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October 2011
Volume 21 Number 3
I N
T H I S
I S S U E
2-channel and 4-channel
pin-selectable I
2
C
multiplexer
11
high efficiency power
supply for Intel IMVP-
6/6+/6.5 CPUs
20
Fast, Accurate Step-Down DC/DC
Controller Converts 24V Directly to
1.8V at 2MHz
Bud Abesingha
3A linear regulator easily
paralleled to spread power
and heat
24
LTspice IV update
36
frequency shifter for
isolated PWM control
40
The continuous march in electronics toward lower supply voltages
and higher load currents puts tremendous pressure on point-of-
load DC/DC converters to maintain a fast pace of performance
improvements. For instance, a lower supply voltage means a
regulator must support a higher step-down ratio from a 12V
or 24V power rail while maintaining high efficiency. Regulation
accuracy also becomes more important as supply voltages
drop—and accuracy must be maintained in the
presence of parasitic IR drops and dynamic load
transients. EMI generated by switching converters
is also of concern, especially in RF applications.
Some applications require that their power supplies meet all of these strin-
gent requirements: high power
,
high efficiency
,
high accuracy
,
high step-
down ratio
,
fast transient performance and low EMI—and that they do
it in a small footprint. The LTC
®
3833 is a high performance synchronous
step-down DC
/
DC controller that steps up to the challenge. Figure
1
shows
a typical application. The LTC3833 accepts an unregulated input volt-
age between 4.5V and 38V (40V abs max) and downconverts it to 0.67%
accurate output voltage between 0.6V and 5.5V (6V abs max).
It features a 20ns minimum on-time
,
enabling a high step-down ratio
(high V
IN
to low V
OUT
) at high frequency (up to 2MH
z
)
,
and its con-
trol architecture is primed for fast transient performance. The
LTC3833 is offered in 20-pin QFN (3mm × 4mm) and TSSOP pack-
ages with exposed pads for enhanced thermal performance.
(continued on page 4)
Caption
Published September 2011 and now available. See page 2.
w w w. li n e ar.co m
In this issue...
COVER STORY
Fast, Accurate Step-Down DC/DC Controller
Converts 24V Directly to 1.8V at 2MHz
Bud Abesingha
1
Linear in the News
ANALOG CIRCUIT DESIGN BOOK PUBLISHED
DESIGN FEATURES
2-Channel and 4-Channel Pin-Selectable I C
Multiplexer Features High Noise Margin, Capacitance
Buffering, Level Translation and Stuck Bus Recovery
2
Rajesh Venugopal
11
Low I
Q
, High Efficiency Dual Output Controllers for
Wide Ranging Input and Output Voltages
Jason Leonard
16
The much-anticipated book
,
Analog Circuit Design: A Tutorial Guide to
Applications and Solutions
was just published by Newnes
,
an imprint of
Elsevier Science & Technology Books. Edited by industry gurus
,
Bob Dobkin
and Jim Williams
,
the 960-page book covers a broad range of analog design
topics. This is the first time that such an extensive collection of applica-
tion notes has appeared in one volume from Linear Technology engineers.
Analog Circuit Design
is a comprehensive source book of circuit design
solutions that will aid systems designers with elegant and practical
design techniques that focus on common circuit design challenges.
The book includes an extensive power management section
,
cover-
ing such topics as switching regulator design
,
linear regulator design
,
high voltage and high current applications
,
powering lasers and illumi-
nation devices
,
and automotive and industrial power design. Other sec-
tions of the book span a broad range of analog design areas
,
including
data conversion
,
signal conditioning and high frequency
/
RF design.
Jim Williams wrote in the book’s Introduction
,
“The nature of ana-
log circuit design is so diverse
,
the devices so sophisticated
,
and user
requirements so demanding that designers require (or at least welcome)
assistance. Ultimately
,
the use of analog IC
s
is tied to the user’s ability to
solve the problems confronting them. Anything that enhances this abil-
ity
,
in both specific and general cases
,
obviously benefits all concerned.”
Bob Dobkin stated in the book’s Foreword
,
“One of the best avenues for learn-
ing analog design is to use the application notes and information from companies
who supply analog integrated circuits. These application notes include circuitry
,
test results
,
and the basic reasoning for some of the choices made in the design
of these analog circuits. They provide a good starting point for new designs.
