1
Linear Technology Chronicle •
October 1998
A Showcase of Linear T echnology’ s Focus Products
Linear Technology Chronicle
October 1998 Vol. 7 No. 10
Product of the Month
region, the RDS(ON) of the device can be
used as a sense resistance. This is tricky to
implement because inductor current informa-
tion is only available when the MOSFET is
turned on (see Linear Technology magazine,
August 1998).
Because the new LTC1625 fully recov-
ers the inductor current information, it
behaves identically to traditional current
mode controllers that use a sense resistor.
Thus the circuit delivers the benefits of cur-
rent mode control without the penalties of a
sense resistor.
Additional Features
The LTC1625 is designed to drive two
N-channel power MOSFETs in synchronous
applications. The controller runs at a nominal
150kHz frequency that also may be set at
225kHz and can be locked to an external
clock. Soft start capability has been built into
the circuit to provide limited inrush current
and a controlled delay for supply sequencing.
Providing gate charge to the power
switches is a major source of efficiency loss.
LOAD CURRENT (A)
0.01
EFFICIENCY (%)
80
90
100
0.1 1 10
1625 TA01
70
60
V
IN
= 10V V
OUT
= 5V
V
OUT
= 3.3V
The LTC1625 reduces that loss. At low load
current, the MOSFETs are turned off for a
few cycles while the output capacitors sup-
port the load. This Burst Mode
TM
operation
saves gate charge at the expense of fixed
frequency operation. (Burst Mode operation
may be overridden.) Also, the gate charge
may be drawn from a more efficient second
voltage source, if available, for further
power savings.
The LTC1625 can be configured for a
wide variety of output voltages. 3.3V and
5V outputs may be selected without an
external resistive divider. With a divider, the
LTC1625 is capable of regulating anywhere
between the input voltage and the 1.19V
internal reference. This is a significant
improvement over prior current mode con-
trollers that were constrained by the
common mode range of the sense amplifier.
The input voltage range is very wide, 3.7V
to 36V, making the LTC1625 an extremely
versatile circuit for efficiency- and control-
sensitive products such as cellular phones,
wireless modems and distributed power
systems.
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.
LTC1625: No R
SENSE
TM
DC/DC Controller
Achieves 97% Efficiency
The LTC
®
1625 is a synchronous step-
down controller that drives external
N-channel MOSFETs using few external
components. Current mode control with
MOSFET VDS sensing eliminates the need
for a sense resistor and improves control and
efficiency (Figure 1).
No Sense Resistor
Many switching regulators require a
sense resistor in order to limit output current
under fault conditions. This resistor reduces
the efficiency of the converter while adding
both cost and complexity to the design.
The LTC1625 enlists “sense resistors”
already in the system. The power MOSFET
switches do double duty as current sense
elements. By measuring the drain-to-source
voltage of a MOSFET operating in the triode
LT®1610: Micropower Boost Converter Offers Smallest Footprint .......................................... 2
LT1614: Tiny, Low Noise DC/DC Inverter Works to 1V Input .................................................. 2
LT1021-5: Ultralow Noise Power Supply ................................................................................. 3
LTC1706-19: Adds Digital Voltage Control to an Entire Family of LTC Regulators ................ 3
LT1636: Rugged Micropower Op Amp—Rail-to-Rail, Over-The-TopTM , 44V Breakdown ....... 4
LT1671: Fast and Low Power Comparator............................................................................. 4
Inside This Issue:
+
+
V
IN
TK
SYNC
LTC1625
RUN/SS
V
OSENSE
TG
SW C
B
0.22µF
D
B
CMDSH-3
C
C
2.2nF
R
C
10k
M2
Si4410DY
D1
MBRS140T3
M1
Si4410DY
C
VCC
4.7µF
1625 F01
L1
10µH
C
IN
10µF
30V
×2
BOOST
INTV
CC
BG
I
TH
V
PROG
SGND
PGND
+
C
OUT
100µF
10V
×3
V
OUT
3.3V
4.5A
V
IN
5V TO
28V
C
SS
0.1µF
Figure 1. High Efficiency Step-Down Converter
,
LTC and LT are registered trademarks of Linear Technology
Corporation. Over-The-Top, Burst Mode and No R
SENSE
are
trademarks of Linear Technology Corporation.
