V01N1 - JUNE

LINEAR TECHNOLOGY
JUNE 1991
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
VOLUME I NUMBER 1
Welcome to
Linear Technology
by Bob Swanson, President & CEO
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.
Linear Technology from LTC
Premiers to Analog World
LTC welcomes all readers to the
premier issue of Linear Technology, a
new publication from Linear Technology 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 customers. “Applications” is not necessarily restricted to just circuits, or
pure hardware functions, they can
and will encompass software in support of LTC devices.
We will try to balance the contents
of each issue around an array of features and speciality items, all presenting 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 articles 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
2
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 issue with an array of “New Device Cameos” which will cover recent LTC devices not otherwise featured in an issue.
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 issue 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 mentioned above. In the main Design Feature 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 feedback (transimpedance) amplifier, an
important achievement for LTC in process capability.
Related by process is the article by
George Feliz, on the LT1220 high speed
op amp, also produced with LTC CB
technology.
by Walt Jung
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 functional 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 represents some new achievements for
size and power efficiency, and opens
up new applications with relative supply voltage freedom.
In the Design Idea section we have
interesting articles for circuit collectors. 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 regulator as a laser pointer driver, followed
by Sean Gold's piece on SCSI active
termination. Jim Williams and Dennis O’Neill finish with an LT1123 regulator/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 regulator ICs, the LT1240 series of high
speed DC-DC converter ICs, the
LTC1272 12 bit, 3µs, parallel A/D
converter, and the LTC1289 low voltage single chip 12-bit DAS.
While it does not use an entirely
new process, George Erdi’s LT1122
fast settling JFET input op amp com-
Linear Technology Magazine • June1991
The LT1223, A New High Speed
Current Feedback Amplifier by Bill Gross
Current Feedback Basics
The distinctions of how current
feedback amplifiers differ from voltage feedback amplifiers are not obvious 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. However their behavior in terms of gainbandwidth tradeoffs and large signal
response is another story.
Unlike voltage feedback amplifiers, small signal bandwidth in a current 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
Linear Technology Magazine • June 1991
capacitor. In fact, the amplifier feedback resistor (Rf) from output to inverting input works with internal junction capacitances of the LT1223 to
set the closed loop bandwidth. Even
though the gain set resistor (Rg) from
inverting input to ground works with
the Rf 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
Rf is held constant and gain is changed
via Rg, the Thevenin equivalent resistance changes by the same amount
as the change in gain. From an overall loop standpoint, this change in
feedback attenuation will in fact produce 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 compensatory change in open loop bandwidth, 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 settings as noted, driving 100 ohms.
Shown for comparison is a plot of the
fixed 100MHz gain bandwidth limitation 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 determines the compensation of the LT1223,
bandwidth and transient response can
be optimized for almost every application. When operating on ±15V supplies,
Rf should be 1k ohms or more for
stability, but on ±5V the minimum
value is 680 ohms, because the junction capacitors increase with lower voltage. For either case, larger feedback
resistors can also be used, but will slow
down the LT1223 (which may be desirable 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 resistor, 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 voltage is 3mV (max), and input bias current is 3µA (max). An 10k pot can be
used for (optional) offset null, connected to pins 1 and 5 with wiper to V+
continued with “LT1223”, page 5
60
VOLTAGE FEEDBACK
100MHz GBW
50
VOLTAGE GAIN (dB)
The LT1223 is the first current
feedback amplifier (CFA) from LTC,
and the second amplifier manufactured on the new complementary bipolar (CB) process. As such, it represents achievements for LTC in both
process capability and an entirely
new amplifier type. Current feedback
amplifiers have distinct performance
contrasts with familiar voltage feedback amplifiers (such as conventional
op amps). Op amps are usually designed for excellent DC performance,
often at the expense of AC parameters. Interestingly, current feedback
amplifiers have intrinsic AC advantages over voltage feedback amplifiers, 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.
