September 2000 - Linear Technology

Linear Technology Chronicle

• September 2000 1

A Showcase of Linear Technology’s Focus Products

Product of the Month

, LTC and LT are registered trademarks of Linear Technology

Corporation. Burst Mode is a trademark of Linear Technology

Corporation. I

C is a trademark of Philips Electronics N.V.

Linear Technology Chronicle

September 2000 Vol. 9 No. 9

Inside This Issue:

Smallest Dual 14-Bit Voltage Output DAC—LTC1654 ........................................................... 2

Measure Resistances Easily, Without Reference Resistor or Current Source—LT1168 ....... 3

SOT-23 Fan Controller Includes SMBus Interface—LTC1695 ............................................... 3

1.5A Charger for Multicell Li-Ion Batteries and Other Chemistries—LT1571 ......................... 4

Tiny Quad 10-Bit Rail-to-Rail V

OUT

DAC Consumes Only 60µA per DAC—LTC1664 ........... 4

Figure 1. High Efficiency Step-Down Converter

Monolithic Synchronous Step-Down Regulators

Pack 500mA Output Current in MSOP Packages—

LTC1877, LTC1878 and LT1612

The LTC

1877, LTC1878 and

1612 are the new generation of synchro-

nous switching regulators capable of supply-

ing more than 500mA of output current.

They are available in the tiny MSOP pack-

age. Their internal switches increase effi-

ciency and eliminate Schottky diodes, saving

components and board space (Figure 1). The

operating ranges of the three devices allow

the use of a single or dual Li-Ion battery and

2- to 6-cell NiCd and NiMH battery packs.

For long battery life, shutdown reduces the

current drain for these products to 1µA.

LTC1877 and LTC1878 Extend Battery

Life with 10µA Quiescent Current

The BiCMOS technology in the

LTC1877 and LTC1878 allows them to

achieve ultrahigh efficiency at low load

currents. Their supply currents are only

10µA while regulating the output voltage.

These devices maintain better than 90%

efficiency over three decades of output

load current (Figure 2).

The LTC1877 is designed for higher

input voltages, 2.65V to 10V, whereas the

LTC1878 is optimized for lower input volt-

ages, 2.65V to 6V. Both incorporate a con-

stant frequency, current mode step-down

architecture with on-chip power MOSFETs.

They include protection against output

overvoltage, short circuit and excessive

power dissipation. The LTC1877 and

LTC1878 switch at 550kHz and may be syn-

chronized to frequencies from 400kHz to

700kHz.

Burst Mode

operation may be enabled

for highest efficiency or disabled for lower

output ripple. The LTC1877 and LTC1878

are capable of turning on continuously

47µF***

LTC1877

GND

10µH*

10µF**

CER

220pF

VOUT

†

3.3V

887k

280k

1877 TA01

SYNC

RUN

2.65V

TO 10V 20pF

*TOKO D62CB A920CY-100M

**TAIYO-YUDEN CERAMIC LMK325BJ106MN

***SANYO POSCAP 6TPA47M

†

OUT

CONNECTED TO V

FOR 2.65V < V

< 3.3V

OUTPUT CURRENT (mA)

0.1

EFFICIENCY (%)

10 1000

100

1877 • TA02

1.0 100

VIN = 3.6V

VIN = 10V

VOUT = 3.3V

L = 10µH

Burst Mode OPERATION

VIN = 7.2V

VIN = 5V

Figure 2. High Efficiency

Over Three Decades of

Output Current

Continued on page 2

Single Resistor Gain

Programmable,

Micropower Precision

Instrumentation

Amplifier—LT1168

The LT1168 is a micropower, precision

instrumentation amplifier that requires only

one external resistor to set gains of 1 to

10,000. The low voltage noise of 10nV/

(at 1kHz) is not compromised by the part’s

low power dissipation (350µA typical for

±15V supplies). The wide supply range of

±2.3V to ±18V allows the LT1168 to fit into

a wide variety of industrial and battery-

powered applications.