Figure 2. Efficiency vs Load Current
2
Linear Technology Chronicle •
October 1998
LT1610: Micropower Boost Converter Offers Smallest Footprint
LOAD CURRENT (mA)
0.1
EFFICIENCY (%)
85
80
75
70
65
60
55
50 1 10 100
1610 TA02
V
OUT
= 3V
V
IN
= 1.5V
V
IN
= 1V
V
IN
= 1.25V
simple, the best charge pump solutions have
output impedances in the 5Ω to 10Ω range
which result in significant regulation prob-
lems when the load increases beyond a few
tens of milliamperes. The LT1614 inductor-
based inverting DC/DC converter uses
closed-loop regulation to obtain an output
impedance of 0.1Ω, eliminating output volt-
age droop under load.
The LT1614 utilizes a low noise topol-
ogy and a fixed 600kHz operating frequency
to ensure an output voltage free from low
frequency switching harmonics, making it
ideal for supplying low noise bias voltages
for MR heads. The LT1614 can generate up
to a 28V differential between input and out-
put from a 1V to 5V input. The LT1614’s
tiny 8-lead MSOP package and small, mini-
mal external components allow it to fit in
tight places.
The circuit contains a low-battery detec-
tor with a 200mV reference and shuts down
to less than 10µA current drain. No load qui-
escent current is 1mA and the internal 0.6Ω
NPN power switch handles a 500mA current
with only a voltage drop of 295mV.
Figure 1 shows the 5V to –5V applica-
tion. The LT1614 operates by driving its
NFB pin to a voltage of –1.24V, allowing
direct regulation of the negative output. This
converter topology, which consists of induc-
tors in series with both input and output,
results in low output noise and also in low
reflected noise on the input supply. Output
ripple is 40mV due to the ESR of the tanta-
lum output capacitor C2. It may be
substantially reduced further by use of a
ceramic capacitor. Efficiency of the circuit is
above 70% from 10mA to 200mA.
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.
The new LT
®
1610 is a versatile,
micropower DC/DC converter that provides
the smallest solution for generating a regu-
lated output voltage from a battery input as
low as 1V.
The LT1610 addresses board space
limitations in several ways. First, the switch-
ing frequency is 1.7MHz, allowing the use of
tiny, inexpensive, minimum height inductors
and capacitors. Second, the frequency com-
pensation components have been integrated
into the IC, eliminating an external RC in
most cases. Finally, the device comes in
LTC’s 8-lead MSOP package, one-half the
size of the 8-lead SO package.
The LT1610’s input voltage range is
from 1V up to 8V. Combined with its 30V,
300mA onboard switch, the circuit is useful
for many regulator configurations, including
boost, SEPIC and flyback. Output voltage
can be up to 28V in the boost design. The
device can generate 3V at 30mA from a
single (1V) cell or 5V at 100mA from two
cells (2V). As a Li-Ion cell to 3.3V SEPIC
converter, the LT1610 can deliver 100mA.
The circuit can also convert 3.3V to 5V at an
output current of 200mA.
Operating quiescent current is only
50µA unloaded. The circuit includes a shut-
down function that reduces the supply cur-
rent to 0.5µA. The LT1610 features Burst
Mode
operation at light loads to provide high
efficiency over a broad range of load current
(Figure 2).
Single Cell to 3V Converter Example
A 1V to 3V boost converter is shown in
Figure 1. The components take up very little
space. The inductor measures 2.5mm by
3.2mm and is only 2mm tall. The tantalum
capacitors measure 1.6mm by 3.2mm and are
1.6mm tall. Circuit efficiency, which reaches
77%, is detailed in Figure 2.
By simply changing the feedback resis-
tor values (to 1MΩ and 322kΩ), the circuit
can generate 5V from 2 cells or from a 3.3V
input. The efficiency of that circuit reaches
83%.
