1
Linear Technology Chronicle •
September 1998
A Showcase of Linear T echnology’ s Focus Products
Linear Technology Chronicle
September 1998 Vol. 7 No. 9
Product of the Month
LT1394: 7ns Comparator
Requires Only 6mA
Supply Current
comparators. The LT1394 is easy to use,
thanks to its stability, single supply
capability and complementary outputs.
For a data sheet and evaluation samples,
contact your local Linear Technology sales
office. Fore more information, visit out web
side at www.linear-tech.com.
LTC1649: High Efficiency Step-Down DC/DC Controller Operates from 3.3V Supply............ 2
LTC1298/LTC1446: Micropower ADC and DAC in SO-8 Give PC 12-Bit Analog Interface .....3
LT1638/LT1639: 200µA, 1.2MHz Rail-to-Rail Op Amps with Over-The-TopTM Inputs..............3
Publications Printed in June and July 1998 ............................................................................ 4
Inside This Issue:
A new ultrahigh speed comparator, the
LT
®
1394, features TTL-compatible comple-
mentary outputs and 7ns response time. The
LT1394 also provides single supply opera-
tion, ground-sensing capability, low power,
high gain and low offset voltage. The
LT1394 is much less prone to oscillation and
other vagaries, even with slow transition
input signals, than previous comparators.
The Underrated Linear Component
The versatility of the IC op amp has
dominated the linear design world while
comparators are perceived as crude devices
with limited usefulness. The LT1394 helps
to open new options to linear circuit design-
ers. “High speed comparators can be used to
implement linear circuit functions which are
as sophisticated as any op amp-based circuit.
Judiciously combining a fast comparator
with op amps is a key to achieving high per-
formance results,” claimed staff scientist Jim
Williams (see Application Note 72).
Proof in the Performance
Nanosecond domain linear circuits are
widely associated with oscillations, mysteri-
ous shifts in circuit characteristics and
unintended modes of operation. The LT1394
is stable in its linear region. Output stage
switching does not appreciably change
power supply current, further enhancing sta-
bility. Current consumption is far lower than
previous devices. These features make the
200GHz gain bandwidth LT1394 consider-
ably easier to use than other fast
comparators.
LTC1258-2.5: Ultralow
Power, Low Dropout
Series Reference
The LTC
®
1258-2.5 is a micropower
bandgap reference that combines high accu-
racy and low drift with very low supply
current and small package size. Low quies-
cent current, 4µA, plus low dropout voltage
of only 200mV make it ideal for 3V and
battery-powered equipment.
A Good Reference
The LTC1258 uses curvature compen-
sation to obtain low temperature coefficient
and trimmed thin-film resistors to achieve
high output accuracy. The reference can
supply up to 10mA and sink up to 2mA,
making it ideal for precision regulator appli-
cations. Initial accuracy is ±0.15%
maximum, good enough to eliminate system
trimming in many applications.
The LTC1258-2.5 is stable without an
output bypass capacitor, but is also stable
with capacitance up to 1µF. This feature is
important in critical applications where
board space is a premium and fast settling is
demanded (see Figure 1).
A “Series” Reference
Series references provide power dissi-
pation advantages over shunt style
references. To operate, shunt references re-
quire a resistor between the power supply
and the output, chosen to supply the maxi-
mum current that is demanded by the circuit
,
LTC and LT are registered trademarks of Linear Technology
Corporation. Over-The-Top, UltraFast and Hot Swap are trade-
marks of Linear Technology Corporation. Pentium is a
registered trademark of Intel Corp. I
2
C is a trademark of Philips
Electronics N.V.
The LT1394 operates on 6mA from a
single 5V supply and delivers 7ns response
with an input range that extends from ground
to 3.5V (see Figure 1). It features low propa-
gation delay, low quiescent current and the
ability to accept low voltage input signals
without amplification or level shifting. No
minimum input slew rate requirement and
low offset voltage (0.8mV) are additional
features. Inputs can exceed the supplies with-
out phase reversal of the outputs.
