LTMag_V18N2_Jun08.pdf

LINEAR TECHNOLOGY

June 2008

VOLuMe XVIII nuMBeR 2

IN THIS ISSUE…

Speed Up Li-ion Battery

Charging and Reduce

Heat with a Switching

PowerPath Manager

by Steven Martin

COVER ARTICLE

Speed Up Li-ion Battery Charging

and Reduce Heat with a Switching

PowerPath™ Manager..........................1

Steven Martin

Linear in the News… ...........................2

DESIGN FEATURES

Hot Swap™ Controller Enables

Standard Power Supplies to

Share Load ........................................6

Vladimir Ostrerov

Introduction

Designers of handheld products race

to pack as many “cool” features as

possible into ever smaller devices. Big,

bright color displays, Wi-Fi, WiMax,

Bluetooth, GPS, cameras, phones,

touch screens, movie players, music

players and radios are just a few of

the features common in today’s bat-

tery powered portable devices. One

big problem with packing so many

features into such a small space is

that the “cool” product must actually

stay cool while in use. Minimizing dis-

sipated heat is a priority in handhelds,

and a signiicant source of heat is the

battery charger.

One component of handhelds has

changed little over the years—the

Li-ion battery. While the capacities

of today’s batteries have increased

from a few hundred milliampere hours

to several ampere-hours to accom-

modate the ever expanding feature

set of modern portable products, the

basic Li-Ion battery technology has

remained unchanged. Why has Li-ion

survived so long? unmatched energy

density (both by mass and volume),

high voltage, low self-discharge, wide

usable temperature range, no memory

effect, no cell reversal, no cell balanc-

ing, and low environmental impact all

make the Li-Ion battery the preferred

1.8

1.6

LINEAR BATTERY CHARGER

1.4

Understanding IP2 and IP3 Issues in

Direct Conversion Receivers for

WCDMA Wide Area Basestations .......10

Doug Stuetzle

1.2

EN ADDITIONAL

POWER AVAILABLE

FOR CHARGING

1.0

0.8

Complete Dual-Channel Receiver

Combines 14-Bit, 125Msps ADCs,

Fixed-Gain Ampliiers and Antialias

Filters in a Single 11.25mm × 15mm

μModule™ Package ...........................14

Todd nelson

0.6

SWITCHING BATTERY CHARGER

0.4

0.2

V IN = 5V

I CHARGE = 1A

3.3

3.4 3.5 3.6 3.7 3.8 3.9 4.0

4.1

BATTERY VOLTAGE (V)

Battery Manager Enables Integrated,

Eficient, Scalable and Testable

Backup Power Systems......................17

Mark Gurries

Figure 1. Reduce battery charge time and keep

handheld devices cool by using a switching

PowerPath manager/battery charger.

500nA Supply Supervisors Extend

Battery Life in Portable Electronics ..22

Bob Jurgilewicz

choice for high performance portable

products.

Charging today’s big batteries,

however, is no small deal. In order to

charge them in a reasonable amount

of time, they should be charged at a

rate commensurate with their capacity

and with a speciic algorithm. For ex-

ample, to fully charge a 1Ah battery in

approximately one hour requires one

amp of charge current. If uSB powered

charging is desired, then only 500mA

of current is available, doubling the

charge time to two hours.

Another problem with higher charge

currents is the additional heat lost in

Quad Output Regulator Meets Varied

Demands of Multiple Power Supplies

.........................................................25

Michael nootbaar

Buck, Boost and LDO Regulators

Combined in a 4mm × 4mm QFN........28

Chris Falvey

DESIGN IDEAS

....................................................30–41

(complete list on page 30)

New Device Cameos ...........................42

Design Tools......................................43

Sales Ofices .....................................44

continued on page

L , LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology

Corporation. Adaptive Power, Bat-Track, BodeCAD, C-Load, DirectSense, easy Drive, FilterCAD, Hot Swap, LinearView,

µModule, Micropower SwitcherCAD, Multimode Dimming, no Latency ΔΣ, no Latency Delta-Sigma, no R SenSe , Operational

Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, True Color PWM,

ultraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the

companies that manufacture the products.

