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Linear Technology Magazine V17N4 - January 2008
LINEAR TECHNOLOGY
January 2008
VOLuME XVII nuMBEr 4
IN THIS ISSUE…
Easy High Density Power:
48A Surface Mount DC/DC
Power Supply Uses Four Parallel
12A µModule Regulators
by Alan Chern
Introduction
Linear Technology’s µModule DC/DC
regulators simplify power supply
design by offering the black box con-
venience of traditional power modules
in an IC form factor. For example,
the LTM4601 µModule regulator is
a complete step-down power module
in a 15mm × 15mm × 2.8mm LGa
package.
The LTM4601 accepts 4.5V to
20V inputs and can produce outputs
anywhere from 0.6V to 5V at 12a.
The wide input and output ranges
and excellent thermal performance
of the LTM4601 allow it to be easily
dropped into a variety of applications
with minimal design effort—just set the
output voltage with a single resistor
and determine the requisite bulk input
and output capacitances.
another signiicant advantage of
the LTM4601 over power-module-
or IC-based systems is its ability to
easily scale up as loads increase. If
load requirements are greater than
one µModule regulator can produce,
simply add more modules in parallel.
The design of a parallel system involves
little more than copying and pasting
the layout of each 15mm × 15mm
µModule regulator. Electrical layout
issues are taken care of within the
µModule package—there are no ex-
ternal inductors, switches or other
components to worry about. Even heat
The LTM4601 µModule
DC/DC regulator is a high
performance power module
shrunk down to an IC
form factor. The usual
external components are
integrated into the LGA
package—including the
PWM controller, inductor,
input and output capacitors,
ultralow R DS(ON) FETs,
Schottky diodes and
compensation circuitry. Only
external bulk input and
output capacitors and one
resistor are needed to set
the output from 0.6V to 5V.
distribution is improved with paral-
lel regulators, thus enabling surface
mount solutions for high power density
applications.
To demonstrate the simplicity and
performance of a paralleled µModule
regulator design, this article discusses
electrical guidelines, layout consid-
erations, and thermal speciics for
designing a compact 48a, 0.6V–5V
V OuT , 4.5V–20V V In converter us-
ing four LTM4601 µModule DC/DC
regulators.
L , LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology
Corporation. adaptive Power, Bat-Track, BodeCaD, C-Load, DirectSense, Easy Drive, FilterCaD, Hot Swap, LinearView,
µModule, Micropower SwitcherCaD, Multimode Dimming, no Latency ΔΣ , no Latency Delta-Sigma, no r SEnSE , Operational
Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCaD, ThinSOT, True Color PWM,
ultraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the
companies that manufacture the products.
DESIGN IDEAS
....................................................27–38
(complete list on page 27)
continued on page
LINEAR TECHNOLOGY
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L LINEAR IN THE NEWS
Linear in the News… robert Dobkin, CTO of Linear Technology, stated,
“Manufactured in a bipolar transistor process, the LT3080
expands the easy-to-use linear regulator into modern high
performance systems. With its low voltage operation and
the ability to parallel devices for higher output, it can do
circuit tricks that no other regulator can. This is a new
general purpose and more useful architecture for regula-
tors that will proliferate with time.”
The LT3080 is a 1.1a 3-terminal linear regulator that can
easily be paralleled for heat spreading and is adjustable to
zero with a single resistor. This new architecture regula-
tor uses a current reference and voltage follower to allow
sharing between multiple regulators with a small length
of PC trace as ballast, enabling multiamp linear regulation
in all surface-mount systems without heat sinks.
The LT3080 achieves high performance with wide input
voltage capability from 1.2V to 40V, a dropout voltage of
only 300mV and millivolt regulation. The output voltage
is adjustable, spanning a wide range from 0V to 40V, and
the on-chip trimmed reference achieves high accuracy of
±1%. The LT3080 really shines in generating multirail
systems.