Figure 1. The LT1394 at a Glance
–
+
LT1394
7
1
4
3
5
V
–
V
+
6
8
9
QOUTPUTS ARE STABLE WHEN THE LT1394
IS IN ITS LINEAR REGION,
REGARDLESS OF HOW SLOWLY THE
INPUT SIGNALS ARE CHANGING
PROP DELAY: 100mV STEP
5mV OVERDRIVE: 7ns TYP, 9ns MAX
DIFFERENTIAL PROP DELAY: 2ns MAX
INPUT OFFSET: 2mV MAX
INPUT OFFSET DRIFT: 2µV/°C TYP
INPUT BIAS CURRENT: 1µA TYP
COMMON MODE RANGE: +V – 1.5V/ –V
GAIN: 1400 MIN
POWER SUPPLY RANGE:
±7V MAX (12V, V
+
/V
–
)
CURRENT CONSUMPTION: 7mA TYP
Q
AN72 F01
Civilized Speed at Work
“The LT1394 permits fast linear circuit
functions that are difficult or impractical us-
ing other approaches,” according to
Williams. Applications include high perfor-
mance NTSC crystal oscillators, single
supply voltage-to-frequency converters and
high speed, high accuracy level detectors.
Other applications include logic switchable
and voltage-controlled crystal oscillators,
tunable clock skew generators, voltage-
controlled delay functions and fast pulse
stretchers.
Innovative circuit design, coupled with
a new 6GHz complementary bipolar process,
has enabled a new benchmark of high speed
Continued on page 2
2
Linear Technology Chronicle •
September 1998
LTC1649: High Efficiency
Step-Down DC/DC
Controller Operates from
3.3V Supply
gate capacitances. The LTC1649 operates
from a 2.7V to 5V supply and provides an
output from 1.27V to 2.5V. Proprietary con-
trol circuitry allows adjustable output current
limiting without requiring a current sense
resistor and provides ±1% output voltage
regulation over line, load and temperature
variations.
Constant Frequency, Synchronous
Switching
The LTC1649 is a voltage feedback
PWM switching regulator controller
designed for use in high power, low input
voltage step-down (buck) converters. It
includes an onboard PWM generator, a pre-
cision reference trimmed to ±0.5%, two high
power MOSFET gate drivers and all neces-
sary control circuitry. It is capable of
delivering output loads from 1A up to 20A
with high efficiency. The constant fre-
quency, 200kHz design minimizes external
component count and size. The controller
may be synchronized to an external clock
source between 260kHz and 500kHz. Other
features include a built-in soft start feature
and thermal protection.
The LTC1649 uses a synchronous
switching architecture, with MOSFET Q3
taking the place of the diode in a classic
buck circuit (Figure 1). This improves effi-
ciency by reducing the voltage drop across
Q3 as compared to the VF of the diode. In
addition, Q3 can conduct current in either
direction. The ability to sink current at the
output allows the circuit to be used with
reactive or other unconventional loads.
The LTC1649 features a shutdown
mode that reduces supply current below
25µA when the SHDN pin is taken low. In
shutdown, the external MOSFET drivers
both go low, keeping the external MOSFETs
off and isolating the output from the input
supply. The CPOUT voltage remains regu-
lated at 5V in shutdown and can be used as a
keep-alive supply.
Applications for LTC1649
Ideal applications for the LTC1649 are
telecommunications supplies: base station
switching, network or ethernet routers; low
voltage CPU and DSP supplies in 3.3V-only
systems; local regulation for distributed
power supplies and high power 3.3V to
1.27V- 2.5V conversion.
Contact your local Linear Technology
sales office for a data sheet and evaluation
samples. For more information, visit our
web site at www. linear-tech.com.
The LTC1649 is the first high power
step-down DC/DC controller that operates
from a 3.3V supply without requiring a sec-
ondary higher supply voltage. An onboard
charge pump generates a 5V source to fully
enhance the external MOSFET gates from an
input as low as 2.7V. This enables the
LTC1649 to operate at greater than 90%
efficiency from a 3.3V supply at loads from
1A to 10A using standard logic level
MOSFETs.
Who Needs 5V?