40
Rg = 10 Ω
30
Rg = 33 Ω
20
Rg = 110 Ω
10
Rg = 470Ω
0
VS = ±15V
Rl = 100 Ω
Rf = 1k Ω
Rg = ∞ Ω
–10
–20
100k
1M
10M
FREQUENCY (Hz)
100M
1G
A6 • F1
Figure 1. LT1223 Voltage Gain vs.
Frequency, Compared to 100MHz Op Amp
3
LT1220 High Speed CB Process
By George Feliz
Op Amp
The LT1220 is the first in a family of
new, high performance amplifiers employing LTC’s advanced complementary 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 voltage sacrifices or other application limitations.
The LT1220 is well suited to virtually any general purpose application
needing extended bandwidth. Examples 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.
Unity-gain stable, the LT1220 op
amp features a 45MHz gain bandwidth 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 currents 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 application, the amplifier is tolerant of
capacitive loading, and can drive 500
ohm loads to ±12V.
For higher closed-loop gain, the
LT1220 family has two other members. 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 compensation 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).
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-follower output buffer maintains this
high gain, even when driving 500
ohms. Low input currents are achieved
by bias cancellation circuitry, meaning 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 accomplished by sensing the load induced
output pole, and adding compensation to the amplifier gain node.
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 configured 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 optimum settling, a 5pF capacitor is used
across the feedback resistor.
Figures 2a and 2b show ± step
settling waveforms at the opamp output as measured at a false sum node
between the output and a -10V reference, 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 additional DC accuracy, an optional nulling amplifier can be used to drive the
LT1220 offset adjust pins to reduce
offset voltage change over temperature.
A SPICE macromodel for the LT1220
is included in the LTC op amp library.
continued with “LT1220”, page 5
5pF
5k*
DAC
INPUT
0-2mA
12
AD565
0-10V OUTPUT
–
MEASUREMENT
NODE
2k
LT1220
TO SCOPE
FET BUFFER AND
GAIN STAGE
+
2k
VREF
–10V
*5kΩ RESISTOR IS INTERNAL TO DAC
A5 • F1
Figure 1. LT1220 High Speed I/V Converter
4
Linear Technology Magazine • June1991
LT1220 continued from page 3
100ns/DIV
INPUT
TO DAC
INPUT
TO DAC
OUTPUT
1LSB/DIV
OUTPUT
1LSB/DIV
100ns/DIV
A5 • F2a
Figure 2a. Positive Step
A5 • F2b
Figure 2b. Negative Step
LT1223 continued from page 3
(like the LT1056 scheme). This trim
shifts inverting input current about
±10µA, effectively producing input voltage 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 current, 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.
Applications
Figure 2 shows the LT1223 in a
typical video cable driver application. With Rf = Rg, 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 often 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 buffers, fast settling I/V converters, fiber
optic LED drivers, photo-diode amplifiers, Rf and IF amplifiers, radar IF
processing, and high gain preamps
which must retain maximum bandwidth per stage.
+
V IN
75Ω
LT1223
Table 1. Current Feedback Amplifier Comparison
Parameter
LT1223
EL2020
BW (MHz)
SR (V/µs)
IO (mA, Min)
VOS (mV, Max)
–IB (µA, Max)
+IB (µA, Max)
R OL (MΩ, Min)
100
1000
50
3
3
3
1.5
50
500
30
10
40
15
0.3
Linear Technology Magazine • June 1991
–
1k
Rf
75Ω
CABLE
VOUT
1k
Rg
75Ω
A6 • F2
Figure 2. LT1223 Video Cable Driver
5
The LT1122 Fast Settling
By George Erdi and Walt Jung
JFET Op Amp
The LT1122 is a new, high performance JFET input op amp. Optimized around high speed and fast
settling performance, the LT1122 uses
a modified bipolar-FET process. The
performance resulting is a combination of not just excellent AC parameters, but low input currents and DC
precision, achieved within a junctionisolated process.
