LTJournal-V21N1-2011-04.pdf

April 2011

Volume 21 Number 1

IN THIS ISSUE

Harvest Energy from a

Single Photovoltaic Cell

Nathan Bourgoine

dual input/output 3A

monolithic buck with

3V–36V input range 10

how to drive low power,

1Msps, 16-bit, differential

input ADC from single-

ended signals 30

To simplify the distribution of wireless communications for

instrumentation, monitoring and control applications, power supply

designers strive for device grid-independence. Batteries, the

immediately obvious solution, offer the illusion of grid independence,

but require replacement or recharging, which means eventual

connection to the grid and expensive human intervention and

maintenance. Enter energy harvesting, where energy is collected

from the instrument’s immediate environment, offering perpetual

operation with no connection to the grid and minimal or no

maintenance requirements.

maximize output power

from current-limited USB

and PCMCIA sources 38

dual output step-down

controller converts 60V

directly to 3.3V 40

POWER

SUPPLY

CONNECTOR

DROPS

WIRING DROPS CONNECTOR

DROPS

LOAD

A variety of ambient energy sources can be harvested to produce electri-

cal power , including mechanical vibration , temperature differential and

incident light. L i near Technology produces power management solutions

that solve the problems speciic to harvesting ambient low energy sources ,

including the LTC ® 3588 for vibration sources , the LTC3 1 08/LTC3 1 09 for

thermal , and now the LTC3 1 05 for photovoltaic energy harvesting applica-

tions. Photovoltaic energy harvesting is widely applicable , given that light

is almost universally available , photovoltaic (PV) cells are relatively low

cost and they produce relatively high power compared to other ambient

energy harvesting solutions. Because of its relatively high energy output ,

photovoltaic energy harvesting can be used to power wireless sensor

nodes , as well as higher power battery charging applications to extend

battery life , in some cases eliminating tethered charging altogether.

CONNECTOR

DROPS

WIRING DROPS

CONNECTOR

DROPS

Figure 1. The simplest model for load regulation over

resistive interconnections.

Caption

While high voltage stacks of series-connected photovoltaic cells are proliic ,

single PV-cell solutions are rare , due to the dificulty of generating useful

power rails from the low voltage produced by a single PV cell under load.

Few boost converters can produce outputs from a low voltage , relatively

(continued on page 2)

www.linear.com

…continued from the cover

In this issue...

COVER STORY

The LTC3105 enables autonomous remote sensor nodes,

data collection systems and other applications that

require grid independence and minimal maintenance.

Harvest Energy from a Single Photovoltaic Cell

Nathan Bourgoine

DESIGN FEATURES

Protect Mobile Devices from Hot Plug

Transients (to 85V) and from Users Who

Use the Wrong Power Adapter

Kevin Wong

( LTC 3105, continued from page 1)

high impedance single PV cell. The LTC3 1 05 ,

however , is designed speciically to meet these

challenges. Its ultralow 250 m V start-up volt-

age and programmable maximum power

point control allow it to generate the typi-

cal voltage rails ( 1 .8V–5V) required for most

applications from challenging PV sources.

V CELL

Monolithic, Dual 3A Input/Output Buck with

3V–36V Operating Range Simpliies and

Shrinks DC/DC Converters in Automotive,

Industrial and Distributed Power Applications

Jonathan Paolucci

–

Figure 1. Simple photovoltaic cell model

3A Output, 96% Eficient Buck-Boost

DC/DC Converter Sets the Standard for

Power Density and Noise Performance

Richard Cook

UNDERSTANDING PHOTOVOLTAIC CELL SOURCES

Photovoltaic sources can be electrically modeled by a current source con-

nected in parallel with a diode as shown in Figure 1 . More complex models

show secondary effects , but for our purposes this model is suficient.

Intermediate Bus Buck Regulator Maintains 5V Gate

Drive During Automobile Cold Crank Conditions

Theo Phillips and Tick Houk

Two common parameters that characterize a PV cell are the open cir-

cuit voltage and the short-circuit current. Typical curves for PV cell cur-

rent and voltage are shown in Figure 2. Note that the short-circuit current

is the output of the model’s current generator while the open circuit volt-

age is the forward voltage of the model’s diode. A s light levels increase ,

the current from the generator increases and the IV curves move up.

