LTJournal-V26N2-2016-05.pdf

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May 2016
Volume 26 Number 2
I N
T H I S
I S S U E
multi-output clock
synthesizer with integrated
VCO and low jitter
12
negative current-reference
linear regulator
20
20A LED Driver with Accurate ±3%
Full Scale Current Sensing Adapts to
Multitude of Applications
Josh Caldwell and Walker Bai
load sharing for three or
four supplies with unequal
voltages
26
monolithic SEPIC/boost
regulators with wide V
IN
range, high efficiency,
and power-on reset and
watchdog timers
28
Rapidly evolving LED lighting applications are replacing nearly
all traditional forms of illumination. As this transformation
accelerates, power requirements for LED drivers increase,
with higher currents making it more challenging to maintain
current sensing accuracy without sacrificing efficiency. LED
drivers must do this while managing current delivery to
multiple independent LED loads at high speeds, and be able
to connect parallel drivers with accurate current sharing.
Some high power
LED
s
have unique mechanical and
electrical considerations, where the anode is electrically
tied to the thermally conducting backtab. In a traditional
LED
driver with a step-down regulator configuration,
where thermal management is achieved by cooling the
chassis, the anode connection to the backtab creates a
mechanical-electrical design challenge. The backtab must
have good thermal conductivity to the heat sink, but
also be electrically isolated from it if the voltage at the
tab is different from the chassis. Since is it difficult for
LED
manufacturers to change processing or packaging,
the
LED
driver itself must meet this design challenge.
One option is to use a 4-switch positive buck-boost
LED
driver, but the additional switching
MOSFET
s
add system
complexity and cost. An inverting buck-boost topology
uses only one set of switching power
MOSFET
s
, and allows
the anode heat sink to be tied directly—electrically and
(continued on page 4)
The LTC4125 5W AutoResonant wireless power transmitter features foreign object
detection and completes linear wireless charging solutions (see page 31).
w w w. li n e ar.co m
In this issue...
COVER STORY
20A LED Driver with Accurate ±3% Full Scale
Current Sensing Adapts to Multitude of Applications
Josh Caldwell and Walker Bai
1
Linear in the News
SINGAPORE TEST FACILITY EXPANDS TO BOOST PRODUCTION
CAPACITY
DESIGN FEATURES
Multi-Output Clock Synthesizer with Integrated
VCO Features the Low Jitter Required to Drive
Modern High Speed ADC and DAC Clock Inputs
Chris Pearson
12
In February, Linear opened the company’s third semiconductor test facility in
Singapore. The additional space for staff, equipment and materials will allow
the company to more than double its production capacity of analog circuits and
µModule
®
(power module) products. This strengthens Linear’s ability to meet the
growing demand for high performance analog integrated circuits worldwide.
The new 87,000 square foot facility is located beside Linear Technology’s
current
1
9
1
,000 square foot facility in Singapore. The company’s Singapore
test operation, started 27 years ago in
1
989, has sophisticated capabilities for
high volume testing of the company’s numerous products, including many
package types, tape and reel, as well as pack and ship to customers worldwide.
The location also includes Linear’s Singapore Design Center and the area sales
office supporting Singapore, Malaysia, India and Australia
/
New Zealand.
Continued Expansion
1.5A, Negative Regulator Expands Family of
Current-Reference Linear Regulators
Dawson Huang
20
DESIGN IDEAS
What’s New with LTspice IV?
Gabino Alonso
24
Easy Balanced Load Sharing for Three or Four
Supplies, Even with Unequal Supply Voltages
Vladimir Ostrerov and Chris Umminger
26
Design Once; Use Twice: Monolithic SEPIC/Boost
Regulators with Wide V
IN
Range Satisfy Requirements
of Both Consumer and Commercial Vehicles
Molly Zhu
28
30
32
Over the years, Linear has continued to expand its Singapore test opera-
tions, with expansion of its first building in
1
997 and a second five-story
building completed in 2005. This third major expansion, adding an addi-
tional 87,000 square foot test facility, is now complete. With head count of
nearly a thousand employees today, Linear has a highly experienced team,
capable of testing the most high performance analog
IC
s
. The facility provides
test capability for over 90 percent of the company’s global demand.
