Power Management for Nanopower Sensor Applications Michael Seeman EE 241 Final Project Spring 2005 UC Berkeley

A look at the application Wireless sensor nodes quickly becoming prevalent Energy collected through scavengers –Must convert to useful voltage Examples: –Tire pressure sensor –Wireless sensor networks (Motes & PicoRadio) Low duty cycle, large power range –5 mA active –5 µA standby Ultracapacitor or battery storage

A nanopower converter Switched-Capacitor design enables full integration Efficiency directly linked to charge conservation: Switching frequency controls impedance and power output

High Voltage 0.13 µm CMOS Triple-well 0.13 µm CMOS process –Floating-body NMOS and PMOS Full utilization of switches and capacitors Level-shift circuitry for gate drive signals –Cascode devices to protect local inverters Rajapandian, Shepard. High-Tension Power Delivery: Operating 0.18um CMOS Digital Logic at 5.4V. ISSCC 2005

Clock Generation Same circuitry must work for many power levels (up to 4 decades). Ultra-low power or fast performance depending on load

Subthreshold performance Frequency is linear in supply current but exponential in supply voltage Current supply eases process variation Regulation and supply switching is easier 11-stage ring oscillator

Digital Control Subthreshold, ultra-low power design Variable (8-bit) frequency divider clocks converter High/Low limit comparators control division Level Shifters convert to 1V logic level

Control Simulation ~ 100 kHz Frequency Initial: 50 µA load 150 µA load step at 150 µs.

Results & Conclusions Power Breakdown (standby mode, 3.5V input) –Gate Drive: 311 nW –Oscillator: 80 nW –Digital Control: 30 nW Approximate Standby efficiency: 78% –Linear: 29% Gate power loss can be improved by using smaller power switches for standby loads Transients can be made faster (esp. for standby) by using a linear control method