Low I Q , Triple Output Boost/Buck/Buck

Synchronous Controller Keeps Electronics

Running Through Battery Transients in Automotive

Start-Stop and Always-On Systems

Joe Panganiban and Jason Leonard

DESIGN IDEAS

How to Drive Low Power, 1Msps, ±2.5V

Differential-Input, 16-Bit ADC with a

Variety of Single-Ended Signals

Guy Hoover

To extract maximum power from the PV cell , the input resistance of the power

converter must be matched to the output resistance of the cell , resulting in opera-

tion at the maximum power point. Figure 3 shows the power curve for a typical

single photovoltaic cell. To ensure maximum power extraction , the output voltage

of the PV cell should be operated at the peak of the power curve. The LTC3 1 05

adjusts the output current delivered to the load in order to maintain the PV cell

Easy, Isolated Low Power Telecom Supply:

No Opto-Isolator Required

Mayur Kenia

4mm × 5mm, Dual Input/Output,

Synchronous Monolithic Buck Regulator

Converts 12V to 1.2V at 4MHz

Phil Juang

(continued on page 4)

Figure 2. Typical photovoltaic cell IV curve

Figure 3. Typical photovoltaic cell power curve

Isolated Flyback Converters Eliminate Opto-Coupler

Yat Tam

250

BRIGHTER

200

Buck-Boost Converter with Accurate Input

Current Limit Maximizes Power Utilization

from USB and PCMCIA Sources

Michael Munroe

150

100

Low I Q , Dual Output Step-Down Controller

Converts 60V Directly to 3.3V

Jason Leonard and Joe Panganiban

DIMMER

product briefs

0.1 0.2 0.3 0.4 0.5

0.6

0.1 0.2 0.3 0.4 0.5

0.6

back page circuits

CELL VOLTAGE (V)

2 × 1 INCH POLYCRYSTALLINE CELL

2 | April 2011 : LT Journal of Analog Innovation

Linear in the news

Linear in the News

JAPAN PRESS CONFERENCE

On December 1 7 , L i near Technology held

a major press conference in Japan to give

27 assembled members of the press an

overview of L i near’s strategy. At the meet-

ing , Executive Chairman Bob Swanson

presented an overview of the company’s

repositioning to focus primarily on three

major market segments—industrial

(including medical , security , factory auto-

mation , instrumentation and industrial

control) , communications infrastructure

(including cellular base stations to support

worldwide growth in wireless networks ,

as well as networking) , and automo-

tive electronics (including battery stack

monitors for hybrid / electric vehicles and

LED lighting systems). CEO Lothar Maier

further elaborated on L i near’s strategy

and product focus in Japan and globally.

EDN Innovation Award Finalists. EDN magazine

has announced inalists for their annual

Innovation Awards , to be presented

on May 2. L i near inalists include:

EDN Hot 100 Products. EDN magazine

highlighted the Hot 1 00 Products

of 20 1 0 , including:

•LT6656 low power voltage reference

•Innovator of the Year: Robert Dobkin

and Tom Hack for the LT ® 4 1 80

Virtual Remote Sense ™ Controller.

•LT4 1 80 Virtual Remote Sense Controller

•LTC3 1 08 energy harvesting

boost regulator

•Power IC s : LT4 1 80 Virtual

Remote Sense Controller

CONFERENCES & EVENTS

EDN Designing with LEDs Conference & Workshop,

San Jose Marriott Hotel, San Jose, California,

May 4. L i near will showcase LED driver

solutions at the booth and Design

Manager Bryan Legates will con-

duct a workshop on LED drivers. Info

at ubmelectronicsvirtualconferences.

businesscatalyst.com / LED / index.html

•Analog IC s : TimerBlox ® IC family

You can read more about the inal-

ists at innovation.edn.com / inalists .

Energy Harvesting Product Awards. Two

UK publications presented L i near

Technology with awards for energy

harvesting products. Electronics Weekly

featured the LTC3 1 09 auto-polarity

energy harvesting power supply as win-

ner of the Renewable Energy Design

Award in their coverage of the Elektra

20 1 0 Awards. In addition , Electronic

Product Design featured the LTC3 1 09

as winner of the Alternative Energy

Award in their 20 1 0 e-Legacy Awards.