Over the past decade, Linear’s products have grown in sophistication
and complexity. This has paralleled the company’s increasing participa-
tion in the key electronics growth markets of industrial and automotive.
Industrial, at 44 percent of the company’s business in the most recent fiscal
year, is a broad market that includes industrial process control, factory
automation, robotics, instrumentation and medical, among others.
Linear has also enjoyed significant growth in its automotive business, reaching
20 percent of the company’s business. This market is driven by the increas-
ing use of electronic systems, replacing mechanical and other systems, in
all aspects of the car, from safety and navigation to electronic steering and
braking and many systems under the hood. This technical transformation
has resulted in analog and power devices that are increasingly complex, and
require sophisticated test systems, now installed in the company’s Singapore
facility. These test systems enable Linear to provide the high quality and
reliability demanded by industrial and automotive manufacturers.
new product briefs
back page circuits
2 | May 2016 :
LT Journal of Analog Innovation
Linear in the news
Linear’s Singapore
test facility expands
to boost production
capacity.
receive the highest quality products
in the shortest possible lead times.
CONFERENCES & EVENTS
Sensor+Test 2016, Nuremberg Exhibition Centre,
Nuremberg, Germany, May 10–12, Booth 241,
Hall 1—
Showcasing Linear’s broad line
of high performance
IC
s
, including
SmartMesh
IP
™
solutions. There will
be working demo applications from
European customers using Linear’s
products.
www.sensor-test.de
/
welcome-to-
the-measurement-fair-sensor-test-20
1
6/
Internet of Things World 2016, Santa Clara
In addition to test capability for the
company’s analog and power
IC
s
, the
Singapore facility has extensive test capa-
bility for Linear’s growing line of µModule
power products. These are complete power
systems-in-package with integrated induc-
tor,
MOSFET, DC/DC
regulator
IC
s
and
supporting components. Each µModule
product is thoroughly tested using Linear’s
stringent electrical, package and thermal
reliability tests. The new Singapore
expansion provides the company with
additional capacity for testing µModule
products to meet growing demand.
Also located in Singapore is the
company’s reliability testing center.
This is a fundamental part of Linear’s
operations, focusing on ensuring
superior product quality for automo-
tive and other demanding markets.
Grand Opening
components of our global manufacturing
operations. Given the excellent conditions
for growth in the region, as well as the
skill and dedication of our employees here,
Singapore will remain as the headquarters
and focal point for all Linear Technology
operations in Asia. We will continue to
enhance our capabilities here to deliver
advanced technology solutions to rapidly
growing markets around the world.”
Lothar Maier, Chief Executive Officer
of Linear Technology, added, “I have
high confidence in our Singapore test
infrastructure and the talent base we’ve
cultivated here over many years, and
we expect to grow it. Our advanced test
operations here ensure that our customers
in the demanding and fast growing auto-
motive and industrial markets continue
to receive the highest quality products.”
Complete Manufacturing Capability
Convention Center, Santa Clara, California,
May 10–12—
Linear’s Dust Networks
®
wireless sensor network products.
https://iotworldevent.com/
Space Tech Expo, Pasadena Convention Center,
Pasadena, California, May 24-26, Booth 8025—
Showcasing products for space and harsh
environments.
www.spacetechexpo.com/
NXP FTF Technology Forum, JW Marriott, Austin,
Texas, May 16–19—
Power management
IC
s
and µModule products.
www.nxp.com
/
support
/
classroom-training-events
/
nxp-ftf-
tech-forum:
NXP-FTF-TECH-FORUM-HOME
Wireless Japan 2016, Tokyo Big Sight, Japan,
May 25–27, Booth 1233—
Demonstrating
Linear’s Dust Networks wireless
sensor network products and their
ecosystem in Japan.
www.wjexpo.com
Sensors Expo & Conference, McEnery Convention
Center, San Jose, California, June 21–23, Booth
940—
Presenting Linear’s energy harvest-
In February, Linear’s management team
was in Singapore for the opening of
the expanded test facility. They toured
the new facility and spent time with
the Singapore operations team.