LINEAR TECHNOLOGY

L LINEAR IN THE NEWS

Linear in the News…

EDN Innovation Award for

Linear Power Device

EDN magazine announced the selection of Linear’s LT3080

3-terminal parallelable low dropout linear regulator as

EDN’s Innovation of the Year in the Power ICs category.

The award was presented at the annual EDN Innovation

Awards ceremony to Linear Technology Vice President

engineering and Chief Technical Oficer Robert Dobkin and

Design engineer Todd Owen, who developed the product.

Other Linear Technology inalists included the LTC6102

current sense ampliier in the Analog ICs category, Robert

Dobkin for Innovator of the Year, and Jim Williams’ article,

“Designing instrumentation circuitry with RMS/DC con-

verters” for Best Contributed Article.

According to Ron Wilson, executive Director of EDN

Worldwide, “Selected by their peers in the design com-

munity for their outstanding results, these innovators

stand in the front rank of the best and brightest electronics

engineering has to offer.”

Robert Dobkin stated, “The LT3080 solves two dificult

problems for linear regulators: spreading heat to eliminate

heat sinks and increasing output current by simply adding

additional devices. The circuit architecture is completely

new and just as easy to use as older devices. I am proud

to introduce this product.”

The LT3080 is a 1.1A 3-terminal linear regulator that can

easily be paralleled for heat spreading and higher output

current, and is adjustable to zero with a single resistor.

This is a new architecture for regulators and uses a current

reference and voltage follower to allow sharing between

multiple regulators, enabling multiamp linear regulation

in all surface-mount systems without heat sinks.

The LT3080 has a wide input voltage capability of 1.2V

to 40V, a dropout voltage of only 300mV and millivolt

regulation. The output voltage is adjustable, spanning

a wide range from 0V to 40V, and the on-chip trimmed

reference achieves high accuracy of ±1%.

Hot Products in Asia

EDN Asia magazine recently announced their list of the

100 Hot Products of 2007. Included in the list are three

Linear Technology products:

q LTC6400 Low noise, Low Distortion ADC Driver

q LT4356 Overvoltage Protection Regulator

q LT3080 3-Terminal Linear Regulator

For more information, visit www.linear.com.

Editor’s Choice Award

Portable Design magazine recently announced their

selections for their annual editor’s Choice Awards. In the

RF/Microwave category, the award went to the LT5575

Direct Conversion I/Q Demodulator. The LT5575, which

was also selected by Electronic Products magazine as

Product of the Year, signiicantly reduces the cost of 3G

and WiMAX basestation receivers. The LT5575’s extended

operating frequency range from 800MHz to 2.7GHz covers

all of the cellular and 3G infrastructure, WiMAX and RFID

bands with a single part. Its capability to convert from

RF directly to baseband at DC or low frequency results

in simpliied receiver designs, reduced component count

and use of lower cost, low frequency components.

The LT5575 offers an outstanding IIP3 of 28dBm and

IIP2 of 54.1dBm at 900MHz, and an IIP3 of 22.6dBm

and IIP2 of 60dBm at 1.9GHz. Moreover, the device has

a conversion gain of 3dB, which when combined with a

DSB noise Figure of 12.7dB, produces excellent receiver

dynamic range. The device’s I (In-phase) and Q (Quadra-

ture phase) outputs have typical amplitude and phase

matching of 0.04dB and 0.6°, respectively, providing an

unprecedented level of demodulation accuracy.

The LT5575 is capable of supporting multiband

basestations covering both the 850MHz GSM/eDGe bands

and the 1.9GHz/2.1GHz 3G wireless services (including

CDMA2000, WCDMA, uMTS, and TD-SCDMA). It is ideal

for single carrier micro- and pico-basestations, where low

cost architectures are key. The LT5575’s performance is

also well suited for 2.6GHz WiMAX basestations, and as

an IF demodulator in a microwave radio link or satellite

receiver. L

Linear Technology Magazine • June 2008

DESIGN FEATURES L

LTC4088/LTC4098, continued from page

the charging process. Since charge

power for these devices usually comes

from a 5V source, such as a uSB port

or 5V wall adapter, power loss can be

signiicant. Assuming a healthy Li-

Ion battery spends signiicant time

at its “happy voltage” of 3.7V during

charging, then charging eficiency via

a linear charging element can at best

be 3.7V/5V or 74%. When the battery

voltage is less than 3.7V, losses are

even worse. even at the maximum

loat voltage of 4.2V, where the bat-

tery spends about 1/3 of the charge

time, charging eficiency can’t be better

than 84%.