Unity-gain stable, the LT1122 features 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 specifications 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) respectively, and an input offset voltage of
0.6mV(max). For driving difficult
loads, the LT1222 has a 40mA current limit, and can drive 600 ohm
loads to ±12V(min).
The LT1122 achieves these combined parameters with a unique polygate JFET. In conventional FET technology, the gate contact is tens of
micrometers away from the actual
channel of the FET, creating a response 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 minimizes gate-drain capacitance.
Atop these and other speed related
circuit improvements, the LT1122 has
very low audio range distortion. For
6
example, THD at an inverting gain of Q12 sources load current greater than
10 is 0.001% or less to 20kHz, with Ii, the control loop turns off, and Q13
non-inverting performance only can then no longer return to sinking
slightly worse. For IM distortion via load current immediately. If unthe CCIF method, the LT1122 has checked, this would lead to crossover
performance as much as 2 orders of glitches, when the output must
magnitude better than typical indus- quickly change from sourcing to sinktry JFET amplifiers such as 156/356 ing load current. Here however, the
types. The low total distortion is largely problem is solved by Q7, an addidue to two design factors. One is the tional main loop drive input which
LT1122 SR
which is not
V+
only high at
R6
80V/µs, but
I3
Ii
which also is
J8
intrinsically
Q11
symmetrical.
This factor
Q12
Q9
eliminates the
even order disR9
tortion effects
present in a toVOUT
pology with
Q10
asymmetric
SIGNAL FROM
transconductQ6
PREVIOUS STAGES
Q13
Q7
ance. Another
factor is the linI4
R8
R7
ear all NPN output
stage,
V–
which features
Figure 1. LT1122 Output Stage
high output
current and very high speed.
can provide the short term sink current required. This allows Q13 to turn
This LT1122 output stage is shown
back on more slowly, but without
in Figure 1 in simplified form, and has
adding distortion.
several features which contribute to
high linearity. One is the local loop
Fast settling is the main feature of
which controls the idle current in the LT1122, and is 100% tested in a
Q12 (Ii). This loop is comprised of settling time test fixture described on
Q12, R9, Q9, R6, J6, Q10 and Q13, the data sheet. The LT1122 is availand it causes the output follower Q12 able in 4 electrical grades, 2 premium
to always conduct the idle current or (A & B) and 2 standard (C & D). Of
more, so providing a low output im- these, the A & C parts are 100% tested
pedance. Without precautions how- for settling, with the others (B & D)
ever, this type of stage can also distort sample tested.
for some conditions. If for example,
A4 • F1
Linear Technology Magazine • June1991
The LT1074 Family of Step Down
by Jim Williams
Switching Regulators
A substantial percentage of DC
regulator requirements involve reduction or step down of a primary voltage.
Linear regulators do this, but they
don’t achieve the efficiency of switchers. 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 switching regulator requiring minimal external 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 controlled saturation Darlington NPN
output switch, with the emitter outcontinued on page 8
INPUT SUPPLY
320 µ A
10 µ A
0.3V
+
µ-POWER
SHUTDOWN
–
6V
REGULATOR
AND BIAS
6V TO ALL
CIRCUITRY
CURRENT
LIMIT
COMP
CURRENT
LIMIT
SHUTDOWN
2.35V
+
0.035
+
C2
250 Ω
–
–
100Ω
I LIM*
SHUTDOWN*
4.5V
10k
EXTLM*
2.4V
–
SYNC
FREQ*
+
SYNC FREQ BOOST
FREQ SHIFT
100kHz
OSCILLATOR
S
SYNC
OUTPUT
VOLTAGE
MONITOR
R
R/S
Q
LATCH
R
VIN
G1
COMOUT*
+
+
STATUS*
2.21V
–
Z
ANALOG
X MULTIPLIER
XY
Z
Y
A1
ERROR
AMP
FB
VC
20V (EQUIVALENT)
400 Ω
15 Ω
C1
–
PULSE WIDTH
COMPARATOR
SWITCH
OUTPUT
(VSW )
*AVAILABLE ONLY ON 11-PIN SIP PACKAGE
Figure 1. LT1074/LT1076 Block Diagram
Linear Technology Magazine • June 1991
7
continued from page 7
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 comparator, which in turn is driven by the
main error amplifier through the analog multiplier. This multiplier allows
a loop gain independent of input voltage, optimizing transient response.