The high accuracy of the LT1168 is due

to a 20ppm maximum nonlinearity and 0.4%

max gain error (G = 10). While most mono-

lithic instrumentation amps cannot handle

low load resistor values, the nonlinearity of

the LT1168 is specified for loads as low as

2kΩ. The LT1168 is laser trimmed for very

low input offset voltage (40µV max), drift

(0.3µV/°C), high CMRR (90dB, G = 1) and

PSRR (103dB, G = 1) and low input bias

Continued on page 2

√Hz

Linear Technology Chronicle

• September 2000

Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.

(100% duty cycle) when the input voltage

falls near the output voltage. All the compo-

nents shown in Figure 3 are surface mount

and minimize the board space and height.

VIN

0.1µF

33k R2

232k

215k

10µH

330pF

10µF

68µF

3.15V

C1: TAIYO-YUDEN JMK325BJ106MN

C2: PANASONIC EEFCDOF680R

L1: SUMIDA CD43-100

1612 F01a

VOUT

1.2V

500mA

100pF

VIN

BOOST

MODE

SHDN

VCGND

LT1612

currents of 250pA max. The output can

handle capacitive loads up to 1000pF in any

gain configuration while the inputs are ESD

protected up to 13kV (human body). With

two external 5k resistors, the LT1168 passes

the IEC 1000-4-2 level 4 specification.

Offered in 8-pin PDIP and SO pack-

ages, this part requires significantly less PC

board area than discrete op amp designs. The

LT1168 is available in both industrial and

commercial temperature ranges.

–

1/2

LT1112

3.5k

1168 TA01

40k

20k

40k

DIGITAL

DATA

OUTPUT

G = 200

249Ω

REF

AGND

ADC

LTC1286

BI TECHNOLOGIES

67-8-3 R40KQ, (0.02% RATIO MATCH)

–

LT1168

Figure 1. Single Supply Pressure Monitor

LTC1877, LTC1878 and LT1612 from page 1

Figure 3. The LT1878 Converter

(Actual Size)

LT1612 Steps Down from 2V Input

For low voltage operation in a buck

regulator, a bipolar NPN top-side power

switch is needed. The LT1612 can supply

500mA with input supply voltages from 2V

to 5.5V (Figure 4). The LT1612 is a low cost

alternative when a quiescent current of

150µA suffices. The internal reference of

0.62V allows outputs below 1V.

The LT1612 is capable of Burst Mode

operation or of constant frequency 800kHz

switching. It exhibits less than one fourth

the power loss of a linear regulator at an

input of 3V.

Figure 4. Compact 2V to 1.2V Converter

LT1168 from page 1

DAC B

1654 BD

OUT B

REFHI B

OUT A

1/2

REFHI A

–

POWER-ON

RESET

DAC A

REFLO AREFLO B

1/2

–

INPUT

LATCH

CONTROL

LOGIC

CS/LD

SCK

SDO

CLR

SDI

INPUT

LATCH

DAC

32-BIT

SHIFT

DAC

Smallest Dual 14-Bit

Voltage Output DAC—

LTC1654

The LTC1654 is a dual 14-bit voltage

output digital-to-analog converter (DAC) in

the narrow 16-pin SSOP package, requiring

the same board space as an SO-8. This tiny

dual DAC draws only 225µA per DAC

(typ) from a single 3V to 5V supply. Two

rail-to-rail output buffers and a 3-wire

cascadable serial interface allow ease of use

and conserve system board space. High

resolution, small size and low power make

this low cost, dual DAC ideal for digital

calibration, industrial process control and

automatic test equipment applications.

The LTC1654 offers two individually

programmable speed modes (3.5µs settling

time and 8µs settling time), which allow the

user to choose speed vs power based on the

application. The reference input can be con-

veniently tied to the supply and the output

can swing from 0V to either VCC or VCC/2,

depending on which gain mode is selected.

The differential nonlinearity (DNL) of

the LTC1654 is only ±1LSB (max), which

guarantees 14-bit monotonic performance,

critical in control loop applications. Linear-

ity is guaranteed over the entire operating

temperature range. The LTC1654 is avail-

able in the narrow 16-pin SSOP package in

both the commercial and industrial tempera-

ture grades.

Figure 1. LTC1654 Dual Rail-to-Rail Voltage Output 14-Bit DAC

Linear Technology Chronicle

• September 2000 3

Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.