3.3V-only systems are becoming more
common. Many systems use “bricks” to
transform –48V to 3.3V bulk supplies and
have no other source readily available. The
LTC1649 eliminates the need to separately
generate a boosted 5V supply to get good
efficiency. The LTC1649’s on-chip drivers
drive all N-channel 5V MOSFETs with large
LTC1258-2.5 from page 1
Figure 1. 3.3V to 2.5V, 15A Converter
supply as low as 2.7V, making it ideal for
3V systems.
Low power, portable and 3V systems
such as are used in data acquisition or
handheld instrumentation are great places to
use the LTC1258-2.5’s advantages:
micropower, low dropout voltage, small
board space and fast settling.
For a data sheet and evaluation samples,
contact your local Linear Technology sales
office. Fore more information, visit our web
site at www.linear-tech.com.
being regulated. The shunt reference must
always sink this current, often resulting in
high power dissipation and short battery life.
A MOSFET Pass Transistor Reference
The LTC1258 features an internal
P-channel MOSFET pass transistor. This
provides several advantages over similar
designs that use a PNP bipolar pass transis-
tor. The LTC1258-2.5 consumes only 4µA
of quiescent current and provides a lower
dropout voltage (200mV maximum) than
PNP-based references. The LTC1258-2.5
will provide a 2.5V reference voltage from a
–
+
2.7V TO
12.6V
12.5V
1258 BD
R2
4
OUT
R1
2.424V
GND
2
IN
0.1µF
Figure 1. LTC1258-2.5
Micropower Reference: Low
Dropout, No Output Capacitor
VCC
VOUT
2.5V
15A
IMAX
SHDN
1µF
G2
FB
VIN
VIN
3.3V
C+
LTC1649
PVCC2
PVCC1 G1
IFB
COMP
SS C–
GND CPOUT
1µF
10µF
MBR0530
0.1µF
CC
0.01µF
IRF7801 = INTERNATIONAL RECTIFIER
MBRO530 = MOTOROLA
RC
7.5k
LEXT
1.2µH
C1
220pF
10µF
MBR0530 50k
22Ω1k
Q3
IRF7801
Q1, Q2
IRF7801
TWO IN 
PARALLEL
COUT
4400µF
CIN
3300µF
SHUTDOWN
R2
12.7k
R1
12.4k
1649 Schem
+0.33µF
+
+
+
3
Linear Technology Chronicle •
September 1998
Application of the Month
Figure 1. Communicating over the Serial Port, the LTC1298 and LTC1446 in SO-8
Create a Simple, Low Power, 2-Channel Analog Interface for PCs
Micropower ADC and DAC in SO-8 Give PC 12-Bit Analog Interface
Need to add two channels of simple,
inexpensive, low powered, compact analog
input/output to a PC computer? Choose the
LTC1298 ADC and LTC1446 DAC. The
LTC1298 and the LTC1446 are the first
SO-8 packaged 2-channel devices of their
kind. The LTC1298 draws just 340µA. A
built-in auto shutdown feature reduces
power dissipation at reduced sampling rates
(to 30µA at 1ksps). Operating on a 5V sup-
ply, the LTC1446 draws just 1mA (typ).
Although the application shown is for PC
data acquisition, these two converters pro-
vide the smallest, lowest power solutions
for any other analog I/O application.
The circuit shown in Figure 1 connects
to a PC’s serial interface using four inter-
face lines: DTR, RTS, CTS and TX. DTR
is used to transmit the serial clock signal,
RTS is used to transfer data to the DAC and
ADC, CTS is used to receive conversion
results from the LTC1298 and the signal on
TX selects either the LTC1446 or the
LTC1298 to receive input data. The
LTC1298’s and LTC1446’s low power dis-
sipation allows the circuit to be powered
from the serial port. The TX and RTS lines
charge capacitor C4 through diodes D3 and
D4. An LT1021-5 regulates the voltage to
5V. Returning the TX and RTS lines to a
logic high after sending data to the DAC or
completion of an ADC conversion provides
constant power to the LT1021-5.