Energy Eficiency & Technology Conference,

Marriott Santa Clara, Santa Clara, California, May

6. L i near will highlight its energy harvest-

ing solutions at the booth , and provide a

speaker at the conference. Info: eetweb.

com / sponsor / energy-eficiency-technology/

The press conference was covered by

several major Japanese publications ,

including Dempa Shimbun , EDN Japan ,

EE Times Japan , Electronic Journal and

Nikkei Electronics , among others.

Electronic Sourcing Live Exhibition, Regency Park

Hotel, Newbury, UK, May 12. L i near will have a

booth promoting µModule power solu-

tions and L i near Express ® product fulill-

ment. Info: www.es-live.co.uk / index.html

LINEAR RECEIVES

PRODUCT AWARDS

Linear continues to receive numer-

ous awards for its innovative , high

performance products. The awards

are determined by independent

groups of technical editors , represent-

ing various print and online publica-

tions around the globe. Following

are a few recent award highlights:

EN-Genius Network Awards. Online publica-

tion , En-Genius Network presented

20 1 0 Product of the Year Awards to:

•Best RF Detector: LTC5583

Dual RMS RF Detector

Sensors Expo, Donald E. Stephens Convention

Center, Rosemont, Illinois, June 6-8. L i near will

showcase energy harvesting solutions at

its booth , and designer Jim Noon will give

a presentation on energy harvesting. Info:

www.sensorsmag.com / sensors-expo n

•Best Advance in Remote Load

Power Sensing: LT4 1 80 Virtual

Remote Sense Controller

•Best Ultralow Input Voltage

DC / DC Power Module: LTM ® 46 11

1 .5V Input 1 5A µModule ® Controller

April 2011 : LT Journal of Analog Innovation | 3

While high voltage stacks of series-connected photovoltaic

cells are proliic, single PV-cell solutions are rare, due

to the dificulty of generating useful power rails from the

low voltage produced by a single PV cell under load.

( LTC 3105, continued from page 2)

LTC3105

voltage at the voltage set by the maxi-

mum power point control pin. Therefore ,

a single programming resistor establishes

the maximum power point and ensures

maximum power extraction from the

PV cell and peak output charging current.

BRIGHTER

BOOST CONVERTER

I PEAK

V CC

10µA

–

MPPC VOLTAGE

MPPC

–

g m

R MPPC

V IN

DIMMER

HOW MUCH POWER IS AVAILABLE?

The amount of power that can be gener-

ated using a photovoltaic cell depends on

a number of factors. The output power of

the cell is proportional to the brightness of

the light landing on the cell , the total area

of the cell , and the eficiency of the cell.

Most PV cells are rated for use under full

direct sunlight ( 1 000W / m 2 ) , but such ideal

conditions are unlikely to occur in most

applications. For devices operating from

sunlight , the peak power available from

the cell can easily change by a factor of ten

from day to day due to weather , season ,

haze , dust , and incident angle of the sun-

light. Typical output power for a crystal-

line cell in full sunlight is about 40 m W per

square inch depending on cell characteris-

tics. A PV cell with an area of a few square

0.1

0.2

0.3

0.4

0.5

0.6

CELL VOLTAGE (V)

2 × 1 INCH POLYCRYSTALLINE CELL

Figure 4. Maximum power point control mechanism

Figure 5. Err on the side of a lower voltage when

choosing a maximum power point voltage to avoid

the steep drop-off

inches is suficient to run many remote

sensors and to trickle charge a battery.

presents design challenges. Even a large

high eficiency crystalline cell with an

area of four square inches generates

only 860 µ W in typical ofice lighting.

In contrast , devices operating from indoor

lighting have far less energy available

to them. Common indoor lighting is

roughly 0.25% as strong as full sunlight

(the huge difference in intensity between

indoor lighting and sunlight is hard to

perceive due to the human eye’s ability

to adjust to a wide range of illumina-

tion levels). The dramatically lower light

levels available to indoor applications

CHOOSING THE MAXIMUM

POWER POINT CONTROL VOLTAGE

Figure 4 shows a model of the maxi-

mum power point control mechanism

used by the LTC3 1 05. Figure 3 shows

the power curve for a PV cell. Note that

PV cell power declines sharply from its

peak as the cell voltage rises away from

peak power. It is thus generally more

desirable to err on the side of a lower-

than-ideal control voltage , rather than a

higher voltage , because the power curve

rolls off more sharply on the high side.

