LTJournal-V25N1-2015-04.pdf

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April 2015
Volume 25 Number 1
I N
T H I S
I S S U E
patent-pending boost-buck
LED driver topology
8
I
2
C programmable
supervisors with EEPROM
12
Industry’s First 0.8µV
RMS
Noise
LDO Has 79dB Power Supply
Rejection Ratio at 1MHz
Amit Patel
18V buck-boost converter
with intelligent PowerPath
control delivers >2A
16
advantages of 75W boost
mode LED driver
22
how to design an isolated,
high frequency, push-pull
DC/DC converter
25
When it comes to powering noise-sensitive analog/RF applications,
low dropout (LDO) linear regulators are generally preferred over their
switching counterparts. Low noise LDOs power a wide range of
analog/RF designs, including frequency synthesizers (PLLs/VCOs),
RF mixers and modulators, high speed and high resolution data
converters (ADCs and DACs) and precision sensors. Nevertheless,
these applications have reached capabilities and sensitivities
that are testing the limits of conventional low noise LDOs.
For instance, in many high end
VCO
s
, power supply noise
directly affects the
VCO
output phase noise (jitter). Moreover,
to meet overall system efficiency requirements, the
LDO
usually post-regulates the output of a relatively noisy
switching converter, so the high frequency power supply
rejection ratio (
PSRR)
performance of the
LDO
becomes
paramount. With its ultralow output noise and ultrahigh
PSRR
performance, the
LT
®
3042 can directly power some
of most noise-sensitive applications while post-regulating
the output of a switching converter, without requir-
ing bulky filtering. Table
1
compares the LT3042’s noise
performance with conventional low noise regulators.
PERFORMANCE, ROBUSTNESS & SIMPLICITY
The LT3042 is a high performance low dropout linear
regulator featuring Linear Technology’s ultralow noise and
ultrahigh
PSRR
architecture for powering noise-sensitive
The LT3042 brings noise-free power to high performance electronics.
(continued on page 4)
w w w. li n e ar.co m
In this issue...
COVER STORY
Industry’s First 0.8µV
RMS
Noise
LDO Has 79dB Power Supply Rejection Ratio at 1MHz
Amit Patel
1
Linear in the News
New Analog Circuit Design Book Published by Elsevier
DESIGN FEATURES
Boost-Buck LED Driver Topology for Automotive
LEDs Operates with Low Input and Output Ripple
Keith Szolusha
8
The third volume in Linear Technology’s
Analog Circuit Design
book series has
been published by Elsevier
/
Newnes Publishers. The book,
Analog Circuit Design,
Volume 3, Design Note Collection,
edited by Bob Dobkin and John Hamburger, is
a compilation of applied circuit design solutions that can be used in a broad range
of applications and systems.
The book is a comprehensive collection of Linear Technology’s highly regarded
Design Notes. The Notes have been written over the past 25 years by some of the
leading lights of analog design, including Jim Williams, Bob Dobkin, Carl Nelson
and George Erdi, among many others. They serve as a framework to provide cir-
cuit design techniques for a range of analog design challenges.
The book is logically organized for easy reference. It includes an in-depth Power
Management section, including such topics as:
• Power management design
• Microprocessor power design
• Switching regulator design for step-down and boost converters, buck-boost
controllers and
DC/DC
controllers
• Linear regulator design
• µModule
®
power design
• Battery management
• Power over Ethernet
• Powering
LED
lighting
• Automotive and industrial power design
The Mixed Signal section spans a range of topics, including:
• Data conversion—analog-to-digital and digital-to-analog
• Data acquisition
• Communications interface design
• Instrumentation design
The Signal Conditioning section covers:
• Operational amplifier design techniques
• Special function amplifier design
• Voltage reference design
• Filter design
• Comparator design techniques
• System timing design
•
RMS-
to-
DC
conversion
I
2
C Programmable Multichannel Voltage
Supervisors with EEPROM
Michael Petersen
12
18V Buck-Boost Converter with Intelligent
PowerPath™ Control Delivers >2A at
95% Efficiency from Dual Inputs
Eddy Wells
16
DESIGN IDEAS
What’s New with LTspice IV?