Using a 486-33 PC, the throughput was
3.3ksps for the LTC1298 and 2.2ksps for
the LTC1446. Your “mileage” may vary.
C code software prompts the user to either
read a conversion result from the ADC’s
CH0 or write a data word to both DAC
channels. The code is available on disk
from LTC.
LT1638/LT1639: 200
µ
A, 1.2MHz Rail-to-Rail
Op Amps with Over-The-Top
Inputs
capacitive loads up to 200pF under all load-
ing conditions (see Table 1).
Table 1. LT1638/LT1639 Typical
Performance, 25
°
C
Parameter Typical Value
Input Offset Voltage 200µV
Input Bias Current 15ns
Input Offset Current 1nA
CMRR 98dB
Open-Loop Gain 1500V/mV
PSRR 100dB
Supply Current per Amp 190µA
An Over-The-Top Application
A battery current monitor powered by a
5V supply (Figure 1) demonstrates the
The LT1638 is Linear Technology’s
latest general purpose, low power, dual rail-
to-rail operational amplifier and the LT1639
is the quad version. The circuit topology is
based on the popular LT1490/LT1491 op
amps but hopped up for five times higher
speed.
Tough, Versatile Op Amps
Users have appreciated the LT1490 for
its “toughness” and other unique features but
some applications require higher gain band-
width or slew rate. The LT1638/LT1639 op
amps meet that need while still qualifying as
micropower circuits. The parts are guaran-
teed to withstand reverse supply voltages to
18V (typically 40V). Their input stages
incorporate protection to prevent the output
from phase reversing when the input is
forced to 22V below the negative supply.
Input protection resistors limit the current
from becoming excessive when the input is
forced to this extreme.
The LT1638/LT1639 operate on single
and split supplies with a total voltage of
2.5V to 44V with specifications tested and
guaranteed at 3V, 5V and ±15V. The out-
put can swing within 30mV of the positive
rail and 5mV of the negative rail with no
load. The gain-bandwidth product is
1.2MHz and the amplifier is stable with
Continued on page 4
1446 F1
V
OUTB
V
CC
GND
A
OUT2
A
OUT1
V
OUTA
CLK
D
IN
CS/LD
LTC1446
D
OUT
8
7
6
5
1
2
3
4
V
CC
INPUT 1
INPUT 2
CLK
D
OUT
D
IN
CS
CH0 0.1µF
CH1
LTC1298
510Ω
510Ω
510Ω
510Ω
4 x 1N914
GND
8
7
6
5
1
2
3
4
Q
CLR
Q
D
PR
CK
1/2 74HC74
LT1021-5
47µFC4
150µF
5V
5
1
6
2
4
3
Q
CLR
Q
D
PR
CK
1/2 74HC74
5
1
6
10
5
13
151k TX
RTS
DTR
CTS
SELECT
D
IN
S
CLK
D
OUT
51k
51k
11
6
12
62
4
9
3
8
4
D3
1N914 D4
1N914
2
4
3
7
0.1µF
14
5V
0.1µF
2
+
+
4
Linear Technology Chronicle •
September 1998
© 1998 Linear Technology Corporation/Printed in USA
Linear Technology Corporation • 1630 McCarthy Blvd. • Milpitas, CA 95035-7417 • (408) 432-1900 • FAX: (408) 434-0507 • www.linear-tech.com • For Literature Only: 1-800-4-LINEAR
Linear Technology
Products Are
Distributed By:
Arrow Electronics
Digi-Key
Electrosonic
Gerber Electronics
Marshall Industries
Phase 1
Publications Printed in June and July 1998
LTC1638/LTC1639 from page 3
LT1638/LT1639’s ability to operate with
their inputs above the positive rail. In this
application, a conventional amplifier would
be limited to a battery voltage between 5V
and ground, but the LT1638 can handle bat-
tery voltages as high as 44V. The LT1638
shuts down with VCC removed and the input
leakage is less than 0.1nA. No damage to the
LT1638 will result from inserting the battery
backwards. The current level and polarity
may be detected and buffered with one quad
amplifier (see Linear Technology magazine,
May 1998).