“Since the applications are aimed at solving problems
,
the application
notes
,
combined with the capability to simulate circuits on Spice
,
pro-
vide a key learning pathway for engineers. The analog information in
most of these application notes is timeless and will be as valid twenty
years from now as it is today. It’s my hope that anyone reading this
book is helped through the science and art of good analog design.”
High Efficiency Power Supply for Intel
IMVP-6/IMVP-6+/IMVP-6.5 CPUs
Jian Li and Gina Le
20
3A Linear Regulator Can Be Easily Paralleled to
Spread Power and Heat
Todd Owen
24
8-Output Regulator Powers Applications Processors
Kevin Ohlson
27
3-Phase Synchronous Step-Down DC/DC
Controller with Stage Shedding™, Active Voltage
Positioning and Nonlinear Control for High
Efficiency and Fast Transient Response
Jian Li and Kerry Holliday
32
DESIGN IDEAS
What’s New with LTspice
®
IV?
Gabino Alonso
36
15A µModule
®
Regulator Solves Thermal Problems
by Converting 12V to 1V with High Efficiency
Eddie Beville
38
Solve Isolated Control Problems by Up-Shifting
Control Frequency with TimerBlox
®
PWM Generator
Tim Regan
40
43
44
product briefs
back page circuits
For more information, go to
www.linear.com/designtools/acd_book.php.
To
purchase
Analog Circuit Design,
click on the Elsevier link for a 30 percent
discount on the cover price, or go to the Amazon link at the bottom of the page.
2 | October 2011 :
LT Journal of Analog Innovation
Linear in the news
Jim Williams’ workbench
COMPUTER HISTORY MUSEUM
EXHIBIT ON ANALOG &
JIM WILLIAMS OPENS
The Computer History Museum
in Mountain View
,
California has
announced a new exhibit opening this
month
,
“An Analog Life: Remembering
Jim Williams.” The exhibit
,
which
runs from October
1
5
,
20
11
until April
1
5
,
20
1
2
,
will focus on
how engineers work.
In their project overview for the exhibit
,
the museum’s curator stated
,
“It is said
that we spend about one-third of our lives
at work. How we work is often reflected
in the way we organize our desks and
workspaces. Analog circuit guru Jim
Williams’ workbench tells us some things
about the way he worked. For example
,
with its years-old strata of past circuits
embedded in a matrix of thousands of
overlapping components and still more
circuits
,
we see someone who worked
iteratively
,
drawing on past designs to con-
tinually invent new circuits and systems.
“Jim Williams’ workbench is an inspi-
rational object that allows Computer
History Museum visitors to explore
engineering work styles
,
be inspired
by an extraordinary person and dis-
cover the world of analog circuitry and
its impact on today’s technology.”
The centerpiece of the exhibit is Jim
Williams’ engineering workbench
,
which
the museum carefully transported from his
Linear Technology lab to the museum. The
display will include interpretive graph-
ics
,
explaining various aspects of analog
design. In addition
,
the exhibit includes
video interviews with engineers who
worked closely with Jim Williams over the
years
,
plus video footage of his labs
,
both
at Linear Technology and at his home lab.
The “Analog Life” exhibit will launch
on October
1
5
,
with an evening event at
the Computer History Museum. Visitors
to the launch will have an opportu-
nity to view the exhibit
,
hear a panel
discussion of analog experts discuss-
ing analog design and Jim Williams’
contributions
,
and attend a book sign-
ing of the new
Analog Circuit Design
book with co-editor Bob Dobkin. For
more information about the exhibit
,
visit
computerhistory.org
/
highlights
/
analoglife.
30 YEARS OF ANALOG INNOVATION
On September 26
,
Linear Technology com-
memorated three decades of innovation
in analog integrated circuits. When the
company was founded in September
1
98
1
,
at the dawn of the
digital revolution
,
some questioned the
wisdom of founding
a company focused
purely on analog
technology. Since
then
,
the analog
market has grown 20-fold with Linear
contributing solutions to all corners of the
electronics industry. The dawn of digital
only
increased
the demand for analog.
