June 1991 - Linear Technology

LINEAR TECHNOLOG

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LINEAR TECHNOLOG

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LINEAR TECHNOLOG

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JUNE 1991 VOLUME I NUMBER 1

IN THIS ISSUE . . .

Welcome to

Linear Technology .............. 1

Bob Swanson

Editor's page ...................... 2

Walt Jung

LT1223 High Speed Current

Feedback Amplifier............. 3

Bill Gross

LT1220 High Speed CB

Process Op Amp .................. 4

George Feliz

LT1122 Fast Settling JFET

Op Amp............................... 6

George Erdi and Walt Jung

LT1074 Family of Step Down

Switching ICs ..................... 7

Jim Williams

LT1073 Single Cell Switching

IC ..................................... 10

Steve Pietkiewicz

LTC485 Line Termination . 11

Bob Reay

Linear High Current Driver 12

Walt Jung and Rich Markell

Single Cell Laser Pointer

Driver ............................... 13

Steve Pietkiewicz

Active SCSI Termination ... 13

Sean Gold

Low Dropout Regulator ..... 14

Jim Williams and Dennis O'Neill

New Device Cameos ........... 15

LTC Marketing

Welcome to

Linear Technology

The founding theme of Linear

Technology Corporation was to create

a company capable of leading and

directing linear circuit technology and

design concepts of the future and thus

become the market's preferred high

performance linear specialist. Since

our inception in late 1981, we have

strived to provide high performance

linear ICs with outstanding quality

and reliability. It is important to

everyone at LTC that the solutions we

provide are delivered not just on time,

but they continue to work properly

and reliably.

Providing high performance ICs

across wide analog product lines

requires not only innovative design

but excellent wafer processing and

test. Our proprietary products are

designed, wafer fabricated and tested

by LTC. Although the Company is

known as the home of many of the

industry's design innovators; we

are equally proud of our total

manufacturing capabilities. In our own

plant, we fabricate both N-Well and

P-Well silicon gate CMOS, general

purpose and high frequency bipolar

devices, and we are now ramping up

production on a full complementary

bipolar process. Our ability to virtually

tailor a process to the product and

combine that with advanced wafer

level trimming bring efficient solutions

to high performance analog problems.

Looking back at our beginning in

1981, we recall that the conventional

wisdom of the time suggested that

only the large, “all product” companies

would have the staying power to make

it through the decade of the 80’s. LTC’s

philosophy at the time was considered

a maverick approach, i.e. we believed

that the “focused” strategy was the

only viable approach to delivering, to

the customer, the best technology in

each functional area. Today it’s

interesting to note that our approach

has become the current conventional

wisdom for companies big and small.

As we get ready to enter a second

decade of operation and expanded

analog product offerings, I am proud

to introduce Linear Technology as a

source of timely technical information

on new products and applications.

During the past five years we've found

our product direction becoming more

and more application specific. Our

attempt to provide more complete

solutions has lead to more complex

products, the benefits of which are

sometimes more difficult to understand

by both our sales force as well as our

customers.

In the 90’s we hope to use this

magazine to provide regular product

introductions along with timely related

circuit information, and applications

ideas on all products. This will

supplement our datasheet and

application note effort which will

continue unabated. Linear Technology

is designed to help you, our customer.

We are interested in your reactions,

and hope you will share them with us.

by Bob Swanson, President & CEO

Linear Technology Magazine

•

June1991

Linear Technology

from LTC

Premiers to Analog World

LTC welcomes all readers to the

premier issue of Linear Technology, a

new publication from Linear Technol-

ogy Corporation. Linear Technology will

focus on the technical highlights of

newly introduced LTC devices and

emphasize how they are most usefully

applied.

At times, this concept of application

orientation may be extended to older

devices as well, especially where newly

developed ideas and techniques serve

particular interests stimulated by cus-

tomers. “Applications” is not neces-

sarily restricted to just circuits, or

pure hardware functions, they can

and will encompass software in sup-

port of LTC devices.

We will try to balance the contents

of each issue around an array of fea-

tures and speciality items, all present-

ing fresh and useful approaches to

linear technology topics. Our “Design

Features” will be articles on recently

introduced LTC ICs, and will serve as

the mainstay of each issue. These ar-

ticles emphasize what is new, unique

or in what way different about each IC.

We will steer this away from a dry

tutorial approach and more towards

what the particular IC makes happen

in circuit. In the latter sense, yes, we

hope there will be lots of interesting

circuits showing how the new ICs are

used.

Somewhat shorter in length will be

a continuing series of brief “Design

Idea” type applications, which can be

new as well as old product related. The

basic objective here is the crux of what

it takes to usefully solve an application

problem in a new or more efficient

manner. We think the ideas for this

first issue illustrate this criteria well,

and it is expected that this section will

be one of the more popular ones of

Linear Technology.

Finally, we will round out each is-

sue with an array of “New Device Cam-

eos” which will cover recent LTC de-

vices not otherwise featured in an is-

sue.

As time goes on and the publication

grows, we hope to stimulate reader

involvement. Let us know your ideas

on what “linear technology” topics will

best serve your analog circuits.

Issue Highlights

Linear Technology leads off this is-

sue with overview remarks by LTC

president Bob Swanson. While these

comments are much less technical

than those surrounding it, they bear

reading for the broader viewpoint they

bring to these pages.

In this premier Linear Technology

issue, we are fortunate to have a bounty

of technical articles of the types men-

tioned above. In the main Design Fea-

ture articles, we have no less than 5

articles of note.

The piece by Bill Gross on his

LT1223 discusses a new high speed

complementary bipolar current feed-

back (transimpedance) amplifier, an

important achievement for LTC in pro-

cess capability.

Related by process is the article by

George Feliz, on the LT1220 high speed

op amp, also produced with LTC CB

technology.

While it does not use an entirely

new process, George Erdi’s LT1122

fast settling JFET input op amp com-

bines fast-settling and high speed with

low input current, DC precision, and

very low distortion.

Jim Williams’ Design Feature on

the LT1074 regulator highlights the

most versatile power switching IC yet

produced within the LTC proprietary

family. The article highlights the func-

tional characteristics of the device and

shows some sample hookups.

Steve Pietkiewicz authors the final

Design Feature, on his LT1073 single

cell switching regulator. This IC rep-

resents some new achievements for

size and power efficiency, and opens

up new applications with relative sup-

ply voltage freedom.

In the Design Idea section we have

interesting articles for circuit collec-

tors. Walt Jung and Rich Markell lead

off with a low distortion driver circuit,

and Bob Reay discusses some RS485

interfacing issues in his Design Idea.

Steve Pietkiewicz describes his LT1110

high speed single cell switching regu-

lator as a laser pointer driver, followed

by Sean Gold's piece on SCSI active

termination. Jim Williams and Den-

nis O’Neill finish with an LT1123 regu-

lator/driver used as a low dropout

driver for a power PNP.