The LT1610’s 1.7MHz operating fre-
quency, internal compensation and tiny
MSOP package result in a low cost and
minimal board space overall solution. For a
data sheet and evaluation samples, contact
your local Linear Technology sales office.
Visit our web site at www.linear-tech.com
for more information.
VIN SW
VCPGND
FB
SHDN
65
2
7
4
1
8
3
GNDCOMP
LT1610 C2
22µF
C1
22µF
1 CELL
L1
4.7µHD1 VOUT
3V
30mA
C1, C2: AVX TAJA226M010R
D1: MOTOROLA MBR0520
L1: MURATA LQH3C4R7M24
1610 TA01
+
+
R1
1M
R2
681k
Figure 1. Single Cell to 3V Converter
Delivers 30mA Figure 2. Single Cell to 3V Converter
Efficiency Reaches 77%
LT1614: Tiny, Low Noise
DC/DC Inverter Works
to 1V Input
The LT1614 is a fixed frequency,
inverting mode switching regulator that pro-
vides a low noise negative output to –24V
from as low as a 1V supply. It is designed
for low power, noise-sensitive, space-limited
portable products.
The inverting DC/DC converter func-
tion is traditionally realized with a
capacitor-based charge pump. Although
V
IN
SW
LT1614
L1
22µH
V
IN
5V
D1
NFB
SHDN 69.8k
24.9k
100k
1nF
V
OUT
–5V
200mA
C1, C2: AVX TAJB336M010
C3: AVX 1206CY106
D1: MBR0520
L1, L2: MURATA LQH3C220
1614 TA01
C1
33µFC2
33µF
V
C
GND
L2
22µH
C3
1µF
+
+
Figure 1. 5V to –5V Converter
3
Linear Technology Chronicle •
October 1998
Application of the Month
Ultralow Noise Power Supply
base clamping. The low noise of the refer-
ence is preserved, giving a 100-fold
improvement over the noise of an equiva-
lent 5V, 3-terminal regulator, plus
improved initial accuracy and long-term
stability. Typical output noise is 7µVP-P
over a 10kHz bandwidth.
The LT1236-5 may also be used in this
circuit. Other output voltages may be
obtained with the wide selection of refer-
ences available from Linear Technology.
For more information, see data sheets and
Application Note 42.
Low noise synthesizers need quiet
power supplies for their VCOs and other
critical circuitry. 3-terminal linear voltage
regulators exhibit far too much noise for
this application, calling instead for a regula-
tor constructed from a voltage reference. A
practical example is shown in Figure 1.
Current through the LT1021-5 refer-
ence is used to drive the base of a PNP pass
device, resulting in an available output cur-
rent of at least 1A. In this example, the
current is intentionally limited to 200mA by
the addition of emitter degeneration and
+
INPUT
9V TO 12V
RED
LED** 220Ω
OUT
IN
GND
LT1021-5
4.7Ω47µF
100µF
TANT
ZBD949*
2Ω
10Ω
1/2W OUTPUT
5V/200mA
* ZETEX INC (516) 864-7630
**GLOWS IN CURRENT LIMIT.
DO NOT OMIT
+
Figure 1. Ultralow Noise 5V, 200mA
Supply Output Noise Is 7µVRMS over a
10Hz to 10kHz Bandwidth. Reference
Noise Is Guaranteed to Less Than
11µVRMS. Standard 3-Terminal
Regulators Have One Hundred Times
the Noise and No Guarantees
LTC1706-19 Adds Digital
Voltage Control to an
Entire Family of LTC
Regulators
The relentless push of microprocessor
development for higher speed and lower
power dissipation, especially in laptop com-
puters, is driving supply voltages to new low
levels. Supply voltage has become so critical
that different processors demand different
yet precise levels to perform optimally.