Gabino Alonso
20
Can’t Find the Right Synchronous Boost LED Driver?
Use a Synchronous Buck Converter Instead: Boost
Mode Topology Drives 25V, 3A LEDs from 12V
Keith Szolusha
22
How to Design an Isolated, High Frequency,
Push-Pull DC/DC Converter
Dawson Huang
25
30
32
new product briefs
back page circuits
2 | April 2015 :
LT Journal of Analog Innovation
Linear in the news
reception of the two prior books in
this series,
Analog Circuit Design, A
Tutorial Guide to Applications and
Solutions,
and
Analog Circuit Design,
Volume 2, Immersion in the Black
Art of Analog Design,
both edited
by Bob Dobkin and Jim Williams.
For more information on the book,
a video interview with Bob Dobkin
and ordering information from either
Elsevier or Amazon, visit
www.linear.
com
/
designtools
/
acd_book.php
AWARDS
Technical Support Award
• The LTC2323-
1
6 Dual,
1
6-Bit, 5
M
sps
Differential Input
ADC
with Wide
Input Common Mode Range was
selected as the Most Competitive
ADC
Product for 20
1
4.
CONFERENCES & EVENTS
IPC & Embedded Expo 2015, Shenzhen Convention
& Exhibition Center, Shenzhen, China, April
15–17—
Linear is presenting “99.999%
Reliable Wireless Sensor Network that
Uses Low Power” at
11
:20 am, April
1
6 at
their 3rd Internet of Things Conference.
More info at
en.embeddedexpo.com/
2nd Annual LTspice Users Conference, Tokyo
Conference Center, Tokyo, Japan, April 17, Halls
A & B—
Mike Engelhardt, a developer of
Analog Circuit Design, Volume 3, Design Note
Collection,
recently published by Elsevier/Newnes
Publishers, is the latest volume in the Linear book
series.
Linear Technology was presented with
the award for Best Technical Support
by
SPDEI,
the French Association of
Electronic Components Distributors
at an event in Paris in December.
EEPW
Editors’ Choice Award
LTspice
®
design tools, will make a pre-
sentation on the latest developments
regarding LTspice software. For registra-
tion and more info, visit
www.ltspice.jp
Wireless Japan 2015, Tokyo Big Site, Tokyo,
Japan, May 27–29—
Presenting customer
The final section of the book covers
a range of topics in Wireless,
RF
and
Communications Design.
In his introduction to the book, Bob
Dobkin,
CTO
& Co-founder of Linear
Technology, states, “The teaching designs
in this
Design Note Collection
help bring
new designers up to speed and give expe-
rienced designers a starting point for even
more sophisticated designs. This book has
two purposes: to speed designs by present-
ing finished examples, as well as provid-
ing a teaching resource for designers.”
Bill Schweber, Contributing Editor to
EE
Times Planet Analog,
stated in his
review of the book, “There are so many
good circuit ideas and topologies along
with analysis of subtleties and design
issues, that anyone who is a serious
student or practitioner of the art and
reality of analog design will receive a
substantial return on time invested.”
Analog Circuit Design, Volume 3, Design
Note Collection,
follows the positive
Linear’s
LTC
®
2338-
1
8
1
8-bit,
1
M
sps
,
±
1
0.24
V
true bipolar
SAR ADC
(analog-
to-digital converter) was selected for
the Best Mixed-Signal Chip Award by
the editors of
Electronic Engineering &
Product World
(China). Operating from
a single 5
V
supply, the LTC2338-
1
8 has a
±
1
0.24
V
true bipolar input range, making
it ideal for high voltage applications which
require a wide dynamic range. The fast
1
M
sps
throughput with no cycle latency
makes the LTC2338-
1
8 ideally suited for a
wide variety of high speed applications.
