Power Management for Nanopower Sensor Applications

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

2. 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 look at the application

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

4. 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

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

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

7. 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

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

9. 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 Results & Conclusions