In the New Device Cameo section,

LTC marketing introduces us to a

several new device types. These are

the LT1103 and LT1105 offline regu-

lator ICs, the LT1240 series of high

speed DC-DC converter ICs, the

LTC1272 12 bit, 3µs, parallel A/D

converter, and the LTC1289 low volt-

age single chip 12-bit DAS.

by Walt Jung

Linear Technology Magazine

•

June 1991

The LT1223 is the first current

feedback amplifier (CFA) from LTC,

and the second amplifier manufac-

tured on the new complementary bi-

polar (CB) process. As such, it repre-

sents achievements for LTC in both

process capability and an entirely

new amplifier type. Current feedback

amplifiers have distinct performance

contrasts with familiar voltage feed-

back amplifiers (such as conventional

op amps). Op amps are usually de-

signed for excellent DC performance,

often at the expense of AC param-

eters. Interestingly, current feedback

amplifiers have intrinsic AC advan-

tages over voltage feedback amplifi-

ers, but their DC performance often

suffers. Although current feedback

amplifiers aren’t new, they have only

recently become popular, driven by

complementary IC processes with fast

PNP’s.

Current Feedback Basics

The distinctions of how current

feedback amplifiers differ from volt-

age feedback amplifiers are not obvi-

ous at first, because from the outside

the differences can be subtle. Both

amplifier types use a similar symbol,

and can be applied on a first order

basis using the same equations. How-

ever their behavior in terms of gain-

bandwidth tradeoffs and large signal

response is another story.

Unlike voltage feedback amplifi-

ers, small signal bandwidth in a cur-

rent feedback amplifier isn’t a straight

inverse function of closed loop gain,

and large signal response is closer to

ideal. Both benefits are because the

feedback resistors determine the

amount of current driving the

amplifier’s internal compensation

capacitor. In fact, the amplifier feed-

back resistor (R

) from output to in-

verting input works with internal junc-

tion capacitances of the LT1223 to

set the closed loop bandwidth. Even

though the gain set resistor (R

) from

inverting input to ground works with

the R

to set the voltage gain just like

it does in a voltage feedback op amp,

the closed loop bandwidth does not

change.

The explanation behind this is fairly

straightforward. The equivalent gain

bandwidth product of the CFA is set

by the Thevenin equivalent resistance

seen at the inverting input and the

internal compensation capacitor. If

is held constant and gain is changed

via R

, the Thevenin equivalent resis-

tance changes by the same amount

as the change in gain. From an over-

all loop standpoint, this change in

feedback attenuation will in fact pro-

duce a change in noise gain, and a

proportionate reduction of open loop

bandwidth (as in a conventional op

amp). With the CFA however, the key

point is that changes in Thevenin

resistance also produces a compen-

satory change in open loop band-

width, unlike a fixed gain bandwidth

amplifier (op amp). As a result, the

net closed loop bandwidth of a CFA

such as the LT1223 remains the same

for various closed loop gains.

Figure 1 shows the LT1223 voltage

gain vs. frequency, for five gain set-

tings as noted, driving 100 ohms.

Shown for comparison is a plot of the

fixed 100MHz gain bandwidth limita-

tion that a voltage feedback amplifier

would have. It is obvious that for

gains greater than one the LT1223

provides 3-20 times more bandwidth.

Because the feedback resistor deter-

mines the compensation of the LT1223,

bandwidth and transient response can

be optimized for almost every applica-

tion. When operating on ±15V supplies,

should be 1k ohms or more for

stability, but on ±5V the minimum

value is 680 ohms, because the junc-

tion capacitors increase with lower volt-

age. For either case, larger feedback

resistors can also be used, but will slow

down the LT1223 (which may be desir-

able in some applications).

The LT1223 delivers excellent slew

rate and bandwidth with better DC

performance than previous CFA’s. On

±15V supplies with a 1k feedback re-

sistor, the small signal bandwidth is

100MHz into a 400 ohm load, and

75MHz into 100 ohms. The input will

follow slew rates of 250V/µs with the

output generating over 500V/µs, and

output slew rate is over 1000V/µs, for

large input overdrive. Input offset volt-

age is 3mV (max), and input bias cur-

rent is 3µA (max). An 10k pot can be

used for (optional) offset null, con-

nected to pins 1 and 5 with wiper to V

continued with “LT1223”, page 5

Figure 1. LT1223 Voltage Gain vs.

Frequency, Compared to 100MHz Op Amp

The LT1223, A New High Speed

Current Feedback Amplifier by Bill Gross

FREQUENCY (Hz)

100k

–20

VOLTAGE GAIN (dB)

–10

10M 100M 1G1M

Ω

V = 15V

Rl = 100

R = 1k

VOLTAGE FEEDBACK

100MHz GBW

R = 110

R = 470

R =

∞

Ω

R = 33

Ω

R = 10

Ω

A6 • F1

Linear Technology Magazine

•

June1991

Figure 1. LT1220 High Speed I/V Converter

By George Feliz

LT1220 High Speed CB Process

Op Amp

The LT1220 is the first in a family of

new, high performance amplifiers em-

ploying LTC’s advanced complemen-

tary bipolar (CB) process. To the user

this means that practical high speed

ICs are now available, with consistent

AC performance between PNP/NPN

transistors, and without supply volt-

age sacrifices or other application limi-

tations.

Unity-gain stable, the LT1220 op

amp features a 45MHz gain band-

width and a 250V/µs slew rate, but

doesn’t sacrifice DC accuracy. The

LT1220 has high open-loop gain of

20V/mV(min), input bias/offset cur-

rents of 300nA(max), and an input

offset voltage less than 1mV. These

DC specifications lend themselves to

data acquisition systems up to 12

bits. Large signal settling time is 90ns

to 0.1%, or 165ns to 0.01%, both

figures for 10V steps. For ease of ap-

plication, the amplifier is tolerant of

capacitive loading, and can drive 500

ohm loads to ±12V.

Some interesting design features

enhance the performance of the

LT1220. Although it has only one gain

stage, high voltage gain is achieved

with a proprietary circuit. Due to the

balanced nature this scheme boosts

voltage gain without contributing to

drift. In addition, a triple emitter-fol-

lower output buffer maintains this

high gain, even when driving 500

ohms. Low input currents are achieved

by bias cancellation circuitry, mean-

ing that the compensation resistor

typically used on the amplifier’s (+)

input isn't needed. Finally, the

amplifier’s ability to remain stable with

increasing capacitive loads is accom-

plished by sensing the load induced

output pole, and adding compensa-

tion to the amplifier gain node.

The LT1220 is well suited to virtu-

ally any general purpose application

needing extended bandwidth. Ex-

amples are active filters, cable drivers

and buffers, data acquisition, and

video applications. It can be used to

upgrade the performance of other

amplifiers such as the HA2505/15/

25, HA2541, AD841, AD847, and the

LM6361.

For higher closed-loop gain, the

LT1220 family has two other mem-

bers. The LT1221 is stable in gains of

+4V/V or greater, and has the same

250V/µs slew rate of the LT1220, but

with higher open-loop gain, 50V/

mV(min). The LT1222 is stable in gains

of +10V/V or more, with no compen-

sation cap. For greater application

flexibility, the LT1222 also has the

compensation node available on pin 5

(an example would be compensation

of 30pF for a stable gain of –1). The

LT1222 has an open-loop gain of

100V/mV(min), and a slew rate of

200V/µs (with no external capacitor).