Power Electronics Technology Names
LT3080 Product of the Year
Power Electronics Technology magazine selected Linear
Technology’s LT3080 3-terminal low dropout linear regu-
lator as Product of the year. The award was presented at
the Power Electronics Technology Conference in Dallas
to Linear Technology Vice President Engineering and
Chief Technical Oficer robert Dobkin, who developed the
product. as a historical note, the LT3080 is a signiicant
reinement over the industry-standard 3-terminal linear
regulators irst developed by robert Dobkin over 30 years
ago.
David Morrison, Editor of Power Electronics Technol-
ogy , stated, “among the hundreds of power components
introduced each year, there are numerous devices with
exciting performance improvements and novel features.
This continuing wave of innovation makes selecting a single
product for special recognition a particularly daunting
challenge. Linear Technology’s LT3080 was selected as this
year’s Product of the year because it offers an intriguing
combination of novelty and usefulness. By redesigning the
low-dropout linear regulator, Linear has given engineers
an extremely lexible building block that should help solve
current and future board-level power challenges.”
Linear Highlights µModule Regulators
in FPGA Net Seminar
Linear Technology power module Development Manager
Eddie Beville recently co-presented a web seminar entitled,
“Xilinx Virtex-5 Power Optimization and Power Design
Guidelines.” The online seminar is designed to teach design-
ers how to leverage
the dedicated blocks
in Virtex-5, using the
Xilinx Power Estima-
tor (XPE) to reduce
power consumption,
increase system reli-
ability and simplify
thermal management
and power supply
design for FPGa-
based systems. It also
demonstrates how
to implement Linear
Technology power
management solu-
tions via real world
design examples for Virtex-5 FPGas. The seminar showed
how to design the power distribution network using Linear
Technology’s µModule DC/DC converters, ultralow noise
VLDOs and other devices for key system functions.
The seminar was conducted on EE Times’ TechOnline
engineering education website. It is currently available for
viewing at www.techonline.com/learning/webinar/. L
2
Linear Technology Magazine January 2008
Big Power in Small Packages
Linear has just completed the launch of a new family of
high voltage µModule™ DC/DC converters. These small,
low proile devices are instant power supplies, packing a
range of power system solutions into surface mount pack-
ages that can be automatically placed on either side of a PC
board. With the introduction of the LTM802X high voltage
µModule regulators, Linear has expanded its offering to
solutions ideal for 24V industrial, 28V medical, automotive
and avionics applications. (For more, see page 36.)
2
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DESIGN FEATURES L
LTM4601, continued from page 1
Figure 1. Designing a high density power supply for a limited space application could not be easier. Here, four LTM4601 µModule
regulators are paralleled in a simple scheme. Board layout is just as easy, since there are so few external components.
Linear Technology Magazine January 2008
3
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L DESIGN FEATURES
DC/DC µModule Regulator:
A Complete System in an
LGA Package
The LTM4601 µModule DC/DC
regulator is a high performance power
module shrunk down to an IC form
factor. It is a completely integrated
solution—including the PWM con-
troller, inductor, input and output
capacitors, ultralow r DS(On) FETs,
Schottky diodes and compensation
circuitry. Only external bulk input
and output capacitors and one resis-
tor are needed to set the output from
0.6V to 5V. The supply can produce
12a (more if paralleled) from a wide
input range of 4.5V to 20V, making it
extremely versatile. The pin compat-
ible LTM4601HV extends the input
range to 28V.
Output features include output
voltage tracking and margining. The
high switching frequency, typically
850kHz at full load, constant on time,
zero latency controller delivers fast
transient response to line and load
changes while maintaining stability.
Should frequency harmonics be a
concern, an external clock can control
synchronization via an on chip phase
lock loop.
100
90
80
70
60
50
40
30
3.3V OUT
2.5V OUT
1.8V OUT
1.5V OUT
1.2V OUT
20
10
0
0
10
20
30
40
50
1µs/DIV
LOAD CURRENT (A)
Figure 2. Individual LTM4601 switching
waveforms for the circuit in Figure 1 shows
the 90° out-of-phase relationship.