The LTC1706-19 takes a 4-bit digital
word from a VID (Voltage Identification)
microprocessor system and precisely con-
trols the output voltage of a high
performance DC/DC converter like the
LTC1435A. The output of the regulator can
be adjusted from 1.3V to 2V in 50mV incre-
ments. The LTC1706-19 is available right
now and works with an entire family of LTC
regulators (listed in Table 1) that have inter-
nal 1.19V references. Also, the LTC1706-19
is fully compliant with the Intel Mobile VID
specification.
High Accuracy, Versatile, Small Package
The digitally programmed resistive
divider in the LTC1706-19 is better than
±0.25% accurate across the 1.3V to 2V range
and is unaffected by temperature. This
results in an overall accuracy of ±1.25%
when coupled with the LTC1435A in the
Intel Mobile Pentium
®
II processor VID
power converter shown in Figure 1. The
four, 40k, pull-up resistors on the VID digi-
tal inputs are built in, and the LTC1706-19
is housed in a space efficient SO-8 package.
The LTC1706-19 can also be used to
program the output voltages of regulators
with different onboard references. Used
with the LT1575, an UltraFast
TM
transient
response, low dropout linear regulator con-
troller with a 1.21V reference, the output will
range from 1.27V to 2.03V in steps of
50.8mV. Using only one N-channel power
MOSFET, the two circuits make an
extremely flexible, efficient, compact power
supply.
The LTC1706-19 is the ideal companion
chip to provide precise, programmable low
voltage outputs for DC/DC converters. Its
compact size, compatibility and high accu-
racy are just the right features for today’s
portable equipment.
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.
Figure 1. Intel Mobile Pentium II Processor Power Converter
+
+
C
C
1000pF
51pF
R
C
10k
C
C2
220pF
C
SS
0.1µF
C
OSC
43pF
R
F
, 4.7Ω
1000pF
L1
3.3µHR
SENSE
0.015Ω
V
IN
4.5V
TO 22V
V
OUT
1.3V TO 2V
7A
0.22µF
C
F
0.1µF
C
IN
10µF
30V
× 2
4.7µF
M1
Si4410DY
M2
Si4410DY
D
B
DB: CMDSH-3
D1: MBRS140T3
D1
V
IN
INTV
CC
C
OSC
SGND
V
OSENSE
TG
SW
PGND
BG
BOOST
RUN/SS
I
TH
SENSE
+
SENSE
–
LTC1435A
13
1
16
2
3
14
12
87
5
15
11
10
6
+
C
OUT
820µF
4V
× 2
V
CC
FB
012
VID
LTC1706-19
FROM µP
3 GND
SENSE
UltraFast is a trademark of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation.
Table 1. Compatible LTC Processor
Power Supply Products
LTC Switcher Potential Application
LTC1433/LTC1434 Low Power Supplies for DSPs or
Palm Computer CPUs
LTC1435A Mobile Pentium II CPU Power
LTC1436A
LTC1436A-PLL Cellular Telephones and Noise-
LTC1437A Sensitive Equipment
LTC1438/LTC1439 Multiple Voltage Systems
LTC1538/LTC1539
LTC1624 High Performance DSP Power
LTC1625 Industrial and Single Board
Computer CPU Power
4
Linear Technology Chronicle •
October 1998
© 1998 Linear Technology Corporation/Printed in USA
Linear Technology
Products Are
Distributed By:
Arrow Electronics
Digi-Key
Electrosonic
Gerber Electronics
Marshall Industries
Phase 1
Rugged LT1636 Micropower Op Amp: Rail-to-Rail,
Over-The-Top, 44V Breakdown
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
The LT1636 is a 55µA rail-to-rail
input/output op amp capable of handling
supply voltages from 2.7V to 44V. It fea-
tures a high level of fault protection and an
output shutdown function. The LT1636 is a
single op amp in a tiny MSOP-8 package
(SO-8 and DIP also available).
Tough Op Amp
An Over-The-Top amplifier, the
LT1636’s unique input stage can handle up
to 44V common mode voltage while operat-
ing on a supply as low as 2.7V. It offers
protection against reverse supply voltages up
to 27V and includes built-in resistors that
protect inputs for fault voltages as great as
22V below the negative supply. This broad
voltage range capability eliminates the need
for protection circuitry, allowing designers
to increase circuit versatility and to reduce
the size and expense of products.