High Speed I/V Converter

An important application class for

the LT1220 is an I/V converter at the

output of a fast settling current mode

DAC. An example of such a DAC buffer

with 12 bits resolution is shown in

Figure 1. Here, the LT1220 is config-

ured with an AD565, a fast 12 bit DAC

which sinks 2mA of current for full

scale and settles in 250ns(max). A 5k

ohm resistor internal to the DAC is

used as the feedback resistor around

the LT1220. To null the 25pF output

capacitance of the DAC and for opti-

mum settling, a 5pF capacitor is used

across the feedback resistor.

Figures 2a and 2b show ± step

settling waveforms at the opamp out-

put as measured at a false sum node

between the output and a -10V refer-

ence, as per Figure 1. The net settling

time is less than 350ns, including the

dominant delay of the DAC plus a few

nsec of delay in the FET buffer. The

key specifications needed for the op

amp are high gain, low input bias

current, and fast settling. For addi-

tional DC accuracy, an optional null-

ing amplifier can be used to drive the

LT1220 offset adjust pins to reduce

offset voltage change over tempera-

ture.

A SPICE macromodel for the LT1220

is included in the LTC op amp library.

–10V

A5 • F1

LT1220

–

AD565

DAC

INPUT

5k*

5pF

0-2mA

0-10V OUTPUT MEASUREMENT

NODE

TO SCOPE

REF

FET BUFFER AND

GAIN STAGE

*5k RESISTOR IS INTERNAL TO DACΩ

continued with “LT1220”, page 5

Linear Technology Magazine

•

June 1991

Table 1. Current Feedback Amplifier Comparison

Parameter LT1223 EL2020

BW (MHz) 100 50

SR (V/µs) 1000 500

IO (mA, Min) 50 30

VOS (mV, Max) 3 10

–IB (µA, Max) 3 40

+IB (µA, Max) 3 15

ROL (MΩ, Min) 1.5 0.3

(like the LT1056 scheme). This trim

shifts inverting input current about

±10µA, effectively producing input volt-

age offset.

The LT1223 also has shutdown

control, optionally available at pin 8.

Pulling more than 200µA from pin 8

drops the supply current to less than

3mA, and puts the output into a high

impedance state. The easy way to

force shutdown is to ground pin 8,

using an open collector (drain) logic

stage. An internal resistor limits cur-

rent, allowing direct interfacing with

no additional parts. When pin 8 is

open, the LT1223 operates normally.

The table summarizes the LT1223

specifications, along with those of the

EL2020, a popular industry type.

LT1220 continued from page 3

LT1223 continued from page 3

Applications

Figure 2 shows the LT1223 in a

typical video cable driver applica-

tion. With R

= R

, the LT1223 has a

gain of two to recover the 6dB loss in

driving the double terminated 75

ohm cable. It is very important to

terminate both ends of the cable or

the frequency response (and time

domain response) will be set by the

length of the cable. Even a 4-5 foot

cable terminated on only one end

will have significant errors at 10MHz.

For this video application, as of-

ten used in studios, the LT1223 CFA

topology performs quite well. For

standard NTSC composite video at

1Vpp input, the differential gain and

phase are 0.02% and 0.12 degrees,

respectively. Other uses for the LT1223

are wideband cable drivers and buff-

ers, fast settling I/V converters, fiber

optic LED drivers, photo-diode am-

plifiers, R

and IF amplifiers, radar IF

processing, and high gain preamps

which must retain maximum band-

width per stage.

Figure 2a. Positive Step Figure 2b. Negative Step

Figure 2. LT1223 Video Cable Driver

Ω

OUT

CABLE

Ω

75Ω

–

LT1223

A6 • F2

100ns/DIV

INPUT

TO DAC

A5 • F2a

OUTPUT

1LSB/DIV

100ns/DIV

A5 • F2b

INPUT

TO DAC

OUTPUT

1LSB/DIV

Linear Technology Magazine

•

June1991

The LT1122 is a new, high perfor-

mance JFET input op amp. Opti-

mized around high speed and fast

settling performance, the LT1122 uses

a modified bipolar-FET process. The

performance resulting is a combina-

tion of not just excellent AC param-

eters, but low input currents and DC

precision, achieved within a junction-

isolated process.

Unity-gain stable, the LT1122 fea-

tures a 14MHz gain bandwidth and a

typical 80V/µs slew rate (SR) with

controlled symmetry. For a 10V step,

it settles to 1mV at the sum node in

340ns(typ), or 540ns max. The

LT1122’s excellent DC accuracy speci-

fications include an open-loop gain of

500V/mV(typ) into a 2k load, (250V/

mV into 600 ohms), input bias/offset

currents of 75 and 40pA(max) respec-

tively, and an input offset voltage of

0.6mV(max). For driving difficult

loads, the LT1222 has a 40mA cur-

rent limit, and can drive 600 ohm

loads to ±12V(min).

The LT1122 achieves these com-

bined parameters with a unique poly-

gate JFET. In conventional FET tech-

nology, the gate contact is tens of

micrometers away from the actual

channel of the FET, creating a re-

sponse pole due to the gate implant

series resistance and capacitance,

thereby limiting bandwidth. In the

LT1122 JFET design, a polysilicon

layer provides a gate contact directly

over the channel, eliminating this pole.

In addition, the circular structure

and small drain diffusion used mini-

mizes gate-drain capacitance.

Atop these and other speed related

circuit improvements, the LT1122 has

very low audio range distortion. For

Q12 sources load current greater than

, the control loop turns off, and Q13

can then no longer return to sinking

load current immediately. If un-

checked, this would lead to crossover

glitches, when the output must

quickly change from sourcing to sink-

ing load current. Here however, the

problem is solved by Q7, an addi-

tional main loop drive input which

can provide the short term sink cur-

rent required. This allows Q13 to turn

back on more slowly, but without

adding distortion.

Fast settling is the main feature of

the LT1122, and is 100% tested in a

settling time test fixture described on

the data sheet. The LT1122 is avail-

able in 4 electrical grades, 2 premium

(A & B) and 2 standard (C & D). Of

these, the A & C parts are 100% tested

for settling, with the others (B & D)

sample tested.

Figure 1. LT1122 Output Stage

example, THD at an inverting gain of

10 is 0.001% or less to 20kHz, with

non-inverting performance only

slightly worse. For IM distortion via

the CCIF method, the LT1122 has

performance as much as 2 orders of

magnitude better than typical indus-

try JFET amplifiers such as 156/356

types. The low total distortion is largely

due to two design factors. One is the

LT1122 SR

which is not

only high at

80V/µs, but

which also is

intrinsically

symmetrical.

This factor

eliminates the

even order dis-

tortion effects

present in a to-

pology with

asymmetric

transconduct-

ance. Another

factor is the lin-

ear all NPN out-

put stage,

which features

high output

current and very high speed.