The error amp of the LT1074 compares a sample of the output presented to the FB pin to an internal
2.21V (±2.5%) reference. Loop compensation is accomplished by a simple
RC network at the amplifier output
(VC pin), to ground.
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 magnetic field is opposite to the originally
applied voltage. As such, the voltage
at the inductor’s left end heads negative, 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 voltage 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 dissipative (e.g. voltage divider) approaches. In this circuit, feedback
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 VOUT, generally as noted in
the figure, with values shown in this
case for 5V.
Performance wise, the circuit operates 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 regulators, 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 inductor. The basic positive to negative
converter of Figure 3 demonstrates
this, essentially grounding the inductor, 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
VOUT = 5V
AT 5A
the negative output, requiring an isolated heat sink.
While the above describes the basic operational loop of the LT1074
design, accessory internal functions
also exist. These are ILIM, FREQ, STATUS, COMOUT, SHUTDOWN and
EXTLIM pins (available only in the 11
pin package). As alluded, there are
multiple power packages
L1✝
used with the LT1074., a 4 V = 10V
50µH
IN
VIN
VSW
TO 40V
lead TO-3 (K), a 5 lead TOR1 **
2.8k
220 (T), and the 11 lead SIP
MBR745
LT1074
1%
package (V), which permits
FB
Feedback is sensed from
the optional clock synchroVC
GND
the
grounded positive outnization, micropower shutR2**
R3
2.21k
put
terminal,
and the regudown, current limit pro2k
1%
lator again forces its feed+ C2
+ C1
+ C3*
gramming and other feaback pin 2.21V above its GND
100µF
0.1µF
500µF
tures. The LT1074 is availpin. Output voltage scaling
able in two basic voltage
* OPTIONAL - USE IF CONVERTER IS MORE THAN 2"
FROM RAW SUPPLY FILTER CAPACITOR
is numerically as in Figure 2,
grades, the LT1074 for
✝ PULSE ENGINEERING, INC. #PE-92114
with a negative sign. Circuit
45V(max) inputs, and the
** VOUT =(R1 + 1)* 2.21
R2
ground is common to input
LT1074HV, usable to 64V.
and output, making system
Figure 2. LT1074 Step Down Regulator (5V output)
There is also a 2.5A rated
use easy.
device, the LT1076.
controls the switch, to regulate outOverall performance is as noted,
put voltage. The switch on-time (e.g. and is similar to the positive buck
Applications
inductor charge time) is varied to
Figure 2 is a practical LT1074 volt- maintain the output against changes converter of Figure 2, but with some
unique distinctions. On the plus side,
age step down or “buck” circuit, us- in either input or loading.
note that the input/output voltages
ing minimum componentry. It closely
With respect to a practical circuit of this configuration are seen in sefollows a voltage step down concepusing the LT1074 regulator, some
continued on page 9
tual model, described as follows.
A3 • F2
8
Linear Technology Magazine • June1991
continued from page 8
ries by the LT1074, as VIN. This has
the effect of allowing a very low absolute 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 instance the LT1074 control pins are
floating off ground, presenting some
potential problems with control interfacing (when necessary).
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. Precision 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 network around A1 gives frequency compensation.