Micropower and Precision
The LT1636’s inputs draw virtually no cur-
rent in battery charging applications when
the wall adapter or the charge source is
removed. Unlike most micropower op amps
the LT1636 can drive heavy loads; its rail-
to-rail output drives 18mA and is unity-gain
stable. The maximum supply current of
55µA is reduced to 4µA in shutdown mode.
The circuit is specified on 3V, 5V and ±15V
supplies.
The LT1636 is a precision amplifier
with less than 225µV input offset voltage
and less than 8nA of input bias current (see
Table 1). The LT1636’s combination of fea-
tures makes it useful in a wide range of
Table 1. LT1636 Overview
Supply Voltage Range 2.7V to 44V
Supply Current (Max) 55µA
Shutdown Current (Max) 12µA
Offset Voltage (Max) 225µV
Input Offset Voltage Drift (Max) 10µV/°C
Input Bias Current (Max) 8nA
Gain Bandwidth (Typ) 220kHz
CMRR (Typ) 110dB
–
+
LT1636
0.1V TO 44V
5V
R1
200Ω
R
S
0.2Ω
R2
2k
V
OUT
(0V TO 4.3V)
1636 TA08
LOAD
I
LOAD
V
OUT
(R
S
)(R2/R1)
I
LOAD
=
Over-The-Top Current Sense
products including battery charging and cur-
rent monitoring, high side current sensing
and charge control in portables and high
speed modems, bias voltage tracking in pas-
sive LCD displays and battery buffering in
high voltage industrial control systems and
electric vehicles.
Dual and quad op amps with similar
features and specifications are the LT1490/
LT1491 and the faster LT1638/LT1639.
For a data sheet and evaluation samples,
contact your local Linear Technology sales
office. For more information, visit our web
site at www.linear-tech.com.
high. The complementary outputs allow for
differential signaling and improved noise
performance.
These features combine to make the
LT1671 well-suited for applications such as
high performance micropower crystal oscil-
lators (Figure 2), single supply voltage-
to-frequency converters and high speed,
high accuracy level detectors. The LT1671
is offered in SO-8 and is pin compatible
with the industry-standard LT1016 and
LT1116 comparators. Contact your local
Linear Technology sale office for a data
sheet and evaluation samples.Visit our web
site at www.linear-tech.com for more
information.
1671 TA01
–
+
LT1671
5V
OUTPUT
0.068µF
2k
2k
2k
1MHz CRYSTAL
(AT-CUT)
Figure 1. Propagation Delay vs Overdrive
Figure 2. 1MHz Crystal Oscillator
LT1671 Fast and Low
Power Comparator
The LT1671 is a low power (450µA),
fast (60ns) comparator designed to operate
on either single 5V or ±5V supplies. It has
maximum offset voltage of 2.5mV, comple-
mentary TTL compatible outputs and output
latch capability. The wide input voltage
range extends from the bottom supply rail to
within 1.5V of the top rail. The LT1671 is
made with Linear Technology’s new 6GHz
complementary bipolar technology, which
results in a dramatically improved speed/
power product compared to industry-
standard comparators developed in slower
NPN-only technologies.
Propagation delay (Figure 1) is less than
80ns (maximum) with 20mV overdrive
(100ns maximum, with 5mV overdrive).
These speeds are great for low power analog
signal processing and data acquisition cir-
cuits. The LT1671’s supply current is only
800µA (maximum).
The LT1671 includes a Latch Enable
function that is useful for sampling signals.
The latch allows the most recent output state
to be retained as long as the latch pin is held
OVERDRIVE (mV)
0
TIME (ns)
140
120
100
80
60
40
20 10 20 30 40
1671 TA02
50
V
S
= ±5V
V
STEP
= 100mV
T
A
= 25°C
R
L
= 1M
FALLING EDGE (t
PDHL
)
RISING EDGE (t
PDLH
)