This LT1122 output stage is shown

in Figure 1 in simplified form, and has

several features which contribute to

high linearity. One is the local loop

which controls the idle current in

Q12 (I

). This loop is comprised of

Q12, R9, Q9, R6, J6, Q10 and Q13,

and it causes the output follower Q12

to always conduct the idle current or

more, so providing a low output im-

pedance. Without precautions how-

ever, this type of stage can also distort

for some conditions. If for example,

The LT1122 Fast Settling

JFET Op Amp By George Erdi and Walt Jung

A4 • F1

Q11

Q12

Q10

Q13

R7R8

SIGNAL FROM

PREVIOUS STAGES

V–

OUT

V+

Linear Technology Magazine

•

June 1991

A substantial percentage of DC

regulator requirements involve reduc-

tion or step down of a primary voltage.

Linear regulators do this, but they

don’t achieve the efficiency of switch-

ers. The theory supporting step down

or “buck” switching regulation is well

established, and has been exploited

for some time. However, conveniently

applied ICs allowing practical imple-

mentations haven’t been available. A

new power IC device, the LT1074,

permits broad application of step down

regulators with minimal complexity

and low cost. Further, more complex

step down regulator functions are

possible with it also.

The LT1074 is a 5A bipolar switch-

ing regulator requiring minimal exter-

nal parts for operation. While the block

diagram of Figure 1. reveals a complex

device, basic operation is still fairly

straightforward. A description of the

main circuit elements and their pin

functions is as follows.

The LT1074 uses a special con-

trolled saturation Darlington NPN

output switch, with the emitter out-

The LT1074 Family of Step Down

Switching Regulators by Jim Williams

Figure 1. LT1074/LT1076 Block Diagram

continued on page 8

6V

REGULATOR

AND BIAS

–

-POWER

SHUTDOWN

10 Aµ320 Aµ

–

2.35V

0.3V

CURRENT

LIMIT

SHUTDOWN

4.5V 10k

–

CURRENT

LIMIT

COMP

400

–

SYNC

FREQ*

2.4V

R/S

LATCH

COMOUT*

–

A1

ERROR

AMP

FB V

100kHz

OSCILLATOR

ANALOG

MULTIPLIER

XY

FREQ SHIFT

SYNC

SYNC FREQ BOOST

PULSE WIDTH

COMPARATOR

20V (EQUIVALENT)

2.21V

STATUS*

SWITCH 

OUTPUT

(V )

0.035

250

100Ω

EXTLM*

INPUT SUPPLY

SHUTDOWN* I *

6V TO ALL

CIRCUITRY

LIM

OUTPUT

VOLTAGE

MONITOR

ΩΩ

Ω

*AVAILABLE ONLY ON 11-PIN SIP PACKAGE

Linear Technology Magazine

•

June1991

When the LT1074 (internal) switch

closes, input voltage appears at the

inductor, and current flowing through

the inductor-capacitor combination

builds over time. When the switch

opens, current flow ceases and the

magnetic field around the inductor

collapses. Faraday teaches that the

voltage induced by the collapsing mag-

netic field is opposite to the originally

applied voltage. As such, the voltage

at the inductor’s left end heads nega-

tive, and is clamped by the diode. The

charge accumulated on the capacitor

has no discharge path, leaving an

output DC potential. This potential is

lower than the input, because the

inductor limits current during the

switch on-time.

Ideally, there are no dissipative

elements in this voltage step down

conversion. Although the output volt-

age is lower than the input, there is

no energy lost in this conversion. In

practice, the circuit elements do have

losses, but step down efficiency is

still higher than with inherently dis-

sipative (e.g. voltage divider) ap-

proaches. In this circuit, feedback

controls the switch, to regulate out-

put voltage. The switch on-time (e.g.

inductor charge time) is varied to

maintain the output against changes

in either input or loading.

With respect to a practical circuit

using the LT1074 regulator, some

put at pin VSW. This switch uses an

isolated design, allowing voltage

swings up to 40V below the ground

pin. In addition, the switch also has

a continuous current monitor. The

oscillator of the LT1074 operates at

100kHz, driving the switch through a

control latch. Duty cycle control

comes from a pulse width compara-

tor, which in turn is driven by the

main error amplifier through the ana-

log multiplier. This multiplier allows

a loop gain independent of input volt-

age, optimizing transient response.

The error amp of the LT1074 com-

pares a sample of the output pre-

sented to the FB pin to an internal

2.21V (±2.5%) reference. Loop com-

pensation is accomplished by a simple

RC network at the amplifier output

(VC pin), to ground.

While the above describes the ba-

sic operational loop of the LT1074

design, accessory internal functions

also exist. These are I

LIM

, FREQ, STA-

TUS, COMOUT, SHUTDOWN and

EXTLIM pins (available only in the 11

pin package). As alluded, there are

multiple power packages

used with the LT1074., a 4

lead TO-3 (K), a 5 lead TO-

220 (T), and the 11 lead SIP

package (V), which permits

the optional clock synchro-

nization, micropower shut-

down, current limit pro-

gramming and other fea-

tures. The LT1074 is avail-

able in two basic voltage

grades, the LT1074 for

45V(max) inputs, and the

LT1074HV, usable to 64V.

There is also a 2.5A rated

device, the LT1076.

Applications

Figure 2 is a practical LT1074 volt-

age step down or “buck” circuit, us-

ing minimum componentry. It closely

follows a voltage step down concep-

tual model, described as follows.

continued from page 7

additional new elements appear. The

RC components at the LT1074 VC

pin provide frequency compensation,

stabilizing the feedback loop. Output

sensing resistors R1/R2 are selected

to scale the output to the desired

voltage V

OUT

, generally as noted in

the figure, with values shown in this

case for 5V.

Performance wise, the circuit op-

erates over an input range of 10-40V,

and has a maximum output of 5A.

Efficiency is about 80% at a current

of 1A, while output ripple is about

25mV with the filtering as shown.

With these and other switching regu-

lators, power components are critical

to performance, and should be rated

for switching use at the currents

anticipated.

Regulated negative outputs with

the LT1074 are easily obtained also,

using a simple two terminal induc-

tor. The basic positive to negative

converter of Figure 3 demonstrates

this, essentially grounding the in-

ductor, steering diode current to what

is now a negative output. This design

accomplishes the plus-to-minus DC

level shift by connecting the

LT1074 GND pin direct to

the negative output, requir-

ing an isolated heat sink.

Feedback is sensed from

the grounded positive out-

put terminal, and the regu-

lator again forces its feed-

back pin 2.21V above its GND

pin. Output voltage scaling

is numerically as in Figure 2,

with a negative sign. Circuit

ground is common to input

and output, making system

use easy.