Linear is consistently at the leading edge
of new electronic markets as they have
emerged. These include the PC revolution
,
laptop and tablet computers
,
industrial
control and robotics
,
network infrastruc-
ture
,
cellular and satellite communications
,
automotive electronics including advanced
displays
,
electronic braking and steering
and now the growth of the hybrid
/
electric
automotive segment
,
to name just a few.
n
October 2011 :
LT Journal of Analog Innovation
| 3
A video interview with Linear Technology co-founders Bob Swanson and Bob Dobkin
and CEO Lothar Maier can be found at
www.linear.com/30yearinterview.
The LTC3833 is a high performance synchronous
step-down DC/DC controller that regulates to 0.67%
output accuracy, operates up to 2MHz switching
frequency and has a 20ns minimum on-time.
(
LTC
3833, continued from page 1)
FAST TRANSIENT PERFORMANCE
AND CONSTANT FREQUENCY
EFFICIENCY
➘
6V
V
IN
12V
15V
24V
FREQUENCY, INDUCTANCE
200kHz, 2.00μH
500kHz, 0.82μH
1MHz, 0.47μH
2MHz, 0.20μH
The LTC3833 uses a new
,
sophisticated
controlled-on-time
architecture—a
variant of the
constant
on-time control
architecture with the distinction that the
on-time is controlled so that the switch-
ing frequency remains constant over
steady state conditions under line and
load. This architecture takes advantage
of all the benefits of a constant on-time
controller
,
namely fast transient response
and small on-times for high step-down
ratios
,
while imitating the behaviors
of a constant frequency controller.
The LTC3833 can respond to a load step
immediately without waiting until the next
switching cycle as in a conventional con-
stant frequency controller. During a load
step
,
the LTC3833 increases its switching
91%
92%
92%
91%
92%
92%
91%
88%
91%
89%
87%
83%
87%
84%
81%
73%
Table 1: Example of efficiency variation over input and frequency. Higher frequencies have lower efficiencies
but allow smaller component size for compact solutions. V
OUT
= 1.8V I
LOAD
= 10A.
frequency to respond faster and reduce
the droop on the output. Similarly
,
during
a load release
,
the LTC3833 reduces the
switching frequency in order to prevent
the input rail from charging the output
capacitor any further. Once the transient
condition subsides
,
the LTC3833 brings the
switching frequency back to the nominal
programmed value
,
or to the external
clock frequency if it is being synchronized.
INTV
CC
V
RNG
R
PGD
100k
LTC3833
PGOOD
RUN
EXTV
CC
V
OUT
SENSE
–
SENSE
+
TG
SW
TRACK/SS
BOOST
D
B
ITH
INTV
CC
BG
RT
SGND
PGND
V
OSNS+
R
FB1
10k
R
FB2
31.6k
INTV
CC
C
VCC
4.7µF
MB
C
B
0.1µF
V
IN
C
IN2
10µF
+
C
IN1
47µF
35V
V
IN
6V TO 28V
The LTC3833’s low minimum off-time
of 90ns allows it to achieve high duty
cycle operation and thus avoid output
dropout when V
IN
is only slightly above
the required V
OUT
. The low minimum
off-time also factors into fast transient
performance. If the switching con-
verter’s control loop is designed for
high bandwidth and high speed
,
the
minimum off-time of the LTC3833 does
not limit performance. That is
,
in a load
step condition
,
the time between con-
secutive on-time pulses can be as low
as 90ns for a high bandwidth design.
Figure 2 shows a low voltage
,
high current
application typical of a microprocessor
power supply where the LTC3833 responds
quickly to a 20A load step and release.
WIDE FREQUENCY RANGE FOR A
MULTITUDE OF APPLICATIONS
1.2MHz
C
SS
0.01µF
C
ITH1
220pF R
ITH
20k
R
T
33.2k
MODE/PLLIN
R
DCR
MT 1.1k
L1
1µH
C
DCR
0.1µF
V
OUT
2.5V
5A
C
OUT1
100µF
V
OSNS–
Figure 1. 28V input, 2.5V output, 5A, 1.2MHz
step-down converter. The high frequency
capabilities of the LTC3833 enable designs
that can squeeze into tight spaces.