With a 1Ah battery charged at a

“1C” rate, we can expect about 1.3W of

power to be lost while delivering 3.7W

to the battery over the longest part of

the charge cycle. note, however, that

the energy delivered to the battery

doesn’t result in any signiicant tem-

perature rise as the battery is storing

the energy for future use. This means

that the predominant source of heat

during charging is generated by the

charger itself. With this in mind, at

a given power level it makes sense to

move to a switching battery charger

for improved charging eficiency, less

charger generated heat and reduced

charge time.

Both the LTC4088 and LTC4098

are examples of single-cell Li-ion bat-

tery chargers from Linear Technology

that not only offer the high eficiency

of a switching battery charger but

also include PowerPath technology.

PowerPath control is a technique

that uses a third, or intermediate,

node to allow instant-on operation,

which provides power to the system

when the battery voltage is below the

system cutoff. Only products like the

LTC4088 and LTC4098 combine a step

down DC/DC switching regulator with

a linear battery charger in a unique

way that ensures high eficiency power

delivery to both the system load and

the battery. Before we delve into these

parts, let’s take a look at how it was

done before.

V OuT is extremely eficient since there

is no signiicant voltage drop across

the pass element. note, however, that

the voltage drop between V OuT (~5V)

and V BAT (say 3.5V) means the linear

charger is running ineficiently. Thus,

power delivered to the load arrives ef-

iciently while power delivered to the

battery arrives ineficiently.

now take the alternate case where

the load current exceeds the input

current limit setting. Here the input

current limit control engages and the

voltage at the intermediate node, V OuT ,

drops to just under the battery volt-

age, thus bringing in the battery as a

source of additional current. Although

this is desired behavior, ensuring

load current is prioritized over charge

current, notice that there is now inef-

iciency at the pass element because

a large voltage difference does exist

between the input pin, again at 5V,

and the output pin, which now may

be about 3.5V.

From these examples we can see

that while a linear PowerPath topology

performs the necessary PowerPath

control functions under all conditions,

it has some inherent ineficiencies.

Speciically, with the linear PowerPath

topology there is likely to be power

wasted in one or the other of the two

linear pass elements under various

conditions. In the next section we’ll see

how a switching PowerPath avoids the

pitfalls of the linear PowerPath.

Old School: Linear PowerPath

The intermediate node topology isn’t

new. Figure 2 shows an example of

a linear PowerPath topology. In this

architecture, a current limited switch

delivers power from an input connector

to both the external load and linear

battery charger. The linear battery

charger then delivers power from the

intermediate node to the battery.

If the load current is far enough

below the input current limit to allow

some current to be directed to battery

charging, the voltage at V OuT is nearly

equal to the input supply voltage, let’s

say 5V. In this case, the path from V In to

TO USB

OR WALL

ADAPTER

V IN

V OUT

TO SYSTEM

LOAD

New School: High Eficiency

with Switching PowerPath

Figure 3 shows an alternative to

the linear PowerPath, a switching

PowerPath. Here a step-down DC/DC

converter delivers power from the

input connector to the intermediate

node V OuT . A linear battery charger is

connected from the intermediate node

to the battery as in the case of the

linear PowerPath. The big difference

from linear PowerPath is that the path

from V In to V OuT maintains relatively

high eficiency regardless of the volt-

age difference since it is a switching,

rather than a linear, path.

Then what about the linear battery

charging path, the other big part of

I SWITCH /

h CLPROG

IDEAL

DIODE

CONSTANT CURRENT

CONSTANT VOLTAGE

BATTERY CHARGER

15mV

–

CLPROG

BAT

SINGLE CELL

Li-Ion

AVERAGE INPUT

CURRENT LIMIT

CONTROLLER

Figure 2. Block diagram of a traditional linear PowerPath,

which has signiicant inherent eficiency limitations.