Overall performance is as noted,

and is similar to the positive buck

converter of Figure 2, but with some

unique distinctions. On the plus side,

note that the input/output voltages

of this configuration are seen in se-

continued on page 9

R2**

2.21k

0.1 Fµ

MBR745

R1 **

2.8k

50 H

µ✝

C3*

100 Fµ

LT1074

VSW

VIN

GND

V = 5V

AT 5A

OUT

V = 10V

TO 40V

+C1

500 Fµ

* OPTIONAL - USE IF CONVERTER IS MORE THAN 2"

FROM RAW SUPPLY FILTER CAPACITOR

PULSE ENGINEERING, INC. #PE-92114

✝

A3 • F2

** V = + 1 * 2.21

OUT ( )

Figure 2. LT1074 Step Down Regulator (5V output)

Linear Technology Magazine

•

June 1991

common. This option uses an op amp

as a precision feedback level shifter.

A1 facilitates loop closure, providing

a scaled inversion of the negative

output to the LT1074 FB pin. Preci-

sion resistors R1/R2 set negative

output voltage as noted (values shown

for a -5V output). The VC pin of the

LT1074 is left open, and the RC net-

work around A1 gives frequency com-

pensation.

Advantages of this circuit com-

pared to Fig. 3 is that the LT1074

package can directly contact a

ries by the LT1074, as V

. This has

the effect of allowing a very low abso-

lute level for the positive input, down

to as low as 5V, making the circuit an

efficient 5V to -5V power converter.

On the down side, note in this in-

stance the LT1074 control pins are

floating off ground, presenting some

potential problems with control in-

terfacing (when necessary).

Figure 4, another variant, is used

when it is desirable to operate the

case (GND) pin of the regulator at

continued from page 8

grounded heat sink. Additionally, this

circuit permits ground referred ad-

dressing of the regulator’s control

pins. Disadvantages are that it re-

quires a higher minimum input volt-

age, plus an additional active device..

Higher Output Currents

with Tapped Inductors

Buck (step-down) convertors have

a switch current at least as high as

regulator output current. This limits

LT1074 output current to 5A in the

simple buck convertor topology. A

slightly modified version (shown on

the data sheet) can double available

output current to 10A when input

voltage is a minimum of 20V. The

modified version uses a 3 to 1 tapped

inductor which generates current gain

in the inductor.

During switch “on” time current

flows through the entire inductor to

the output and can have a maximum

value of 5A. When the switch turns

off, the voltage at the tap flies nega-

tive and current flows to the output

through just 1/4 of the inductor.

Energy conservation requires that

this current be four times the current

which was flowing in the entire in-

ductor. Average current delivered to

the output is between 5A and 20A, as

determined by operating switch duty

cycle. For low input voltages, switch

duty cycle is very high, and maxi-

mum output current is only slightly

above 5A. For input voltage above

20V, duty cycle is low enough to

deliver 10A output current.

The voltage on the LT1074 switch

pin flies negative to about 17V during

switch “off” time due to the trans-

former action of the inductor. Leak-

age inductance, however, would

cause, at switch turn-off, the switch

voltage to briefly fly negative without

limit. Clamps are needed to protect

the LT1074.

Figure 4. Positive to Negative Converter with Op Amp Level Shift

1000µ

IN4148

55 H

µ✝

LT1074

L1 = PULSE ENGINEERING, INC. #PE-92116

✝

A3 • F4

* V = – 2.21

OUT

( )

LT1006

12V INPUT

4.7k 0.33

10k 1%

MBR745

V = –5V

OUT

R1*

22.6k

12V

INPUT

–

GND

Fµ

Figure 3. Positive to Negative Converter

C3*

200

A3 • F3

Fµ

12V TO 40V

L1

22 Hµ

†

R1

2.80k

C1**

1000 Fµ

–5V

1.5A

††

R2

2.21k

†

††

OPTIONAL – USE IF CONVERTER IS MORE THAN 2"

FROM RAW SUPPLY FILTER CAPACITOR

LOWER OUTPUT RIPPLE CAN BE OBTAINED BY 

PARALLELING SEVERAL LOWER VALUE CAPACITORS.

AN OUTPUT FILTER OF 5µH, 100µF WILL GIVE

20:1 RIPPLE ATTENUATION WITH AN ESR OF 0.1Ω

ON THE 100µF CAPACITOR

PULSE ENGINEERING, INC. #PE-51590

MAXIMUM OUTPUT CURRENT IS 1.5A AT V

= 5V

3A AT V

= 15V AND 3.5A AT V

= 30V

D1

MBR745

GND FB

LT1074

Linear Technology Magazine

•

June1991

The LT1073 Single Cell

Switching Regulator

components beyond the LT1073-5 it-

self are required for this practical

step-up converter. In this straightfor-

ward circuit, the LT1073-5’s SENSE

pin monitors the output voltage. When

the voltage drops below 5V, the oscil-

lator switches on, causing current to

alternately build in L1, then dump

into C1, raising output voltage. A built-

in small hysteresis precludes need for

frequency stabilization.

The circuit delivers 5V at 40mA for

a cell voltage as low as 1.25V, and at

1V it still delivers 10mA. Conversion

efficiency is 65% as shown, with still

higher efficiency possible using a larger

inductor (at some expense of maxi-

mum output current). Note that for

these circuit types, the diode, induc-

tor and capacitor specified are perfor-

mance-critical “power” components,

so here substitutions aren’t advised.

The same general performance ad-

vantages of this circuit can be ex-

tended to other voltages with the use

of the LT1073, and two external resis-

tors for voltage setting.

tor NPN output (AO) for various ancil-

lary uses. The I

LIM

pin is used to

control current limit, from a maxi-

mum internal limit of 1A downward.

The LT1073 is most notable for its

ability to operate efficiently at low

input voltages, from as low as 1.0V up

to 12V, while consuming less than

100µA. A related device, the LT1173

works over a 2-36V range. Operable in

either boost or buck modes, LT1073

family devices come in 8 pin plastic

DIP or surface mount packages, and

in 3 electrical versions. These are the

LT1073 and LT1173, general purpose

voltage programmable parts, and the

LT1073-5 and LT1073-12, fixed volt-

age versions. The latter come pre-

configured with internal resistors for

output voltages of 5 and 12V with 5%

guaranteed accuracy. In the circuit

examples below are shown the two

basic LT1073 operating modes.

Applications

The LT1073-5 functions nicely as a

1.5V single-cell to 5V boost converter,

shown in Figure 2a. Just three more

Battery-powered electronic equip-

ment continues to become more popu-

lar. Miniaturization levels have in-

creased, due largely to trends toward

surface mount devices and increased

IC functionality. Component sizes have

now been reduced to a point where the

battery can become a significant per-

centage of system volume, forcing size

reduction efforts in this area. The

LT1073, a new micropower switching

regulator circuit, generates a wide

range of output voltages from inputs

as low as 1.5V (single cell). This en-

ables attendant reductions in both

battery volume and net power con-

sumption.