Figure 4, another variant, is used
when it is desirable to operate the
case (GND) pin of the regulator at
Advantages of this circuit compared to Fig. 3 is that the LT1074
package can directly contact a
+
C3*
200µF
L1†
22 µ H
12V TO 40V
VIN
R1
2.80k
VSW
grounded heat sink. Additionally, this
circuit permits ground referred addressing of the regulator’s control
pins. Disadvantages are that it requires a higher minimum input voltage, 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.
LT1074
GND
VC
FB
D1
MBR745
+
C2
C1**
1000 µF
R2
2.21k
R3
–5V ††
1.5A
A3 • F3
*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 VIN = 5V
3A AT VIN = 15V AND 3.5A AT VIN = 30V
Figure 3. Positive to Negative Converter
MBR745
12V
INPUT
VOUT = –5V
L1✝
55µH
VSW
VIN
4.7k
LT1074
VC
GND
+
0.33µ F
The voltage on the LT1074 switch
pin flies negative to about 17V during
switch “off” time due to the transformer action of the inductor. Leakage inductance, however, would
cause, at switch turn-off, the switch
voltage to briefly fly negative without
limit. Clamps are needed to protect
the LT1074.
R1*
22.6k
1%
R2
10k 1%
FB
1000µF
12V INPUT
–
✝ L1 = PULSE ENGINEERING, INC. #PE-92116
* VOUT = –
IN4148
LT1006
NC
A1
+
2.21)
( R1
R2 *
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 negative 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 inductor. 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 maximum output current is only slightly
above 5A. For input voltage above
20V, duty cycle is low enough to
deliver 10A output current.
A3 • F4
Figure 4. Positive to Negative Converter with Op Amp Level Shift
Linear Technology Magazine • June 1991
9
The LT1073 Single Cell
Switching Regulator
Battery-powered electronic equipment continues to become more popular. Miniaturization levels have increased, 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 percentage 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 enables attendant reductions in both
battery volume and net power consumption.
The LT1073 consists of the elements 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 “Auxiliary Gain Block” with an open collec-
tor NPN output (AO) for various ancillary uses. The ILIM pin is used to
control current limit, from a maximum 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 voltage versions. The latter come preconfigured 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
components beyond the LT1073-5 itself are required for this practical
step-up converter. In this straightforward circuit, the LT1073-5’s SENSE
pin monitors the output voltage. When
the voltage drops below 5V, the oscillator switches on, causing current to
alternately build in L1, then dump
into C1, raising output voltage. A builtin 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 maximum output current). Note that for
these circuit types, the diode, inductor and capacitor specified are performance-critical “power” components,
so here substitutions aren’t advised.
The same general performance advantages of this circuit can be extended to other voltages with the use
of the LT1073, and two external resistors for voltage setting.
continued with “LT1073” on page 11
L1
82µH
+
SET
1.5V
AO
–
V IN
D1
VOUT = 5V
A2
ILIM
GAIN BLOCK/ERROR AMP
I LIM
SW1
212mV
REFERENCE
A1
COMPARATOR
FB
Figure 1. LT1073 Block Diagram
V IN
SW1
+
C1
LT1073 – 5
GND
OSCILLATOR
GND
10
by Steve Pietkiewicz
SENSE
SW2
Q1
DRIVER
SW2
A1 • F1
L1 = GOWANDA GA10-822k
D1 = 1N5818
C1 = SANYO 0S-CON (SAN DIEGO, CA)
A1 • F2a
Figure 2a. Single Cell to 5V Step Up Converter
Linear Technology Magazine • June1991
LTC485 Line Termination
The termination of the data line
connecting LTC485 transceivers is very
important because an improperly terminated line can cause data errors.
