An Ultra-Low-Power Power Management IC for Wireless Sensor Nodes Michael D. Seeman, Seth R. Sanders, Jan M. Rabaey EECS Department, University of California, Berkeley, CA 94720 {mseeman, sanders, jan} @ eecs.berkeley.edu Abstract– A power interface IC is designed and demonstrated to convert and manage power for a wireless tire pressure sensor node. Power conversion is performed using on-chip switched- capacitor converters with size-optimized devices and level-shifting gate drivers. A synchronous rectifier efficiently harvests energy from an electromagnetic shaker and control circuitry regulates the output voltage while minimizing power consumption. The converters achieve efficiencies approaching 80%. I. I NTRODUCTION Wireless Sensor Nodes (WSNs) are using less power and are becoming smaller as this technology matures. Scavenged- power sensor nodes are now a reality with modern proces- sor, sensor and radio technology [1]. The efficiency of the scavenger-battery-load power interface path, especially at low power, is critical to the performance of such a sensor node. A custom IC is designed in this work to perform scavenger-to- battery and battery-to-load power conversion, while meeting power and size constraints of the system. II. APPLICATION DESCRIPTION This paper describes a power interface integrated circuit for a wireless tire pressure sensor (TPS), running from energy scavenged from a magnetic shaker [1]. The energy consumers, or loads, include a TI MSP430 microcontroller, an Infineon pressure and acceleration sensor, and a custom PicoRadio radio transmitter [2]. The microcontroller and sensor run at a minimum 2.1 V supply and the radio requires a precise 0.65 V supply. A small NiMH coin cell with a nominal capacity of 18 mAh is used as an energy buffer. The electromagnetic shaker utilizes the rotation of the tire to generate energy to power the sensor. WSNs often run at very low duty cycles to minimize power consumption. In the TPS application, tire pressure is measured once every six seconds. Power consumption for a single 14ms measurement/transmission period is shown in fig. 1. Each 14ms measurement/transmission cycle uses approximately 29 μJ, yielding a time-averaged power consumption of 6 μW. Peak powers of several mW are required. The performance of a self-powered WSN is often defined by the sample rate or the number of samples per second the node can acquire and transmit. For a given fixed energy per packet, and a fixed average power supply (defined by the capabilities of the battery or energy scavenger), the sample rate is highly dependent on the efficiency of the power interface circuits. Since WSNs spend the vast majority of time in standby mode, power efficiency at microwatt levels is critical but often lacking Fig. 1. Measurement and transmission power in current solutions. This power interface IC aims to improve this efficiency. III. CONVERTER ARCHITECTURE The architecture of the power interface IC is given in fig. 2. The synchronous rectifier interfaces the electromagnetic shaker (scavenger), which puts out a pulsed waveform, to the battery. Details about its use and implementation are in section VI. Two switched-capacitor power converters convert the battery voltage, nominally 1.2 V, to 2.1 V for the microcontroller and sensors and to 0.7 V to power the radio. The design of the power stages of these converters is detailed in section IV, while the gate drive techniques used are described in section V. A linear regulator is used as a post-regulator to more-precisely set the radio voltage to 0.65 V and to smooth the ripple from the switched-capacitor converter. The design of the linear regulator is not novel, so it will not be described in depth. Fig. 2. Block diagram of the converter IC 567 IEEE 2007 Custom Intergrated Circuits Conference (CICC) 1-4244-1623-X/07/$25.00 ©2007 IEEE TP-04-1