The LT1073 consists of the ele-

ments shown in the functional block

diagram of Figure 1. In operation, it

acts as a gated oscillator switcher,

enabling the oscillator as needed to

maintain a given output voltage. The

regulation loop of the device consists

of a 212mV reference, a comparator,

an oscillator, a driver, and a controlled

saturation NPN output switch. The

uncommitted amplifier A2 is an “Aux-

iliary Gain Block” with an open collec-

L1 = GOWANDA GA10-822k

SW2

SW1

LIM

SENSE

GND

LT1073 – 5

µD1

V = 5V

OUT

1.5V

A1 • F2a

D1 = 1N5818

C1 = SANYO 0S-CON (SAN DIEGO, CA)

Figure 2a. Single Cell to 5V Step Up Converter

Figure 1. LT1073 Block Diagram

continued with “LT1073” on page 11

A1 • F1

GND

SET

GAIN BLOCK/ERROR AMP

212mV

REFERENCE A1

DRIVER

–

SW1

SW2

LIM

OSCILLATOR

COMPARATOR

by Steve Pietkiewicz

Linear Technology Magazine

•

June 1991

The LT1073 shows off its buck mode

versatility in Figure 2b, configured

here as a 9V to 5V step down con-

verter. This circuit deploys the cur-

rent limit feature of the LT1073, used

here to let the converter work with

wide input ranges. With most gated-

oscillator switchers, the switch stays

on for a fixed time period. With input

voltages too high, currents can build

to levels of inductor saturation. Or,

under worst-case conditions, the de-

vice can destruct.

In this circuit, current limiting in

the LT1073 monitors switch current

and turns it off when current reaches

a predetermined level. R3, the 220

ohm resistor between the I

LIM

and V

pins sets this current limit to about

400mA. For a minimum of output

Figure 2b. 9V to 5V Step Down Converter

LT1073 continued from page 10

ripple, the LT1073’s gain block is used

as a pre-amplifier in front of the com-

parator input pin, FB. This measure

reduces output ripple to 100mV p-p.

The output voltage of this buck type

LT1073 converter is programmed by

resistors R2 and R1. As noted in the

diagram, these two resistors scale the

212mV reference up to the final out-

put voltage, or 5V. The circuit delivers

5V at 90mA, working from a 6.5 to

12.6V input.

LTC485 Line Termination by Bob Reay

The termination of the data line

connecting LTC485 transceivers is very

important because an improperly ter-

minated line can cause data errors.

The data line is usually a 120Ω shielded

twisted pair of wires which is termi-

nated at each end with a 120Ω resistor

(Figure 1). For some applications a

problem occurs when the output of

the drivers are forced into a high im-

pedance state because the termina-

tion resistors short the inputs to the

receivers. Since the receivers are dif-

ferential comparators with built in

hysteresis, their output will remain in

the last logic state.

For the applications which must

force the outputs of the receivers to a

known state, but still maintain low

power consumption, the cable can be

terminated as in figure 2. A capacitor

(typically 0.1µF) has been connected

in series with the 120Ω termination

resistor R2, and two bias resistors (R1

and R3) have been added. When data

is being transmitted, the capacitor

looks basically like a short circuit and

a differential signal is developed across

the termination resistor. When the

drivers are forced into a high imped-

ance state, the bias resistors force the

receiver into a logic 1 state. The re-

ceiver inputs can be reversed when

the output must be a logic 0.

Because the capacitor is in series

with the bias string, no DC current

flows when data is not being transmit-

ted. Care must be taken to transmit

data at a high enough data rate to

prevent the bias resistors from charg-

ing the capacitor to the wrong state

before the next data bit arrives. Also

note that differences in the V

sup-

plies or grounds will cause DC current

to flow in the cable, but this can be

kept to a minimum by using high

value bias resistors.

Figure 2. AC Coupled Termination

Figure 1. DC Coupled Termination

120Ω

0.1 F

25kΩ

+5V

120ΩDD

0.1 F

25kΩ

+5V

LTC485 • F2

120Ω

120ΩDD

LTC485 • F1

L1 = GOWANDA GA10-472k

47 H

A1 • F2b

GND

SW2

SET

SW1

LIM

100 F

1N5818

470k

V = 5V

OUT

909k*

40.2k*

LT1073

220

C1 = SANYO OS-CON

* 1% METAL FILM

V = 212mV

OUT

R2 + 1

()

Linear Technology Magazine

•

June1991

Walt Jung and Rich Markell

Among linear applications not usu-

ally seen are those which require high

speed combined with either very low

DC error, or high load current. Such

applications can be solved by combin-

ing the best attributes of two ICs,

either one of which may not be ca-

pable by itself of the entire require-

ment.

A case in point is the line driver of

Figure 1, which uses an LT1122 JFET

input op amp as the gain element

combined with an LT1010 buffer. This

provides the output current of the

LT1010 (typically 150mA) but with

the basic DC and low level AC charac-

teristics of the LT1122. The circuit is

capable of driving loads as low as 100

ohms with very low distortion. The

input referred DC error is the low DC

offset of the LT1122, typically 0.5mV

or less. Large signal characteristics

are also very good, due to the 80V/µs

symmetrical SR of the LT1122.

The circuit as shown is configured

as a precise gain of 5 non-inverting

amplifier by gain set resistors R2 and

R1, with the LT1010 unity gain volt-

age follower inside the overall feed-

back loop. This provides current buff-

ering to the op amp, allowing it to

operate most linearly. Small signal

bandwidth is set by the time constant

of R2 and C1, and is 1MHz as shown,

with a corresponding risetime of about

400ns.

Performance with ±18V supplies is

shown in Figures 2a and 2b, with

output generally 5Vrms or equiva-

lent, driving 100 ohms directly. THD

is shown in Figure 2a, with input level

swept up output clipping level, at a

fixed 10kHz frequency. The distortion

is generally well below 0.01%, and

improves substantially for lower fre-

quencies.

CCIF IM distortion performance

of the circuit for similar loading is

shown in Figure 2b, driving a load of

100 ohms at a swept level, again up

to output clipping. The LT1122 am-

plifier is represented by the lower of

the two curves, with distortion around

the 0.0001% level. Also shown for

comparison in this plot is the distor-

tion of a type 156 JFET op amp (also

driving the LT1010 buffer with other

conditions the same). The 156 op

amp uses a design topology with an

intrinsically asymmetric SR. This

manifests itself as rising even order

distortion for methods such as this

CCIF test. For this example, the dis-

tortion is more than an order of mag-

nitude higher than that of the faster

and symmetric slewing LT1122 for

the same conditions.

Applications of this circuit include

low offset linear buffers such as for

A/D inputs, line drivers for instru-

mentation use, and audio frequency

range buffers such as very high qual-

ity headphone use.