Over-The-Top Value
With its 1.2MHz speed, Over-The-Top
capability, reverse-battery protection and
rail-to-rail input and output features, the
LT1638 and LT1639 are ideal candidates
for multiple general-purpose op amp
applications.
Contact your local Linear Technology
sales office for a data sheet and evaluation
samples. For more information, visit our web
site at www. linear-tech.com.
Catalog: Spring/Summer New Products Catalog
Data Sheets:
LT1167 Low power, precision instrumentation amplifier that requires only one external resistor to set gains of 1 to 10,000
LTC1326-2.5 Triple supply monitor for systems with multiple supply voltages. Micropower operation, small size, high accuracy, glitch immunity
LT1374 500kHz monolithic buck mode switching regulator. 4.5A current mode for fast transient response and good loop stability
LTC1416 2.2µs, 400ksps, 14-bit sampling A/D converter that draws only 75mW from ±5V supplies. High dynamic range and precision reference
LTC1418 Low power, 200ksps, 14-bit A/D converter. Data output is selectable for 14-bit parallel or serial format
LTC1435A Synchronous step-down switching regulator controller that drives external N-channel power MOSFETs. 99% duty cycle
LT1506 500kHz monolithic buck mode switching regulator. 4.5A switch, current mode for fast transient response and good loop stability
LTC1517-3.3 Micropower charge pump DC/DC. Extremely low operating current (typically 6µA with no load) and low external parts count
LTC1541 Micropower amplifier, comparator and bandgap reference in an 8-pin package
LTC1542 Micropower amplifier and comparator in an 8-pin package. Single 2.5V to 12.6V or dual supply. The input current is 10pA typical
LT1575/77 UltraFastTM transient response, low dropout regulator controllers, drive N-channel MOSFETs without tantalum or electrolytic capacitors
LTC1595/96 Serial input, 16-bit multiplying current output DACs. Pin and hardware compatible with the 12-bit LTC8043 and LTC8143/LTC7543
LTC1604 333ksps, 16-bit sampling A/D converter, 220mW from ±5V supplies, high dynamic range sample-and-hold, high speed parallel output
LT1610 Fixed frequency 1.7MHz, single cell micropower DC/DC converter, internal 300mA switch
LT1614 Inverting fixed frequency 600kHz switching regulator, internal 500mA switch, 1V to 5V input
LTC1623 SMBus switch controller is a slave device that controls two high side N-channel MOSFETs on either the SMBus or the I2CTM bus
LT1634 Micropower precision shunt voltage reference, 10µA operating current, 0.05% initial accuracy, 10ppm/°C maximum drift
LT1640 Hot SwapTM controller for insertion and removal from a live backplane. Inrush current is limited to a programmable value
LTC1650 Deglitched rail-to-rail voltage output 16-bit (DAC). 16-bit monotonicity over temperature, 3-wire cascadable serial interface
LTC1660 Octal 10-bit DAC, 16-pin narrow SSOP package. 56µA total supply current per DAC. DC output currents in excess of 5mA
LT1671 60ns, low power, single supply, ground-sensing comparator
LTC1706-19 Four VID inputs, output voltage between 1.3V and 2V is programmed in 50mV increments for the Intel Mobile Pentium®II
–
+
+
1/4
LT1639
A
+
–
1/4
LT1639
C
RA
2k
RS
0.2Ω
QA
CHARGER
VOLTAGE
VBATTERY = 12V
VSUPPLY = 5V, 0V
LOGIC
RA'
2k
–
+
1/4
LT1639
B
RB
2k
RB'
2k
RL
RG
10k
QB
LOGIC HIGH (5V) = CHARGING
LOGIC LOW (0V) = DISCHARGING
NOTE: RA = RB
VOUT
RS RG/RA GAIN
VOUT
GAIN
()
()
()
I
BATTERY = = AMPS
–
+
1/4
LT1639
D
90.9k
10k
S1 S1 = OPEN, GAIN = 1
S1 = CLOSED, GAIN = 10
VOUT
Figure 1. LT1639 Battery Current Monitor—An Over-The-Top Application