Linear Technology Magazine • June 2008

L DESIGN FEATURES

TO USB

OR WALL

ADAPTER

V BUS

3.3µH

3.5V TO

(BAT + 0.3V)

TO SYSTEM

LOAD

10µF

V OUT

1 0µF

PWM AND

GATE DRIVE

I SWITCH /

h CLPROG

IDEAL

DIODE

–

Si2333DS

OPTIONAL

EXTERNAL

IDEAL DIODE

PMOS

CONSTANT CURRENT

CONSTANT VOLTAGE

BATTERY CHARGER

GATE

15mV

–

8.2

CLPROG

0.3V

–

+ –

BAT

0.1µF

2.94k

1.188V

3.6V

AVERAGE INPUT

CURRENT LIMIT

CONTROLLER

AVERAGE OUTPUT

VOLTAGE LIMIT

CONTROLLER

SINGLE CELL

Li-Ion

LTC4088

Figure 3. Switching PowerPath block diagram. The big advantage of a switching PowerPath scheme over a linear

PowerPath is that the path from V IN to V OUT maintains relatively high eficiency regardless of the V IN /V BAT ratio.

the total eficiency picture? Voltage

drops between V OuT and the battery

would pretty much erase the eficiency

gains made by the switching regulator.

Total eficiency remains high with the

the LTC4088 and LTC4098 because

of a feature called Bat-Track™. With

Bat-Track, the output voltage of the

switching regulator is programmed to

track the battery voltage plus a few

hundred millivolt difference. Since

the output voltage is never signii-

cantly above the battery voltage, little

power is ever lost to the linear battery

charger. The battery charger pass ele-

ment leaves most of the voltage control

duties to the switching regulator and

exists merely to control charge current,

loat voltage and battery safety moni-

toring—tasks at which it excels.

support a higher power 1A setting for

wall adapter applications.

For products with large batteries,

uSB current control can be the limit-

ing factor in determining how much

power is delivered to the battery for

charging. With a linear PowerPath

topology, input and output are current

limited—the sum of the load current

and the battery charging current

cannot exceed the input current. In

this case, a switching PowerPath has

a signiicant advantage over a linear

PowerPath. In a switching PowerPath

topology the input is still current

limited, but this only limits available

power to the load and charger. This

is an important distinction. Figure 4

shows an example of how the LTC4088

can provide up to a 40% increase in

charge current over a linear PowerPath

design.

notice that while the uSB current is

limited to 500mA, it’s possible for the

charge current to be above 500mA due

to the high eficiency of the switching

PowerPath system. So not only does the

higher eficiency produce little heat,

but it also reduces charge time.

The input current limited topology

of the LTC4088 and LTC4098 offers

a big advantage over devices that use

an output current controlled topology

to maintain uSB compliance. This is

because as the battery voltage rises

throughout the charge cycle, the ef-

fective power consumed by the battery

also rises, assuming a constant cur-

rent. In order to retain uSB compliance

in an output current controlled sys-

tem (assuming perfect eficiency) one

would have to limit the battery charge

current to its power-limited value at

the highest battery voltage.

For example, to remain below 2.5W

(5V In • 500mA) of power delivery at a

4.2V battery voltage, the charge cur-

rent must not exceed 595mA. This

current limit is overly conservative

when the battery voltage is low, say

3.4V, where it would be possible to

deliver 735mA without violating the

uSB speciication. Input current lim-

ited devices designed speciically for

uSB compliance, such as the LTC4088

USB-Based

Constant-Power Charging

These days, an important feature

in many portable products is the

convenience of charging from a uSB

port. The LTC4088 and LTC4098

have a unique control system that

allows them to limit their input cur-

rent consumption for uSB compliant

applications while maximizing power

available to the load and battery

charging. These two devices not only

have low and high power uSB settings

of 100mA and 500mA, but they also

700

600

500

V BUS = 5V

R PROG = 1k

R CLPROG = 2.94k

400

300

200

100

5x USB SETTING,

BATTERY CHARGER SET FOR 1A

2.7

3.0

3.3

3.6

3.9

4.2

BATTERY VOLTAGE (V)

Figure 4. Input power limited charge current

Linear Technology Magazine • June 2008

DESIGN FEATURES L

and LTC4098, allow the charger to use

this additional available current. In

contrast, an output current regulated

switching charger designed for uSB

compliance must be programmed to

limit battery charging current to the

high voltage case (595mA), thus ham-

stringing it at low battery voltages. Said

another way, an input current limited

switching charger always extracts as

much power from the input source as

is allowed, whereas an output current

controlled one does not.