A Fast, Linear, High Current Line Driver

Figure 2b. CCIF IM Distortion vs. Input

Level

Figure 1. Line Driver

Figure 2a. THD vs. Input Level

INPUT LEVEL (V)

0.001

CCIF 1M DISTORTION (%)

0.010

0.1

0.1 1 5

A • F2b

0.0001

INPUT LEVEL (V)

0.1

0.001

TOTAL HARMONIC DISTORTION (%)

0.010

0.1

110

A• F2a

1 F

µ1 F

10k

INPUT

39pF

OUT

100

–

Ω

49.9

(OPTIONAL)

Ω

1 F

µ1 F

µR

LT1010

LT1122

–

BOOST

A • F1

Linear Technology Magazine

•

June 1991

A Single Cell Laser Diode Driver

Using The LT1110

Figure 1. SCSI Active Termination

The active terminator shown in Fig-

ure 1 uses an LT1117 low dropout

three terminal regulator to control a

logic supply. The LT1117's line regu-

lation makes the output immune to

variations in TERMPWR. After ac-

counting for resistor tolerances and

variations in the LT1117's reference

voltage, the absolute variation in the

2.85V output is only 4% over tem-

perature. When the regulator drops

out at TERMPWR-2.85, or 1.25V, the

output linearly tracks the input with a

1V/V slope. The regulator provides

effective signal termination because

the 110 ohm series resistor closely

matches the transmission line's char-

acteristic impedance, and the regula-

tor provides a good AC ground.

In contrast to a passive terminator,

2 LT1117s require half as many termi-

nation resistors, and operate at 1/15

the quiescent current or 20mA. At

these power levels, PC traces provide

adequate heat sinking for the LT1117's

SOT-223 package. Beyond solving ba-

sic signal conditioning problems, this

LT1117 terminator handles fault con-

ditions with short circuit current lim-

iting, thermal shutdown, and on chip

ESD protection.

Steve Pietkiewicz

The Gain Block output of the

LT1110 functions with Q1 as an error

amplifier. The differential inputs com-

pare the photodiode current devel-

oped as a voltage across R2 to the

212mV reference. The amplifier drives

Q1, which modulates current into the

LIM

pin. This varies oscillator frequency

to control average current.

Overall frequency compensation is

provided by R1 and C1, values care-

fully chosen to eliminate power-up

overshoot. The value of current sense

resistor R2 determines the laser diode

power, as shown the 1000 ohms re-

sults in about a 0.8 milliwatt output.

Recently available visible lasers can

be operated from 1.5V supplies, given

appropriate drive cirtcuits. Because

these lasers are exceptionally sensi-

tive to overdrive, power to the laser

must be carefully controlled lest it be

damaged. Over-currents as brief as 2

microseconds can cause damage.

In the circuit of Figure 1, an LT1110

switching regulator serves as the con-

troller within the single-cell powered

laser diode driver. The LT1110 regula-

tor is a high speed LT1073 (see “The

LT1073..” this issue). It is available in

an 8 pin miniature SOIC.

The LT1110 is used here as an FM

controller, driving a PNP power switch

Q2, with a typical “ON” time of 1.5

microseconds. Current in L1 reaches a

peak value of about1.0A. The output

capacitor C2 has been specified for low

ESR, and should not be substituted

(damage to the laser diode may result).

Low Dropout Regulator Simplifies

Active SCSI Terminators Sean Gold

Figure 1. LT1110 Laser Diode Driver Operating from a Single Cell

2.2 H

A2 • F1

100 F

1N5818

4.7k

Ω

220Ω

2N3906

22nF

1.5V

1N4148 R5

2Ω

GND SW2

SET

SW1

LIM

LT1110

MJE210

L1 = TOKO 262LYF - 0076K

C1 = SANYO OS-CON

LZ1 = TOSHIBA TOLD9211

LZ1

0.1 F

CERAMIC

10 F

TANTALUM

LOGIC

SUPPLY

10 F

TANTALUM

LT1117

2.85V

SOT-223

1N5817

CONNECTOR

SCSI

BUS

TERMPWR

110

Ω

110

Ω

110

Ω

110

Ω

A10 • F1

Linear Technology Magazine

•

June1991

Switching regulator post regula-

tion, battery powered apparatus, and

other applications often require low

-V

OUT

, or dropout, linear regula-

tors. For post regulators this is needed

for high efficiency. In battery circuits

lifetime is significantly effected by regu-

lator dropout. The LT1123, a new low

cost reference/control IC, is designed

specifically for cost-effective duty in

such applications. Used in conjunc-

tion with a discrete PNP power tran-

sistor, the 3 lead TO-92 unit allows

very high performance positive leg

regulator designs. The IC contains a

5V bandgap reference, error ampli-

fier, NPN darlington driver, and cir-

cuitry for current and thermal limit-

ing.

A low dropout example is the simple

5V circuit of Fig. 1, using the LT1123

and an MJE1123 silicon PNP. In op-

eration, the LT1123 sinks Q1 base

current through the DRIVE pin, to

servo control the FB (feedback) pin to

5V. R1 provides pull-up current to

turn Q1 off, and R2 is a drive limiter.

The 10µF output capacitor (Cout) pro-

vides frequency compensation. The

LT1123 is designed to tolerate a wide

range of capacitor ESR so that low

cost aluminum electrolytics can be be

used for C

OUT

. If the circuit is located

more than 6 inches from the input

source, the optional 10µF input ca-

pacitor (C

) should be added.

Normally, such configurations re-

quire external protection circuitry.

Here, the MJE1123 has been coopera-

tively designed by Motorola and LTC

for use with the LT1123. The device is

specified for saturation voltage for

currents up to 4 amperes, with base

drive equal to the minimum LT1123

drive current specification. In addi-

tion, the MJE1123 is specified for

min/max beta at high current. Be-

cause of this factor and the defined

LT1123 drive, simple current limiting

is practical. In limit, excessive output

current causes the LT1123 to drive Q1

hard until the LT1123 current limits.

Maximum circuit output current is

then a product the LT1123 current

and the beta of Q1. The foldback char-

acteristic of the LT1123’s drive cur-

rent combined with the MJE1123 beta

and safe area characteristics provide

reliable short circuit limiting. Thermal

limiting can also be accomplished, by

mounting the active devices with good

thermal coupling.

Performance of the circuit is no-

table, as it has lower dropout than any

monolithic regulator. Line and load

Figure 1. The LT1123 5V regulator features ultra-low dropout.

DRIVE

LT1123 FB

GND

10 F

OUT

µ R1

600Ω

INPUT +5V

OUT

* = OPTIONAL (SEE TEXT)

MJE 1123 = MOTOROLA

A8 • F1

MJE1123

20Ω

Figure 2. LT1123 regulator dropout voltage

vs. output current

An LT1123 Ultra Low Dropout 5V

Regulator Jim Williams and Dennis O'Neill

regulation are typically within 5 milli-

volts, and initial accuracy is typically

inside 1%. Additionally, the regulator

is fully short circuit protected, with a

no load quiescent current of 1.3mA.

Figure 2 shows typical circuit drop-

out characteristics, in comparison with

other IC regulators. Even at 5A the

LT1123/MJE1123 circuit dropout is

less than 0.5V, decreasing to only

50mV at 1A. Totally monolithic regu-

lators cannot approach these figures,

primarily because their power tran-

sistors do not offer the MJE1123 com-

bination of high beta and excellent

saturation. For example, dropout is

ten times lower than in 138 types, and

significantly better than all the other

IC types. Because of Q1’s high beta,

base drive loss is only 1-2% of output

current even at high output currents.

