International Symposium on Low Power Electronics and Design Switched-Capacitor Boost Converter Design and Modeling for Indoor Optical Energy Harvesting with Integrated Photodiodes Stanley W. Hsu, Erin Fong, Vipul Jain, Travis Kleeburg, Rajeevan Amirtharajah University of California, Davis

Introduction/Motivation Integrated Photodiode Switched-Capacitor Boost Converter (SCBC) Delta-Sigma Modulator (DSM) Supply Ripple Effects on DSM Summary 2 Outline

Introduction/Motivation Integrated Photodiode Switched-Capacitor Boost Converter (SCBC) Delta-Sigma Modulator (DSM) Supply Ripple Effects on DSM Summary 3 Outline

4 Introduction Ultra-low voltage sensor circuits powered by free-space optics (Kleeburg, 2010) Integrated photovoltaics for optical power, data, and clock delivery Subcutaneous medical implants Ultra-low duty cycle sensor (Ayazian, 2012) Integrated photovoltaics (2.5 mm x 2.5 mm) Off-chip capacitive and resistive transducers, and electrodes

–Rectified AC mains at 120 Hz 5 Energy Harvesting from Indoor Lighting –Pulse-width-modulated dimming at > 200 Hz Low light intensity limits harvested energy Issue: light flickering Source: ksj.mit.edu Source:

6 Integrated Photodiode Power Electronics Vdd Domain Circuits Light Vdd Indoor Lighting-Powered Sensor Bypass Capacitor Supply ripple Cost, Area/Volume Circuit performance

Introduction/Motivation Integrated Photodiode Switched-Capacitor Boost Converter (SCBC) Delta-Sigma Modulator (DSM) Supply Ripple Effects on DSM Summary 7 Outline

8 Integrated Photodiode Designs P+/NWP+/DNW Voc523 mV508 mV Isc density 134  A/mm 2 52  A/mm 2 Power generated/area  W/mm  W/mm 2 P+/NWP+/DNW

9 Integrated Photodiode Results 3 P+/DNW photodidoes stacked in series (no bypass capacitor) Increasing frequency or duty cycle decreases ripple.

Introduction/Motivation Integrated Photodiode Switched-Capacitor Boost Converter (SCBC) Delta-Sigma Modulator (DSM) Supply Ripple Effects on DSM Summary 10 Outline

Phase 2 – Charge capacitors to VIN Phase 1 – Boost output to 4x VIN 11 Switched-Capacitor Boost Converter S4

12 Buck Converter Model Fast Switching Limit: Slow Switching Limit : (Seeman, 2008) Combined Output Impedance:

13 Proposed Boost Converter Model Model accounts for bottom plate parasitic effects and allows cascading of multiple stages N=4

14 SCBC Output vs. Switching Frequency Model is accurate to within 10%, from 500 Hz to 5 MHz

SCBC Simulated Ripple to Output Ratio 15

Introduction/Motivation Integrated Photodiode Switched-Capacitor Boost Converter (SCBC) Delta-Sigma Modulator (DSM) Supply Ripple Effects on DSM Summary 16 Outline

17 Conventional 1 st Order DSM Design Integrator Pre- Amp + 1-bit DAC Analog input Digital output - error Latch Comparator

18 Proposed 1 st Order DSM Design Low Pass Filter Pre- Amp + 1-bit DAC Analog input Digital output - error Latch Removed!

19 Proposed 1 st Order DSM Schematic Attenuates input! Gain <1 Switched-capacitor low pass filter 1b DAC feedback Dynamic Comparator No pre-amplifier

20 DSM Die Photo and Measured Results 1 Technology180 nm Supply Voltage1.4 V1.8 V Sampling Rate50 kHz1.6 MHz Nyquist Rate4 kHz -7dBFS input ~27 dB~50dB SNDR ~27 dB

Introduction/Motivation Integrated Photodiode Switched-Capacitor Boost Converter (SCBC) Delta-Sigma Modulator (DSM) Supply Ripple Effects on DSM Summary 21 Outline

Sampling switch behaves as passive mixer (Cook, 2006) Distortion due to passive mixing –Sampling switch Mixing between input and ripple –1b DAC feedback switch Mixing between ripple and itself 22 Supply Ripple Effects on DSM

23 Measured DSM Lower Sideband Spectrum

24 Measured DSM SNDR vs. Ripple Vdd = 1.4V Sampling Rate = 50 kHz Input Amplitude = -7dBVdd ~4.5 bits ~2 bits

Introduction/Motivation Integrated Photodiode Switched-Capacitor Boost Converter (SCBC) Delta-Sigma Modulator (DSM) Supply Ripple Effects on DSM Summary 25 Outline

P+/NW integrated photodiodes achieves µW/mm 2 with Voc=523 mV Switched-capacitor boost converter model for predicting output voltage to within 10% accuracy from 500Hz to 5 MHz Supply ripple effects on passive delta-sigma modulator results in IM2 distortion at 26 Summary

27 Integrated Photodiode Power Electronics Vdd Domain Circuits Light Vdd Summary Bypass Capacitor  If DSM can tolerate an increased supply ripple from 10% to 21% of Vdd, bypass capacitor can be reduced from 56.5 nF to 3.86 nF.

Texas Instruments for chip fabrication –William McIntyre –Arun Rao –Keith Schoendoerfer –Greg Winter –Bijoy Chatterjee 28 Acknowledgements