4.5

junction with an external MOSFeT,

provide signiicant input protection to

the low voltage (uSB/WALL) input.

4.2

3.9

3.6

NO LOAD

High Voltage Input Controller

The LTC4098’s external input control

circuit recognizes when a second input

supply is present and prioritizes that

input in the event that both it and

the uSB/WALL input are powered

simultaneously. Furthermore, the

LTC4098 interfaces with a number

of Linear Technology high voltage

step-down switching regulators to

allow for higher voltage inputs, such

as an automotive battery. using the

same Bat-Track technique described

above, the auxiliary input controller

commands the high voltage regula-

tor to develop a voltage at V OuT that

tracks just above the battery. Again,

this technique results in high charging

eficiency even when charging from a

fairly high voltage.

300mV

3.3

3.0

2.7

2.4

2.7

3.0

3.3

3.6

3.9

4.2

BAT (V)

Figure 5. V OUT vs BAT

Instant-On

(Low Battery System Start)

Figure 5 shows the instant-on feature

of the switching PowerPath topology.

When the battery voltage is very low

and the system load does not exceed

the available programmed power,

the output voltage is maintained at

approximately 3.6V. This prevents

the system from having to wait for

the battery voltage to come up before

turning on the device—a frustrating

scenario to the end user.

This is the primary reason for having

a decoupled output node and battery

node (i.e. the 3-terminal topology).

This feature can be used to power

the system in a low power mode. For

example, it may be just enough power

to start up and indicate to the user

that the system is charging.

cuit, the input current does not exceed

its programmed limit. Rather the

battery charger shuts off completely

and the extra power is drawn from the

battery via the ideal diode.

When the ideal diode is engaged, the

conduction path from the battery to the

output pin is approximately 180mΩ.

If this is suficient for the applica-

tion, then no external components

are needed. If greater conductance

is necessary, however, an external

MOSFeT can be used to supplement

the internal ideal diode. The LTC4088

and LTC4098 both have a control pin

for driving the gate of the optional

external transistor. Transistors with

resistance of 30mΩ or lower can be

used to supplement the internal ideal

diode.

Overvoltage Protection

The LTC4098 includes an overvoltage

protection controller that can be used

to protect the low voltage uSB/Wall

input from the inadvertent applica-

tion of high voltage or from a failed

wall adapter. This circuit controls

the gate of an external high voltage

n-type MOSFeT. By using an external

transistor for high voltage standoff, the

protection level is not limited to the

process parameters of the LTC4098.

Rather the speciications of the exter-

nal transistor determine the level of

protection provided.

Automatic Load Prioritization

The current delivered to the system

at V OuT , as well as the battery charge

current, form a combined load on the

switching regulator. If this combined

load does not exceed the power level

programmed by the input current limit

circuit then the switching PowerPath

topology happily delivers charge and

load current without concern. If,

however, the total load exceeds the

available power, the battery charger

automatically gives up some or all of

its share of the power to support the

extra load. That is, the system load is

always prioritized and battery charging

is only performed opportunistically.

This algorithm provides uninterrupted

power to the system load. even if the

system load alone exceeds the power

available from the input limiting cir-

Full Featured Battery Charger

The LTC4088 and LTC4098 both

include a full featured battery char-

ger. The battery chargers feature

programmable charge current, cell

preconditioning with bad-cell detec-

tion and termination, CC-CV charging,

C/10 end of charge detection, safety

timer termination, automatic recharge

and a thermistor signal conditioner for

temperature qualiied charging.

Conclusion

The LTC4088 and LTC4098 represent

a new paradigm in power management

and battery charging. Both optimize

power delivery by combining constant

input power limiting with a high

eficiency switching regulator and

Bat-Track battery charging. Other

beneits include instant-on system

starting, automatic load prioritization

and unmatched charging eficiency.

The LTC4098 goes a step beyond

with an auxiliary input controller for

higher input voltages (such as a car

battery) and an overvoltage protection

controller. L

LTC4098 Enhancements

The LTC4098 has a few features that

the LTC4088 does not. First, it sup-

ports the ability to control an external

high voltage switching regulator to

receive power from a second input sup-

ply such an automobile battery. It also

includes an independent overvoltage

protection module that can, in con-

Linear Technology Magazine • June 2008

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