Application of the Month

Measure Resistances Easily Without Reference

Resistor or Current Source—LT1168

SOT-23 Fan Controller

Includes SMBus

Interface—LTC1695

The LTC1695 fan speed controller inte-

grates a low dropout linear regulator, 6-bit

DAC and a simple 2-wire SMBus/I2C

digital interface into a tiny SOT-23 package.

The LDO offers 180mA of output current for

driving 5V brushless DC fans. The DAC and

SMBus interface allow easy communication

with a power management controller to regu-

late the speed of the fan. A user selectable

“boost start” feature automatically forces full

power to the fan for the first 250ms of opera-

tion, ensuring clean start-up. Only two small,

inexpensive external capacitors are required

to form an operational circuit, greatly reduc-

ing board space requirements for laptop

computers and portable battery-powered

equipment.

The LTC1695 employs a linear

regulator approach as opposed to a PWM

LTC1695

GND

SCL

OUT

SDA

SYSTEM

CONTROLLER

4.7µF

10µF

5V DC FAN

1695 • TA01

Figure 1. LTC1695 Controls 5V DC Fan

technique. This fan speed control method

greatly reduces rotor vibrations resulting in

lower mechanical noise and longer fan life.

It also prevents electrical noise from being

introduced on the 5V supply line.

In a typical system design, the micro-

controller uses available temperature data to

program the LTC1695 for the required fan

speed to maintain a system’s temperature

profile. The LTC1695 also includes output

current limiting and thermal shutdown as

well as status monitors that can be read back

by the power management controller during

fault conditions.

The LTC1695 is available in a 5-lead

SOT-23 package specified over the commer-

cial temperature range.

Measuring the resistance of a device, a

thermistor for example, usually requires

biasing it with a precision current source or

combining it with several other precision

resistors in a bridge. The circuit of Figure 1

shows how to use the new LT1168 instru-

mentation amplifier to achieve a precision

resistance-to-voltage conversion as simply

as possible.

Normally the resistor across Pin 1 and

Pin 8 is the gain-set resistor and the voltage

across Pin 3 and Pin 2 is the variable to be

measured. In this case, however, the 1.25V

reference establishes a fixed input voltage so

that the variable to be measured is now the

resistance. The equation for VOUT vs RT is

VOUT = 1.25V • 49.4kΩ/RT. Given the limi-

tation on output swing (with the supply volt-

ages shown), the smallest measurable

resistance is about 4.5k. The highest resolv-

able resistance is limited to about 200M by

the 300mV output offset voltage of the

LT1168. The 0.05% accuracy of the LT1634

is not an issue here because it is subtracted at

the REF pin of the LT1168 and only contrib-

utes to gain error.

Figure 2 shows output voltage vs tem-

perature for 10k and 100k (at 25°C)

thermistors from two manufacturers. Ther-

mistors are difficult to linearize; although

the output is still not linear with tempera-

ture, it can at least be read directly by an

ADC and compared against a lookup table.

The circuit has good noise immunity but

does not tolerate capacitance at Pin 1 or Pin

8 and so it is not ideal for resistive devices

placed remotely from the LT1168.

–

15V

–15V

LT1634-

1.25

LT1168

REF

OUT

= 1.25 • 4.49kΩ

22k

THERMOMETRICS

DC95F103W THERMO-

METRICS

DC95G104Z

–40 –20 0 20 40 60 80 100 120

TEMPERATURE (°C)

OUTPUT VOLTAGE (V)

YSI #44011

YSI #44006

Figure 1. Simple Resistance-to-Voltage

Converter

Figure 2. Output Voltage vs Tempera-

ture for Thermistors from Two Manu-

facturers. Curves are Approximations to

Aid Design—Contact Manufacturers for

Exact Lookup Tables:

YSI (800) 765-4974

Thermometrics (723) 287-2870

http://www.linear-tech.com/ezone/zone.html

Articles, Design Ideas, Tips from the Lab…

Linear Technology Chronicle

• September 2000

Linear Technology Corporation • 1630 McCarthy Blvd. • Milpitas, CA 95035-7417 • (408) 432-1900 • FAX: (408) 434-0507 • www.linear-tech.com • For Literature Only: 1-800-4-LINEAR

Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.