Robert Swanson, Linear Technology
co-founder and Executive Chairman,
who flew to Singapore for the opening,
said, “The facilities in Singapore are vital
Linear’s Singapore test operation is just
one part of the company’s advanced
manufacturing operations. These
include two wafer manufacturing
facilities, located in Milpitas, California
and Camas, Washington, as well as
a wafer sort and package assembly
operation in Penang, Malaysia. These
facilities ensure that Linear customers
ing family and low power wireless sensor
networks.
www.sensorsexpo.com/
IEEE Nuclear and Space Radiation Effects
Conference, DoubleTree and Oregon Convention
Center, Portland, Oregon, July 11–15, Booth
33—
Showcasing products for space and
harsh environments.
www.nsrec.com/
n
May 2016 :
LT Journal of Analog Innovation
| 3
To meet high performance demands, the LT3744 can be configured
as a synchronous step-down or inverting buck-boost controller
to drive LED loads at continuous currents exceeding 20A. The
supply input for the LT3744 is designed to handle 3.3V to 36V.
(
LT
3744, continued from page
1
)
300
250
NUMBER OF UNITS
200
150
100
50
0
NUMBER OF UNITS
mechanically—to the chassis ground,
eliminating the need for electrical isola-
tors on the heat sink, and simplifying
the mechanical design of the system.
To meet high performance demands, the
LT
®
3744 can be configured as a synchro-
nous step-down or inverting buck-boost
controller to drive
LED
loads at continu-
ous currents exceeding 20
A.
The supply
input for the LT3744 is designed to handle
3.3
V
to 36
V.
A
s
a step-down converter,
it regulates
LED
current from 0
V
up
to the supply voltage. A
s
an inverting
buck-boost converter, the LT3744 can
accurately regulate
LED
currents with
output voltages from 0
V
down to −20
V.
Full-range analog current regulation
accuracy is 3%, and even at
1
/20th scale,
it is better than ±30%. The LT3744 has
three independent analog and digital
control inputs with three compensation
and gate drive outputs for a wide range
of
LED
configurations. By separating
the inductor current sense from the
LED
current sense, the LT3744 can be config-
ured as a buck or inverting buck-boost.
For ease of system design, all input
signals are referenced to board ground
(
SGND,
signal ground), eliminating the
need for complex discrete level-shifters.
In the inverting buck-boost configura-
tion, the total
LED
forward voltage
can be higher than the input supply
voltage, allowing high voltage
LED
strings to be driven from low voltage
supplies. When
PCB
power density calls
for spreading the component power
125°C
25°C
–45°C
380 TYPICAL UNITS
V
CTRL1
= 0V
100
90
80
70
60
50
40
30
20
10
125°C
25°C
–45°C
380 TYPICAL UNITS
V
CTRL1
= 1.5V
–300 –200 –100
0
100
200
300
REGULATED V
LED_ISP
- V
LED_ISN
VOLTAGE (µV)
0
59
59.4
59.8
60.2
60.6
61
REGULATED V
LED_ISP
- V
LED_ISN
VOLTAGE (mV)
Figure 1. The LED current regulation amplifier
in the LT3744 has a typical offset of ±300µV with
V
CTRL
 = 0V.
Figure 2. At full current, the LED current regulation
loop has a typical accuracy of ±1.7% with V
CTRL
=
1.5V.
dissipation, the LT3744 can be easily
paralleled with other LT3744s to drive
high pulsed or
DC
currents in
LED
loads.
HIGH ACCURACY CURRENT
SENSING
The LT3744 features a high accuracy
current regulation error amplifier, which
achieves accurate analog dimming down
to
1
/20th of the total current control
range. This is critical in applications
where the total digital
PWM
dimming
range is limited—or in applications where
very high dimming range is required.
A
s
an example, with a
1
00
H
z
PWM
dimming frequency and a
1
MH
z
switch-
ing frequency, the LT3744 is capable of
1
250:
1
PWM
dimming, which can be
combined with 20:
1
analog dimming to
extend the total diming range to 25,000:
1
.
Figure
1
shows the production consistency
of the LT3744 with regard to offset voltage
over temperature, in this case 380 typical
units when the analog control input is
at 0
V.
With the low offset of the error
amplifier, the control loop is capable of
a typical accuracy of ±
1
0% at
1
/20th
scale analog dimming. The distribution
of the regulated voltage across the
LED
current sensing pins with the control
input equal to
1
.5
V
is shown in Figure 2.
