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: www.dlsound.net

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+/NW P+/DNW

Voc 523 mV 508 mV

Isc density 134 A/mm2 52 A/mm2

Power generated/area 557.5 W/mm2 210 W/mm2

P+/NW P+/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)

i

swi

ic

out

outSSL fC

a

i

vR

2, )(

i iriFSL aRR 2

, )(2

22FSLSSLo RRR

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 1st Order DSM Design

Integrator Pre-Amp

+

1-bit DAC

Analoginput

Digitaloutput

-error

Latch

Comparator

18

Proposed 1st Order DSM Design

Low Pass Filter

Pre-Amp

+

1-bit DAC

Analoginput

Digitaloutput

-error

Latch

Removed!

19

Proposed 1st Order DSM Schematic

Attenuates input!Gain <1

Switched-capacitorlow pass filter

1b DAC feedback

Dynamic ComparatorNo pre-amplifier

20

DSM Die Photo and Measured Results1

Technology 180 nm 180 nm

Supply Voltage 1.4 V 1.8 V

Sampling Rate 50 kHz 1.6 MHz

Nyquist Rate 4 kHz 4 kHz

SNDR @ -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

)(tVin )()()( tVtVtV inclkout

)(tVclk

23

Measured DSM Lower Sideband Spectrum

24

Measured DSM SNDR vs. Ripple

Vdd = 1.4VSampling Rate = 50 kHzInput 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

557.5 µW/mm2 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

ripplein Nff

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