1.5A Charger for Multicell Li-Ion Batteries

and Other Chemistries—LT1571

SW V

PROG

4.93k 100k

11V

TO 26V

1571 F04

0.22µF

10µF

L1**

33µH

LT1571-2

MBRM120T3

MMBD914L

SELECT

SENSE

GND

CAP

OUT

22µF

TANT 8.4V

0.3µF

0.1µF

1µF

NOTE: COMPLETE LITHIUM-ION CHARGER, NO TERMINATION REQUIRED

TOKIN OR MARCON CERAMIC SURFACE MOUNT

COILTRONICS CTX33-2

BOOST

BAT

BAT2

FLAG

300Ω

Figure 1. 200kHz Charging Li-Ion Batteries

(Efficiency at 1A Exceeds 87%)

Table 1. Constant Current, Constant Voltage,

Single Chip Battery Chargers

Charge Current <1A 1A 1.5A >1.5A

No End-of- LTC1731 LT1510-5 LT1769 LT1769

Charge Indicator LT1511

With End-of- LTC1731 LT1571-5 LT1571-1or -2 LT1505

Charge Indicator (500kHz) (200kHz) Up to 6A

The LT1571-1 and LT1571-2 current-

mode PWM battery chargers deliver up to

1.5A charge current for products such as

handheld computers and portable instru-

ments. The chargers feature 1% constant

voltage and 5% constant current accuracy

over the full operating temperature range.

The LT1571-2 includes preset charge volt-

ages of 8.2V or 8.4V (pin selectable) while

the LT1571-1 is adjustable from 2.5V to

24V using external resistors.

Both versions includes an internal cur-

rent sense resistor which allows the charge

current to be easily adjusted using a single

external resistor, DAC or PWM signal. They

also include adjustable soft start, undervolt-

age lockout and an end-of-charge flag output

with a threshold that is programmable from

7.5% to 20% of the full charge current. A

shutdown mode (with VIN present) drops the

quiescent current to 150µA, and a sleep

mode drops the battery drain current to 5µA

when VIN is removed.

The LT1571-2 is designed to charge

two Li-Ion cells (Figure 1), while the

LT1571-1, with its adjustable output, can be

used to charge three or four Li-Ion cells as

well as NiCd, NiMH and SLA batteries. The

previously introduced 500kHz LT1571-5,

in a 16-lead SSOP with a maximum charge

current of 1A, charges 4.1V or 4.2V single-

cell Li-Ion systems.

The wide 8V to 28V input range of the

LT1571 series accommodates most wall

adapter and automotive input voltage ranges.

An onboard 2A saturating switch produces

up to 87% efficiency. Its built-in

undervoltage lockout avoids overloading

wall adapter supplies at start-up. Designed to

be used with all surface mount components

including compact low profile inductors, the

200kHz battery charger circuit occupies a

minimum of board space. The LT1571

devices are packaged in a fused lead frame,

narrow 28-lead SSOP package for good ther-

mal performance in a small space.

Other chargers are optimized for higher

or lower charge currents, with or without the

end-of-charge function (Table 1).

The LTC1664 is a quad, 10-bit, rail-to-

rail voltage output micropower DAC. The

part consumes only 60µA per DAC during

normal operation and in sleep mode the sup-

ply current drops to only 1µA. This ultralow

power consumption makes the LTC1664

ideal for battery-powered applications. The

LTC1664 is available in the tiny 16-pin nar-

row SSOP package, fitting in the footprint of

a standard SO-8.

The LTC1664’s reference input pin has

a constant impedance over all codes. This

feature eliminates external reference buffers

necessary with other DACs to ensure con-

stant linearity over the entire full scale. In

addition, its reference input can be tied to

the supply pin for ratiometric rail-to-rail out-

put. The LTC1664 has a differential nonlin-

earity (DNL) of only ±0.75LSB (max),

guaranteeing the monotomic performance

critical in control-loop applications. Its

3-wire cascadable serial interface simplifies

transmitting through isolation barriers or to

remote locations and allows several

LTC1664s to be connected together for

daisy chain operation.

In addition to the 16-pin SSOP, the

LTC1664 is also available in the 16-pin

PDIP package. Both are available in com-

mercial and industrial temperature ranges.

Tiny Quad 10-Bit Rail-to-Rail V

OUT

DAC Consumes

Only 60

A per DAC—LTC1664

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