The data line is usually a 120Ω shielded
twisted pair of wires which is terminated 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 impedance state because the termina-
DI
RO
tion resistors short the inputs to the
receivers. Since the receivers are differential 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
120Ω
D
120Ω
D
R
+5V
DI
D
+5V
R1
25kΩ
R2
120Ω
R2
120Ω
0.1µ F
RO
R
RO
LTC485 • F1
R1
25kΩ
D
DI
0.1µ F
R3
25kΩ
R3
25kΩ
RO
R
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 impedance state, the bias resistors force the
receiver into a logic 1 state. The receiver 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 transmitted. Care must be taken to transmit
data at a high enough data rate to
prevent the bias resistors from charging the capacitor to the wrong state
before the next data bit arrives. Also
note that differences in the V + supplies 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.
DI
R
Figure 1. DC Coupled Termination
by Bob Reay
LTC485 • F2
Figure 2. AC Coupled Termination
LT1073 continued from page 10
The LT1073 shows off its buck mode
versatility in Figure 2b, configured
here as a 9V to 5V step down converter. This circuit deploys the current limit feature of the LT1073, used
here to let the converter work with
wide input ranges. With most gatedoscillator 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 device 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 ILIM and VIN
pins sets this current limit to about
400mA. For a minimum of output
Linear Technology Magazine • June 1991
ripple, the LT1073’s gain block is used
as a pre-amplifier in front of the comparator 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 output voltage, or 5V. The circuit delivers
5V at 90mA, working from a 6.5 to
12.6V input.
L1
47µH
9VIN
VOUT = 5V
R4
470k
R3
220
I LIM
FB
C2
22µF
+
VIN
SW1
LT1073
AO
GND
D1
1N5818
C1
100 µ F
+
R2
909k*
SW2
SET
R1
40.2k*
L1 = GOWANDA GA10-472k
C1 = SANYO OS-CON
* 1% METAL FILM
VOUT = 212mV
(R2R1 + 1)
A1 • F2b
Figure 2b. 9V to 5V Step Down Converter
11
A Fast, Linear, High Current Line Driver
Walt Jung and Rich Markell
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 characteristics 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 feedback loop. This provides current buffering 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 equivalent, 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 frequencies.
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 amplifier 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 distortion 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 distortion is more than an order of magnitude 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 instrumentation use, and audio frequency
range buffers such as very high quality headphone use.
1
TOTAL HARMONIC DISTORTION (%)
Among linear applications not usually seen are those which require high
speed combined with either very low
DC error, or high load current. Such
applications can be solved by combining the best attributes of two ICs,
either one of which may not be capable by itself of the entire requirement.
0.1
0.010
0.001
0.1
V+
1
10
INPUT LEVEL (V)
A• F2a
R3
100 Ω
1%
+
INPUT
U1
LT1122
R IN
10k
1µ F
R BOOST
30
1%
R4
49.9 Ω
1%
(OPTIONAL)
U2
LT1010
VOUT
–
1µ F
1µ F
C1
39pF
V–
0.1
0.010
0.001
R2
4k
1%
0.0001
0.1
R1
1k
1
5
INPUT LEVEL (V)
A • F1
Figure 1. Line Driver
12
1
CCIF 1M DISTORTION (%)
1µ F
Figure 2a. THD vs. Input Level
A • F2b
Figure 2b. CCIF IM Distortion vs. Input
Level
Linear Technology Magazine • June1991
A Single Cell Laser Diode Driver
Steve Pietkiewicz
Using The LT1110
Recently available visible lasers can
be operated from 1.5V supplies, given
appropriate drive cirtcuits. Because
these lasers are exceptionally sensitive 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 controller within the single-cell powered
laser diode driver. The LT1110 regulator is a high speed LT1073 (see “The
LT1073..” this issue). It is available in
an 8 pin miniature SOIC.
The Gain Block output of the
LT1110 functions with Q1 as an error
amplifier. The differential inputs compare the photodiode current developed as a voltage across R2 to the
212mV reference. The amplifier drives
Q1, which modulates current into the
ILIM pin. This varies oscillator frequency
to control average current.