The accuracy at full range is better than
PWM1
5V/DIV
SW
10V/DIV
I
LED
1.67A/DIV
I
L
20A/DIV
1µs/DIV, 5-MINUTE PERSISTENCE
Figure 3. The LT3744 features flicker-free LED
dimming.
4 | May 2016 :
LT Journal of Analog Innovation
design features
In projection systems, reducing the turn-on time of the light source reduces timing
constraints. With a reduction in timing constraints, the image refresh rate can
increase, allowing higher resolution images and a reduction in the rainbow effect
from fast-moving white objects. The LT3744 is capable of transitioning between
the different output current states in less than three switching cycles.
Figure 4. The LT3744 is capable of driving
a single LED with three different current
levels.
EN/UVLO
EN/UVLO
PWM1
PWM2
PWM3
CTRL1
CTRL2
CTRL3
V
REF
R
HOT
45.3k
R
NTC
680k
100k
FAULT
CTRLT
SGND
BG
VEE
ISP
ISN
V
IN
TG
1µF
220nF
M1
V
IN
56µF 24V
×4
L1: IHLP-5050FD-ER1R2M01
R
S
: WSL28163L000D
R
SLED
: WSL28163L000J
M1: BSC050NE2LS
M2: SiR438DP
M3, M4, M5, M6, M7, M8: Si7234DP
D1, D2. D3, D4: BAT54A
C1, C2, C3: 10T4B330M
C1
330µF
M4
M6
D1
D2
D3
D4
M3
M5
M7
M8
10µF
C2
330µF
2V
2.2µF
BOOST
SW
LT3744
INTV
CC
22µF
L1
1.2µH
R
S
3m
20A MAXIMUM
M2
C3
330µF
51k
BLUE
SYNC PWM_OUT1
SS
RT
PWM_OUT2
V
FNEG
PWM_OUT3
LED_ISP
LED_ISN
FB
V
C2
V
C3
287k
10nF
287k
10nF
10nF
82.5k
VEE
V
C1
287k
10nF
R
SLED
3m
10k
±3%, which corresponds to ±
1
.8
m
V
on
the 60
m
V
full-scale regulation voltage.
FLICKER-FREE PERFORMANCE
One of the most important metrics in
LED
driver performance is in the recovery of
the
LED
current during
PWM
dimming.
The quality of the end product is highly
dependent on the behavior of the driver
in the first few switching cycles after the
rising edge of the
PWM
turn-on signal.
The LT3744 uses proprietary
PWM,
compensation and clock synchronization
technology to provide flicker-free perfor-
mance—even when driving
LED
s
to 20
A.
Figure 3 shows a 5-minute capture of the
LED
current recovery with a
1
2
V
supply
delivering 20
A
to a red
LED.
The switch-
ing frequency is 550
k
H
z
, the inductor
is
1
µ
H,
the
PWM
dimming frequency
is
1
00
H
z
with an on-time of
1
0µsec
(
1
000:
1
PWM
dimming). Roughly 30,000
dimming cycles are shown, with no
jitter in the switching waveform—every
recovery switching cycle is identical.
HIGH SPEED DIMMING BETWEEN
THREE DIFFERENT REGULATED
CURRENTS
In projection systems, reducing the
turn-on time of the light source reduces
timing constraints. With a reduction in
timing constraints, the image refresh rate
can increase, allowing higher resolution
images and a reduction in the rainbow
effect from fast-moving white objects.
The LT3744 is capable of transitioning
between the different output current
states in less than three switching cycles.
The LT3744 features three regulated
current states, allowing color-mixing
system designers to sculpt the color
temperature of each
LED.
Color mixing
delivers high color accuracy, corrects
inaccurate
LED
colors, and eliminates
variations in production systems. While
the LT3743 has low and high current states,
the LT3744 features three current states so
that all three
RGB LED
colors can be mixed
with each other at their own light outputs
to independently correct the other colors.
Figure 4 shows a 24
V
input/20
A
output,
single
LED
driver with three different regu-
lated currents—determined by the analog
voltages on the
CTRL
and the digital
state of the
PWM
pins. Note that since
R
S
is only used for peak inductor current
and absolute overcurrent protection,
May 2016 :
LT Journal of Analog Innovation
| 5

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