C
IN1
: KEMET T521X476M035ATE070
D
B
: DIODES INC. SDM10K45
L1: VISHAY IHLP2525CZ-1µH
MT, MB: VISHAY/SILICONIX Si4816BDY
The LTC3833 is capable of a full decade
of switching frequency
,
from 200
k
H
z
to 2MH
z
(programmed with an external
resistor on the RT pin). This wide range
allows the LTC3833 to meet the require-
ments of a wide variety of applications
,
from low frequency applications that
require high efficiency
,
to higher frequency
4 | October 2011 :
LT Journal of Analog Innovation
design features
INTV
CC
V
IN
C
IN2
22µF
×2
V
IN
4.5V TO 14V
C
IN1
180µF
16V
+
Figure 2a. 14V input, 1.5V output, 20A, 300kHz step-down converter. The
LTC3833 excels in low voltage, high current applications such as these,
which are typical of a microprocessor power supply. It can respond
quickly to sudden, high slew current requirements of the microprocessor.
R
PGD
100k
LTC3833
PGOOD
V
OUT
SENSE
–
RUN
V
RNG
MODE/PLLIN
EXTV
CC
SENSE
+
TG
SW
BOOST
D
B
C
B
0.1µF
INTV
CC
C
VCC
4.7µF
BG
ITH
RT
SGND
PGND
V
OSNS+
V
OSNS–
MB
MT
L1
0.47µH
R
SENSE
1.5m
R
FB2
15k
R
FB1
10k
C
OUT2
100µF
×2
applications that require smaller solu-
tion size
,
to 2MH
z
applications that stay
above the AM radio band while being
able to downconvert from a high input
rail and deliver high output current.
The choice of operating frequency is a
tradeoff between efficiency and compo-
nent size. Lower frequencies are more
efficient due to a reduction of switching-
related losses in the converter. On the
other hand
,
lower frequencies require
larger inductors and capacitors to achieve
a given output ripple. At higher frequen-
cies
,
smaller components can be used to
achieve the same output ripple
,
but at the
cost of efficiency. Table
1
illustrates the
trade-offs between efficiency and induc-
tor size required to maintain output ripple
when the LTC3833 is used to generate a
1
.8V output at several frequencies and
input voltages. A
s
seen from the table
,
switching losses are exacerbated at higher
frequencies and higher V
IN
,
mainly due to
the higher V
DS
across the high side MOSFET.
C
SS
0.1µF
C
ITH2
47pF
C
ITH1
220pF
R
ITH
84.5k
R
T
137k
TRACK/SS
INTV
CC
V
OUT
1.5V
20A
C
OUT1
330µF
2.5V
×2
+
C
IN1
: SANYO 16SVP180M
C
OUT1
: SANYO 2R5TPE330M9
D
B
: CENTRAL CMDSH-3
L1: PULSE PA0515.471NLT
MB: RENESAS RJK0330DPB
MT: RENESAS RJK0305DPB
The LTC3833’s wide frequency range also
helps minimize EMI interference from
the switching regulator. The switching
frequency can be chosen
,
and held over
line and load
,
such that the operating fre-
quency and harmonics of the regulator fall
outside of the frequency band of the end
application. This allows the end applica-
tion to easily filter out switching noise of
the DC
/
DC converter. Figure 3 shows an
example of a 5.5V application that operates
above the AM radio band (f
SW
>
1
800
k
H
z
)
that could be used to power electronics
in an automotive infotainment system.
The LTC3833 provides an additional
safeguard against EMI and noise interfer-
ence by allowing it to be synchronized
to an external clock applied to the
MODE
/
PLLIN pin. This way
,
the end appli-
cation has control over the DC
/
DC con-
verter’s switching cycles and timing so
it does not interfere during critical time
periods in the application where sensi-
tive signal processing might occur.
Figure 2b. The LTC3833 can respond quickly to sudden, high slew current requirements.
I
LOAD
20A/DIV
I
LOAD
20A/DIV
I
LOAD
20A/DIV
V
OUT
50mV/DIV
V
OUT
50mV/DIV
V
OUT
50mV/DIV
I
L
20A/DIV
50µs/DIV
V
IN
= 12V
V
OUT
= 1.5V
LOAD TRANSIENT = 0A TO 20A
I
L
20A/DIV
5µs/DIV
V
IN
= 12V
V
OUT
= 1.5V
LOAD STEP = 0A TO 20A
I
L
20A/DIV
5µs/DIV
V
IN
= 12V
V
OUT
= 1.5V
LOAD RELEASE = 20A TO 0A
October 2011 :
LT Journal of Analog Innovation
| 5

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