Overall frequency compensation is
provided by R1 and C1, values carefully chosen to eliminate power-up
overshoot. The value of current sense
resistor R2 determines the laser diode
power, as shown the 1000 ohms results in about a 0.8 milliwatt output.
LZ1
R1
4.7k
C1
22nF
D1
1N4148
I LIM
AO
1.5V
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).
R3
220Ω
Q1
2N3906
Q2
MJE210
+
R4
10Ω
V IN
SW1
C2
100 µ F
R5
2Ω
D2
1N5818
LT1110
FB
GND
SET
SW2
R1
1k
L1
2.2 µ H
L1 = TOKO 262LYF - 0076K
C1 = SANYO OS-CON
LZ1 = TOSHIBA TOLD9211
A2 • F1
Figure 1. LT1110 Laser Diode Driver Operating from a Single Cell
Low Dropout Regulator Simplifies
Active SCSI Terminators
The active terminator shown in Figure 1 uses an LT1117 low dropout
three terminal regulator to control a
logic supply. The LT1117's line regulation makes the output immune to
variations in TERMPWR. After accounting for resistor tolerances and
variations in the LT1117's reference
voltage, the absolute variation in the
2.85V output is only 4% over temperature. 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 characteristic impedance, and the regulator provides a good AC ground.
Linear Technology Magazine • June 1991
In contrast to a passive terminator,
2 LT1117s require half as many termination 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 basic signal conditioning problems, this
LT1117 terminator handles fault conditions with short circuit current limiting, thermal shutdown, and on chip
ESD protection.
CONNECTOR
TERMPWR
5V
LOGIC
SUPPLY
1N5817
LT1117
2.85V
SOT-223
10 µ F
TANTALUM
Sean Gold
110Ω
2%
110Ω
0.1 µ F
CERAMIC
10 µ F
TANTALUM
SCSI
BUS
110Ω
2%
110Ω
A10 • F1
Figure 1. SCSI Active Termination
13
An LT1123 Ultra Low Dropout 5V
Jim Williams and Dennis O'Neill
Regulator
A low dropout example is the simple
5V circuit of Fig. 1, using the LT1123
and an MJE1123 silicon PNP. In operation, 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) provides 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 COUT. If the circuit is located
more than 6 inches from the input regulation are typically within 5 millisource, the optional 10µF input ca- volts, and initial accuracy is typically
inside 1%. Additionally, the regulator
pacitor (CIN) should be added.
is fully short circuit protected, with a
Normally, such configurations reno load quiescent current of 1.3mA.
quire external protection circuitry.
Figure 2 shows typical circuit dropHere, the MJE1123 has been cooperatively designed by Motorola and LTC out characteristics, in comparison with
for use with the LT1123. The device is other IC regulators. Even at 5A the
specified for saturation voltage for LT1123/MJE1123 circuit dropout is
currents up to 4 amperes, with base less than 0.5V, decreasing to only
drive equal to the minimum LT1123 50mV at 1A. Totally monolithic regudrive current specification. In addi- lators cannot approach these figures,
tion, the MJE1123 is specified for primarily because their power tranmin/max beta at high current. Be- sistors do not offer the MJE1123 comcause of this factor and the defined bination of high beta and excellent
LT1123 drive, simple current limiting saturation. For example, dropout is
is practical. In limit, excessive output ten times lower than in 138 types, and
current causes the LT1123 to drive Q1 significantly better than all the other
hard until the LT1123 current limits. IC types. Because of Q1’s high beta,
Maximum circuit output current is base drive loss is only 1-2% of output
then a product the LT1123 current current even at high output currents.
and the beta of Q1. The foldback char- This maintains high efficiency under
acteristic of the LT1123’s drive cur- the low VIN-VOUT conditions the circuit
rent combined with the MJE1123 beta will typically see. As an exercise, the
and safe area characteristics provide MJE1123 was replaced with a 2N4276
reliable short circuit limiting. Thermal germanium device. This provided even
limiting can also be accomplished, by lower dropout performance, but limitmounting the active devices with good ing couldn't be production guaranthermal coupling.
teed without screening.