China Electronic Market
Editors’ Choice
Awards
case studies of Linear’s Dust Networks
®
products, as well as industrial wire-
less sensor network solutions at the
Linear Technology booth. More info at
www8.ric.co.jp
/
expo
/
wj
/
en
/
index.html
Sensors Expo/Energy Harvesting Pavilion, Long
Beach Convention Center, Long Beach, CA,
June 9–11, Booth 649—
Presenting Linear’s
Two Linear Technology products were
selected by the editors of
China Electronic
Market
magazine for Editors’ Choice
Awards:
• The LTC3355 20
V
1
A
Buck
DC/DC
with
Integrated Supercap Charger and Backup
Regulator was chosen as the Most
Competitive Power Product for 20
1
4.
energy harvesting family and low power
wireless sensor networks. Sam Nork
is presenting “Energy Harvesting for
Battery-Operated Applications” and
Joy Weiss is presenting “Low Power
Wireless Sensor Networks for IoT
.
” For
more info, visit
www.sensorsexpo.com
April 2015 :
LT Journal of Analog Innovation
| 3
The LT3042 is a high performance low dropout regulator featuring Linear’s ultralow
noise and ultrahigh PSRR architecture for powering noise-sensitive applications.
Even with its high performance, the LT3042 maintains simplicity and robustness.
(
LT
3042, continued from page
1
)
Table 1. The LT3042 vs traditional low noise LDOs
applications. Even with its high per-
formance, the LT3042 maintains sim-
plicity and robustness. Figure
1
is a
typical application and Figure 2 shows
a complete demonstration circuit. The
LT3042’s tiny 3mm × 3mm
DFN
pack-
age and minimal component require-
ments keep overall solution size small.
Designed as a precision current refer-
ence followed by a high performance
voltage buffer, the LT3042 is easily paral-
leled to increase output current, spread
heat on the
PCB
and further reduce
noise—output noise decreases by the
square-root of the number of devices
in parallel. Its current-reference based
architecture offers wide output volt-
age range (0
V
to
1
5
V)
while maintaining
unity-gain operation, thereby providing
virtually constant output noise,
PSRR,
bandwidth and load regulation, indepen-
dent of the programmed output voltage.
In addition to offering ultralow noise and
ultrahigh
PSRR
performance, the LT3042
includes features desired in modern
PARAMETER
LT1763
LT3062
LT3082
LT3042
RMS Noise (10Hz to 100kHz)
Spot Noise (10kHz)
PSRR at 1MHz
Minimum PSRR (DC to 1MHz)
Directly Parallelable
Programmable Current Limit
Programmable Power Good
Fast Start-up Capability
Rail-to-Rail Output Range
Quiescent Current
20µV
RMS
35nV/√Hz
22dB
22dB
30µV
RMS
80nV/√Hz
55dB
30dB
33µV
RMS
100nV/√Hz
45dB
40dB
L
0.8µV
RMS
2nV/√Hz
79dB
77dB
L
L
L
L
L
30µA
45µA
300µA
2mA
systems, such as programmable cur-
rent limit, programmable power good
threshold and fast start-up capability.
Furthermore, the LT3042 incorporates
protection features for battery-powered
systems. Its reverse input protection
circuitry tolerates negative voltages at
the input without damaging the
IC
or
developing negative voltages at the out-
put—essentially acting as if an ideal diode
is connected in series with the input. In
battery backup systems where the output
can be held higher than the input, the
LT3042’s reverse output-to-input protec-
tion circuitry prevents reverse current
flow to the input supply. The LT3042
includes internal foldback current limit,
as well as thermal limit with hysteresis
for safe-operating-area protection.
Figure 1. Typical LT3042
application
V
IN
5V ±5%
4.7µF
IN
100µA
LT3042
Figure 2. LT3042
demonstration
circuit
200k
PG
SET
4.7µF
33.2k
4 | April 2015 :
LT Journal of Analog Innovation
+
–
GND
ILIM
499
EN/UV
OUT
OUTS
PGFB
4.7µF
V
OUT
3.3V
I
OUT(MAX)
200mA
450k
50k
design features
Designed as a precision current reference followed by a high performance
voltage buffer, the LT3042 is easily paralleled to increase output current,
spread heat on the PCB and further reduce noise—output noise
decreases by the square-root of the number of devices in parallel.
