An ultra-low power voltage regulator system for wireless sensor networks powered by energy harvesting

Abstract

Student Paper (oral) An Ultra-low Power Voltage Regulator System for Wireless Sensor Networks Powered by Energy Harvesting Chao Wang, Myung Park, Wei Zhao, Guannan Liu, Zeynep Dilli, and Martin Peckerar a Department of Electrical and Computer Engineering, University of Maryland, College Park, MD, USA, Email: chaowang@umd.edu A sensor network is composed of individual sensor nodes that can monitor environmental and physical conditions by communicating with other nodes. Sensor nodes may be powered by harvesting ambient energy. A typical radio frequency (RF) energy harvesting system is shown in Fig. 1 [1]. The rectenna is a hybrid antenna/rectifying diode assembly which captures RF energy. The charge pump boosts the voltage level to charge the battery, and then the voltage is regulated by a DC-DC converter before providing it to the sensor. Because the RF power density is very low (e.g. only a few tens to hundreds of uW/cm), one needs to minimize power consumptions within individual subsystem blocks. A sensor node usually has at least two working modes (active mode and standby mode). In active mode, a node consumes high current to collect data or communicate with other nodes. In standby mode, it consumes little current. When sensor node changes between the two modes, the DC-DC converter senses the load change and through feedback a constant voltage is provided to the load. The commonly used DCDC converter is a low-dropout linear regulator, which is composed of an error amplifier, a pass element and the feedback network [2]. Because of the low input power level, one needs to minimize the quiescent current of the regulator to reduce its power consumption. However, the quiescent current cannot be reduced without bound, otherwise it takes a long time to charge the gate capacitance of the pass element, so the system is slow and may fail to respond to rapid change in load current. Sensor nodes in different operating modes demand different currents. We propose an ultra-low power hybrid regulator system which utilizes sensor mode information. The hybrid regulator system is composed of two switches and two regulators with different amounts of quiescent current consumption. The system block diagram is shown in Fig. 2. The key idea is to switch between the two regulators depending on the operating state of the sensor. When sensor is in active mode, it needs moderate current. In this case, switch SW is closed and regulator I delivers to the sensor the active mode current. At the same time, regulator I consumes a few uA of quiescent current itself. When sensor changes into standby mode, only a small current is needed, so SW is open and SW/ is closed, and regulator II is used to deliver the standby mode current to the sensor. Regulator II only consumes a nA-range quiescent current itself. The average quiescent current ( avg q I , ) of the hybrid regulator system can be expressed as