Performance of the circuit is notable, as it has lower dropout than any
monolithic regulator. Line and load
3.0
2.5
Q1
MJE1123
INPUT
+
CIN
10µF
+5VOUT
+
R1
600Ω
COUT
10µF
R2
20Ω
DRIVE
DROPOUT VOLTAGE (V)
Switching regulator post regulation, battery powered apparatus, and
other applications often require low
VIN-VOUT, or dropout, linear regulators. For post regulators this is needed
for high efficiency. In battery circuits
lifetime is significantly effected by regulator dropout. The LT1123, a new low
cost reference/control IC, is designed
specifically for cost-effective duty in
such applications. Used in conjunction with a discrete PNP power transistor, the 3 lead TO-92 unit allows
very high performance positive leg
regulator designs. The IC contains a
5V bandgap reference, error amplifier, NPN darlington driver, and circuitry for current and thermal limiting.
LT138
2.0
1.5
LT1084
1.0
LT1123/MJE1123
U1
FB
LT1123
0
0
GND
* = OPTIONAL (SEE TEXT)
MJE 1123 = MOTOROLA
1
2
3
4
5
OUTPUT CURRENT (A)
A8 • F2
A8 • F1
Figure 1. The LT1123 5V regulator features ultra-low dropout.
14
LT1123/2N4276
LT1185
0.5
Figure 2. LT1123 regulator dropout voltage
vs. output current
Linear Technology Magazine • June1991
New Device Cameos
LT1103 and LT1105: Offline Switching Regulator Control ICs Need No
Opto Feedback
The LT1103 and LT1105 are
switching regulators designed for 15200W 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 isolation allows 1% line/load regulation
with the LT1103. Current mode operation provides easy frequency compensation, and the device is well
suited for both transformer isolated
flyback and forward converter topologies. The IC contains the oscillator, control, and protection circuitry,
and it needs only a few external components for a fully functional, efficient offline converter. Internal bootstrap circuitry requires only 200µA
startup current.
Related to the LT1103 is the companion 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 8pin, current mode, pulse width modulation switching controllers. LT1240
devices are manufactured using LTC’s
high speed process, and are pin compatible upgrades to industry standard 1842/3842 products. LT1240
series units have improvements over
the older counterparts in such areas
as speed, lower quiescent and startup currents, and new operational
Linear Technology Magazine • June 1991
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 include 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 Sampling A/D Converter
The LTC1272 A/D is a substantially improved pin compatible upgrade 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 interfaced 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 (including S/H, reference, and quick conversion functions). The LTC1272 replaces AD7572 and PM7572 units in
demanding new (or old) designs where
the designer has speed and/or power
issues with the older AD7572.
By LTC Marketing
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 product features all of the system functions of the LTC1290, but is implemented with a low voltage CMOS
process, with guaranteed operation
down to +2.7V.
Key accuracy specifications of the
LTC1289 include ±1/2 LSB linearity/gain errors, and ±1.5LSB offset
error for the “B” electrical part. In
addition to the above functional features, the device’s strength is a 26µs
conversion time, achieved with only
1mA of supply current. This combination makes the LTC1289 a versatile yet high performance front end
for portable low voltage equipment,
such as 3.3V battery-powered systems. 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.
Information furnished by Linear Technology Corporation is
believed to be accurate and reliable. However, no responsibility 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.
15
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 working circuit examples using the models, and a demonstration
copy of PSPICETM 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 CUSTOM mode for more esoteric filter responses. SAVE and
LOAD utilities are used to allow quick performance comparisons 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.
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Linear Databook — This 1,600 page collection of data
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scope photography. A special feature in this edition includes 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
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Application Notes.
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16
© 1991 Linear Technology Corporation/ Printed in U.S.A./125M
Linear Technology Magazine • June1991