1000
C
SET
= 0.047µF
C
SET
= 0.47µF
C
SET
= 1µF
C
SET
= 4.7µF
C
SET
= 22µF
9
8
RMS OUTPUT NOISE (µV
RMS
)
7
6
5
4
3
2
1
1k
10k 100k
FREQUENCY (Hz)
1M
10M
0
0.01
0.1
1
10
SET PIN CAPACITANCE (µF)
100
V
IN
= 5V
R
SET
= 33.2k
C
OUT
= 4.7µF
I
L
= 200mA
50µV/DIV
OUTPUT NOISE (nV/√Hz)
100
10
LT1763
LT3042
10ms/DIV
Figure 3. Output noise: 10Hz to 100kHz
1
0.1
V
IN
= 5V
R
SET
= 33.2k
C
OUT
= 4.7µF
I
L
= 200mA
10
100
ULTRALOW OUTPUT NOISE
Figure 4. Noise spectral density
With its 0.8
µ
V
RMS
output noise* in
1
0
H
z
to
1
00
k
H
z
bandwidth, the LT3042 is the
industry’s first sub-
1
µ
V
RMS
noise regulator.
Figure 3 compares the LT3042’s integrated
output noise from
1
0
H
z
to
1
00
k
H
z
to
that of the LT
1
763, Linear’s lowest noise
regulator for over a decade. The LT3042’s
ultralow noise performance opens up
applications that were previously not
possible, or otherwise required expen-
sive and bulky filtering components.
The
SET
pin capacitor (
C
SET
)
bypasses the
reference current noise, the base cur-
rent noise (of the error amplifier’s input
stage) and the
SET
pin resistor’s (
R
SET
)
inherent thermal noise. A
s
shown in
Figure 4, low frequency noise performance
is significantly improved with increasing
C
SET
.
With a 22
µ
F C
SET
,
the output noise
is under 20
n
V/
√
H
z
at
1
0
H
z
. Note that
capacitors can also produce
1
/f noise,
particularly electrolytic capacitors. To
minimize
1
/f noise, use ceramic, tanta-
lum or film capacitors on the
SET
pin.
Actively driving the
SET
pin with either a
battery or a lower noise voltage reference
Figure 5. Integrated RMS output noise (10Hz to
100kHz)
reduces noise below
1
0
H
z
. Doing so
essentially eliminates the reference cur-
rent noise at lower frequencies, leaving
only the extremely low error amplifier
noise. This ability to drive the
SET
pin is
another advantage of the current-reference
architecture. The integrated
RMS
noise
also improves as the
SET
pin capacitance
increases, dropping below
1
µ
V
RMS
with
just 2.2
µ
F C
SET
,
as shown in Figure 5.
Increasing
SET
pin bypass capacitance
for lower output noise generally leads
to increased start-up time. But the
LT3042’s fast start-up circuitry allevi-
ates this trade-off. The fast start-up
circuitry is easily configured using two
resistors; Figure 6 shows the signifi-
cant improvement in start-up time.
ULTRAHIGH PSRR PERFORMANCE
Figure 6. Fast start-up capability
OUTPUT
WITH FAST
START-UP
(SET AT 95%)
500mV/DIV
OUTPUT WITHOUT
FAST START-UP
500mV/DIV
LT3042’s high
PSRR*
is important when
powering noise-sensitive applications.
Figure 7 shows the LT3042’s incredible
low and high frequency
PSRR
perfor-
mance—approaching almost
1
20
d
B
at
1
20
H
z
, 79
d
B
at
1
MH
z
, and better than
70
d
B
all the way to 3
MH
z
.
PSRR
per-
formance is even better with decreasing
load currents, as shown in Figure 8.
Unlike conventional
LDO
s
whose
PSRR
performance deteriorates into the
1
0s
of
d
B
as you approach dropout, the
LT3042 maintains high
PSRR
at even low
input-to-output differentials. A
s
Figure 9
illustrates, LT3042 maintains 70
d
B PSRR
PULSE EN/UV
2V/DIV
V
IN
= 5V
R
SET
= 33k
C
OUT
= 4.7µF
100ms/DIV
C
SET
= 4.7µF
R
L
= 16.5
April 2015 :
LT Journal of Analog Innovation
| 5

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