This maintains high efficiency under

the low V

-V

OUT

conditions the circuit

will typically see. As an exercise, the

MJE1123 was replaced with a 2N4276

germanium device. This provided even

lower dropout performance, but limit-

ing couldn't be production guaran-

teed without screening.

OUTPUT CURRENT (A)

DROPOUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

12 5

A8 • F2

LT1123/MJE1123

LT1185

LT1084

LT138

LT1123/2N4276

Linear Technology Magazine

•

June 1991

Information furnished by Linear Technology Corporation is

believed to be accurate and reliable. However, no responsi-

bility is assumed for its use. Linear Technology Corporation

makes no representation that the interconnection of its

circuits as described herein will not infringe on existing

patent rights.

New Device Cameos

LT1103 and LT1105: Offline Switch-

ing Regulator Control ICs Need No

Opto Feedback

The LT1103 and LT1105 are

switching regulators designed for 15-

200W offline applications. Using an

external source-driven FET switch,

the LT1103 is optimized for 15-100

watt applications. A unique feedback

technique not requiring opto isola-

tion allows 1% line/load regulation

with the LT1103. Current mode op-

eration provides easy frequency com-

pensation, and the device is well

suited for both transformer isolated

flyback and forward converter to-

pologies. The IC contains the oscilla-

tor, control, and protection circuitry,

and it needs only a few external com-

ponents for a fully functional, effi-

cient offline converter. Internal boot-

strap circuitry requires only 200µA

startup current.

Related to the LT1103 is the com-

panion LT1105, used in FET gate

driven designs. The LT1105 allows

for up to 200W offline converters

using an external sense resistor, with

other features similar to the LT1103.

Both devices come in 11 pin plastic

SIP packages, and the data sheet

includes several offline circuits.

LT1240 Series: High Speed DC-DC

Converter Control ICs

The LT1240 series devices are 8-

pin, current mode, pulse width modu-

lation switching controllers. LT1240

devices are manufactured using LTC’s

high speed process, and are pin com-

patible upgrades to industry stan-

dard 1842/3842 products. LT1240

series units have improvements over

the older counterparts in such areas

as speed, lower quiescent and start-

up currents, and new operational

features. Current spikes in the totem

pole output have been eliminated

and internal delay times reduced,

making 500kHz operation practical.

Several new features contained in

the LT1240 series make use easier

than with previous ICs. These in-

clude leading-edge blanking of the

current sense comparator to prevent

premature tripping, eliminating the

input filter and allowing minimum

delays, trims in the oscillator for

both frequency and sink current (with

these parameters tightly specified),

an output stage which is clamped to

a voltage maximum. The drive and

reference outputs are actively pulled

low during under voltage lockout.

LTC1272: 12-Bit, 3µs, 250kHz Sam-

pling A/D Converter

The LTC1272 A/D is a substan-

tially improved pin compatible up-

grade to the industry standard 12-bit

AD7572. The LTC1272 offers true

single supply operation, improved

conversion time (3µs vs. 5µs), and an

on-chip S/H (transparent to those

not needing it). The device, LTC’s first

parallel out 12-bit A/D, is easily in-

terfaced due to the ability of data

transfer in either two 8-bit bytes or a

single 12-bit word. On chip clock

circuitry supports timing with either

an external crystal or from a TTL/

CMOS clock source up to 4MHz. While

the LTC1272 is faster, it runs on a

single 5V supply with typical power

consumption of only 75mW (includ-

ing S/H, reference, and quick con-

version functions). The LTC1272 re-

places AD7572 and PM7572 units in

demanding new (or old) designs where

the designer has speed and/or power

issues with the older AD7572.

LTC1289: Low Voltage Single Chip

12-bit Data Acquisition System

The LTC1289 is a complete data

acquisition system IC that includes

an A/D, sample and hold, 8-channel

MUX, serial I/O port, all operating on

a single +3V supply. This new prod-

uct features all of the system func-

tions of the LTC1290, but is imple-

mented with a low voltage CMOS

process, with guaranteed operation

down to +2.7V.

Key accuracy specifications of the

LTC1289 include ±1/2 LSB linear-

ity/gain errors, and ±1.5LSB offset

error for the “B” electrical part. In

addition to the above functional fea-

tures, the device’s strength is a 26µs

conversion time, achieved with only

1mA of supply current. This combi-

nation makes the LTC1289 a versa-

tile yet high performance front end

for portable low voltage equipment,

such as 3.3V battery-powered sys-

tems. The LTC1289 currently stands

alone as a data acquisition system

operational on 3V.

For further information on the

above, or any other devices mentioned

in this issue of Linear Technology, use

the reader service card supplied. Or,

call the LTC literature service number,

(800) 637-5545. Ask for pertinent data

sheets and app notes.

By LTC Marketing

Linear Technology Magazine

•

June1991

DESIGN TOOLS

Applications on Disk

NOISE DISK

This IBM-PC (or compatible) progam allows the user to

calculate circuit noise using LTC op amps, determine the

best LTC op amp for a low noise application, display the

noise data for LTC op amps, calculate resistor noise, and

calculate noise using specs for any op amp.

SPICE MACROMODEL DISK

This IBM-PC (or compatible) high density diskette contains

the library of LTC op amp SPICE macromodels. The

models can be used with any version of SPICE for general

analog circuit simulations. The diskette also contains work-

ing circuit examples using the models, and a demonstration

copy of PSPICE

by MicroSim.

FILTERCAD DISK

FilterCAD is a menu-driven filter design aid program which

runs on IBM-PCs (or compatibles). This collection of design

tools will assist in the selection, design, and implementation

of the right switched capacitor filter circuit for the application

at hand. Standard classical filter responses (Butterworth,

Cauer, Chebyshev, etc.) are available, along with a CUS-

TOM mode for more esoteric filter responses. SAVE and

LOAD utilities are used to allow quick performance com-

parisons of competing design solutions. GRAPH mode,

with a ZOOM function, shows overall or fine detail filter

response. Optimization routines adapt filter designs for

best noise performances or lowest distortion. A design time

clock even helps keep track of on-line hours.

Technical Books

Linear Databook — This 1,600 page collection of data

sheets covers op amps, voltage regulators, references,

comparators, filters, PWMs, data conversion and interface

products (bipolar and CMOS), in both commercial and

military grades. The catalog features well over 300 devices.

$10.00

Linear Applications Handbook — 928 pages chock full of

application ideas covered in-depth through 40 Application

Notes and 33 Design Notes. This catalog covers a broad

range of “real world” linear circuitry. In addition to detailed,

systems-oriented circuits, this handbook contains broad

tutorial content together with liberal use of schematics and

scope photography. A special feature in this edition in-

cludes a 22-page section on SPICE macromodels.

$20.00

Monolithic Filter Handbook — This 232 page book comes

with a disk which runs on PCs. Together, the book and disk

assist in the selection, design and implementation of the

right switched capacitor filter circuit. The disk contains

standard filter responses as well as a custom mode. The

handbook contains over 20 data sheets, Design Notes and

Application Notes. $40.00

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