Figure 3. Efficiency of the four
parallel LTM4601s remains high
over a wide range of outputs
V IN
C IN
C IN
GND
SIGNAL
GND
C OUT
C OUT
V OUT
Figure 4. The LTM4601’s pin layout promotes simple power
plane placement and uncomplicated part paralleling
48A from Four
Parallel µModule Regulators
Figure 1 shows a regulator compris-
ing four parallel LTM4601s, which
can produce a 48a (4 ×12a) output.
The regulators are synchronized but
operate 90° out of phase with respect
to each other, thereby reducing the
amplitude of input and output ripple
currents through cancellation. The at-
tenuated ripple in turn decreases the
external capacitor rMS current rating
and size requirements, further reduc-
ing solution cost and board space.
Synchronization and phase shift-
ing is implemented via the LTC6902
oscillator, which provides four clock
outputs, each 90° phase shifted (for 2-
or 3-phase relationships, the LTC6902
can be adjusted via a resistor.). The
clock signals serve as input to the
PLLIn (phase lock loop in) pins of the
four LTM4601s. The phase-lock loop
of the LTM4601 comprises a phase
detector and a voltage controlled os-
Figure 5. Top layer planes for 4-parallel µModule system
Figure 6. Bottom layer planes for 4-parallel µModule system
4
Linear Technology Magazine January 2008
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DESIGN FEATURES L
become paramount, so the important
parameters are spacing, vias, airlow
and planes.
The LTM4601 µModule regulator
has a unique LGa package footprint,
which allows solid attachment to the
PCB while enhancing thermal heat
sinking. The footprint itself simpliies
layout of the power and ground planes,
as shown in Figure 4. Laying out four
parallel µModule regulators is just as
easy, as shown in Figures 5 and 6.
V OUT
GND
V IN
Figure 7. Thermograph of four
parallel LTM4601s without airflow
(20V input to 1.5V output at 40A)
Figure 8. Via placement (cross marks)
under a single µModule regulator
cillator, which combine to lock onto
the rising edge of an external clock
with a frequency range of 850kHz
± 30%. The phase lock loop is turned
on when a pulse of at least 400ns
and 2V amplitude at the PLLIn pin is
detected, though it is disabled during
start-up. Figure 2 shows the switching
waveforms of four LTM4601 µModule
regulators in parallel.
Only one resistor is required to set
the output voltage in a parallel setup,
but the value of the resistor depends
on the number of LTM4601s used.
This is because the effective value of
the top (internal) feedback resistor
changes as you parallel LTM4601s.
The LTM4601’s reference voltage is
0.6V and its internal top feedback
resistor value is 60.4k Ω , so the re-
lationship between V OuT , the output
voltage setting resistor (r FB ) and the
number of modules (n) placed in
parallel is:
Figure 9. Thermograph of four parallel LTM4601s with 200LFM
bottom-to-top airflow (20V input to 1.5V output at 40A)
60 4
k
+
R
FB
n
V
=
0 6
V
OUT
R
FB
Figure 10. Thermograph of four parallel LTM4601s with 400LFM right-to-left
airflow in 50°C ambient chamber (12V input to 1V output at 40A)
Figure 3 illustrates the system’s
high eficiency over the vast output
current range up to 48a. The system
performs impressively with no dipping
in the eficiency curve for a broad range
of output voltages.
Layout
Layout of the parallel µModule regula-
tors is relatively simple, in that there
are few electrical design consider-
ations. nevertheless, if the intent of
a design is to minimize the required
PCB area, thermal considerations
Figure 11. Thermograph of four parallel LTM4601s with BGA heat sinks and 400LFM
right-to-left airflow in a 75°C ambient chamber (12V input to 1V output at 40A)
Linear Technology Magazine January 2008
5
.
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