 to 5V of input voltage, and makes the propagation delay of a subthreshold digital circuit almost insensitive to temperature and process variations. Comparing the oscillation frequency of a ring oscillator both with a constant supply voltage and with our DC-DC converter, when the DC-DC converter is used, the temperature coefficient of the oscillation frequency is 176ppm/℃. In the case of a constant supply voltage, the TC of the frequency is 5~20%/℃, and the variations in different design corner change from 2670% to 26.5%. The efficiency of the DC-DC converter reaches 79% in condition of 700uW load.