Figure 6. Li-ion charging circuit

10µH

L1: COILCRAFT MSS5131-103MX

225mV TO 500mV

V IN

V OUT

–

PHOTOVOLTAIC

CELL

1 0µF

V OUT

4.1V

When selecting the MPPC tracking volt-

age , various operating conditions must be

considered. Typically , the maximum power

point does not move substantially with

changes in illumination. A s a result , it is

LTC3105

1020k

PGOOD

LDO

FBLDO

Li-ION

MPPC

332k

OFF

SHDN

AUX

2.2V

1 0µF

40.2k

1 µF

GND

4.7µF

4 | April 2011 : LT Journal of Analog Innovation

design features

The LTC3105’s integrated maximum power point control and low voltage start-up

functionality enable direct operation from a single PV cell and ensure optimal energy

extraction. The LTC3105 can be used to directly power circuitry or for charging

energy storage devices to allow operation through dark or low light periods.

LI-ION BATTERY CHARGING

IN OUTDOOR LIGHTING

One of the challenges faced by applica-

tions using a photovoltaic source is the

lack of input power during darkness and

low light conditions. For most applica-

tions this necessitates use of energy

storage elements such as a supercapaci-

tor or rechargeable battery that is large

enough to provide power throughout

the longest expected dark period.

CHOOSING THE RIGHT

ENERGY STORAGE DEVICE

There are many alternatives for storing

harvested energy , including a wide variety

of rechargeable battery technologies and

high energy density capacitors. No one

technology is perfect for all applications.

When selecting the storage element for

your application , consider a number of

factors , including the self-discharge rate ,

maximum charge and discharge current ,

voltage sensitivity , and cycle lifetime.

SUNNY DAY

BUILDING

SHADOW

–1

RAINY OVERCAST DAY

7:00

10:00

13:00

16:00

19:00

TIME OF DAY

2 × 1 INCH POLYCRYSTALLINE CELL

Figure 7 shows the measured charging

current proile using a 2” × 1 ” polycrys-

talline PV cell to charge a L i -ion bat-

tery using the LTC3 1 05 circuit shown in

Figure 6. The upper curve of Figure 7

shows the charging current on a typical

clear day with full sun. The lower curve

shows the charging current observed

over the course of a heavily overcast day.

Even under these low light conditions a

charging current of 250 µ A or more was

maintained throughout the day totaling

6 m A h of charge delivered to the battery.

Figure 7. Charging proiles for two square inch

photovoltaic cell

The self-discharge rate is particularly

important in photovoltaic applications.

Given the limited amount of charging

current available in most photovoltaic

power applications , a high self-discharge

rate may consume a large portion of the

available energy from the PV source. Some

energy storage elements , such as large

supercapacitors , may have self-discharge

current in excess of 1 00 µ A , which could

dramatically reduce the net charge

accumulated over a daily charge cycle.

possible to choose a single tracking voltage

that provides operation near the maximum

power point for a wide range of illumina-

tion levels. Even though the operating

point will not be precisely at the maxi-

mum power point at extreme levels of illu-

mination , the reduction in output power

from the ideal is usually only 5%– 1 0%.

For the power curve shown in Figure 5 , an

MPPC voltage of 0.4V yields performance

near the maximum power point at either

illumination extreme. The voltage dif-

ference from the maximum power point

is approximately 20 m V in both cases ,

resulting in a power loss of less than 3%.

Figure 8. A Li-ion trickle charger operates from a single photovoltaic cell

10µH

L1: COILCRAFT MSS5131-103MX

225mV TO 500mV

A s a rule of thumb , the maximum power

point control voltage should be around

75%–80% of the open circuit voltage for

the PV cell. Tracking the cell to this volt-

age results in a cell output current that is

75%–80% of the short-circuit current.

–

V IN

V OUT

PHOTOVOLTAIC

CELL

1 0µF

V OUT

LTC3105

1020k

V CC

ADJ

PGOOD

LDO

FBLDO

LTC4071

BAT

MPPC

332k

NTC

OFF

SHDN

AUX

2.2V

1 0µF

40.2k

Li-ION

LBSEL

GND

1 µF

GND

4 .7µF

April 2011 : LT Journal of Analog Innovation | 5

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