Understanding
Silent Switcher
Technology:
High Efficiency,
Low EMIVISIT ANALOG.COM/POWER

Table of Contents
4 How the Smart Hardware Engineer Can Easily
Design Power Supplies: Mini Tutorial
11 4-Switch Buck-Boost Controller Layout for Low
Emissions—Single Hot Loop vs. Dual Hot Loop
16 High Power Density in a Small Form Factor
9 Article Spotlight►►►

Improving Power Supply Design Using Semi-Automation—Five Steps to Quickand Efficient Design
Ultralow Noise, High Current, Compact DC-to-DC Converter Solution for Signal andData Processing Circuits
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Matched Resistor Networks

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Technical Article

How the Smart
Hardware Engineer
Can Easily Design Power
Supplies: Mini Tutorial
Frederik Dostal
Abstract

Input Voltage

This mini tutorial gives an overview of the possibilities for power supply design.
It will address the basic and commonly used isolated and nonisolated power
supply topologies along with their advantages and disadvantages. We will also
cover electromagnetic interference (EMI) and filtering considerations. This minitutorial aims to provide a simplified understanding and renewed appreciation
for the art of power supply design.

Introduction
Most electronic systems require some sort of voltage conversion between thevoltage of the energy supply and the voltage of the circuitry that needs to be
powered. As batteries lose charge, the voltage will drop. Some DC-to-DC conversion can ensure that much more of the stored energy in the battery is used to
power the circuitry. Also, for example, with a 110 V AC line, we cannot powera semiconductor such as a microcontroller directly. Since voltage converters,also named power supplies, are used in almost every electronic system, they
have been optimized for different purposes over the years. Certainly, some of
the usual targets for optimization are solution size, conversion efficiency, EMI,and cost.

VIN

VOUT
Linear Regulator
FB
GND

Input Voltage

Output Voltage
Adjustable Resistor

Figure 1. A linear regulator converts one voltage into another.

For many years, a typical power converter consisted of a 50 Hz or 60 Hz transformer, connected to the power grid, with a certain windings ratio to generate anonregulated output voltage, a few volts higher than the needed supply voltage
in a system. Then, a linear regulator was used to convert this voltage to a wellregulated one as needed by the electronics. Figure 2 shows the block diagramof this concept.
110 V ACLine Voltage

Intermediate
Voltage

The Simplest Power Supply: The LDO
One of the simplest forms of a power supply is the low dropout (LDO) regulator. LDOs are linear regulators as opposed to switching regulators. Linear
regulators put a tunable resistor between the input voltage and the output voltage, which means the output voltage is fixed independent of how
the input voltage changes and which load current is running through thedevice. Figure 1 shows the basic principle of this simple voltage converter.

4

Output Voltage

VIN
VOUT
Linear Regulator
FB
GND

OutputVoltage

Figure 2. A line transformer followed by a linear regulator.

The problem with the basic setup in Figure 2 is that the 50 Hz/60 Hz transformer
is relatively bulky and expensive. Also, the linear regulator dissipates quite a lotof heat, so the total system efficiency is low and getting rid of the generated
heat is difficult with high system power.

How the Smart Hardware Engineer Can Easily Design Power Supplies: Mini TutorialSwitch-Mode Power Supplies to the Rescue
To avoid the disadvantages of a power supply as shown in Figure 2, switchmode power supplies were invented. They do not rely on 50 Hz or 60 Hz AC
voltage. They take a DC voltage, sometimes rectified AC voltage, and generatea much higher frequency AC voltage to use a much smaller transformer or, innonisolated systems, to rectify the voltage with an LC filter to generate a DC
output voltage. The advantages are small solution size and relatively low cost.
The AC voltage being generated does not need to be a sine voltage waveform. A
simple PWM signal shape will work just fine and is easy to generate with a PWM
generator and a switch.
Up until the year 2000, bipolar transistors were the most commonly used
switches. They would work well but had relatively slow switching transition
speed. They were not very power efficient, limiting the switching frequency to
50 kHz or maybe 100 kHz. Today we use switching MOSFETs instead of bipolartransistors, allowing for much faster switching transitions. This in turn gives us
lower switching losses, allowing for switching frequencies of up to 5 MHz. Suchhigh switching frequencies enable the use of very small inductors and capacitors in the power stage.
Switching regulators bring a lot of benefits. They generally offer a power
efficient voltage conversion, allow voltage step-up and step-down, and offer
relatively compact and low cost designs. The disadvantages are that they
are not so simple to design and optimize, and they generate EMI from theswitching transitions and the switching frequency. The availability of switchmode power supply regulators, along with power supply design tools such as
LTpowerCAD® and LTspice®, has greatly simplified this difficult design process.
With such tools, the circuit design process of a switch-mode power supply canbe semi-automated.

Isolation in Power Supplies
When designing a power supply, the first question to answer is whether or not
galvanic isolation is required. Galvanic isolation is used for multiple reasons. It
can make circuits safer, it allows for floating system operation, and it prevents
noisy ground currents from spreading through different electronic devices in
one circuitry. The two most common isolated topologies are the flyback andforward converters. However, for higher power, other isolated topologies suchas push-pull, half-bridge, and full-bridge are used.
If galvanic isolation is not required, in most cases a nonisolated topology isused. Isolated topologies always require a transformer and such a device tends
to be expensive, bulky, and often difficult to get off-the-shelf with the exact
requirements that a custom power supply requires.

Most Common Topologies When Isolation Is
Not Required
The most common nonisolated switch-mode power supply topology is thebuck converter. It is also known as the step-down converter. It accepts a
positive input voltage and generates an output voltage lower than the input
voltage. It is one of the three most basic switch-mode power supply topologies that only require two switches, an inductor, and two capacitors. Figure
3 shows the basic principle of this topology. The high-side switch pulses
a current from the input and generates a switch node voltage alternating
between the input voltage and ground voltage. The LC filter takes that pulsed
voltage on the switch node and generates a DC output voltage. Depending on
the duty cycle of the PWM signal controlling the high-side switch, a different level of DC output voltage is generated. This DC-to-DC buck converter isvery power efficient, is relatively easy to build, and requires few components.

Input Voltage

Output Voltage

Figure 3. Concept of a simple buck step-down converter.

The buck converter pulses current on the input side, while the output side hascontinuous current coming from the inductor. This is the reason why a buckregulator is very noisy on the input side and not so noisy on the output side.
Understanding this is important when low noise systems need to be designed.
Besides the buck topology, the second basic topology is the boost, or step-up,
topology. It uses the same five basic power components as the buck converter,
but rearranged, so that the inductor is placed on the input side and the highside switch is placed on the output side. The boost topology is used to step up acertain input voltage to an output voltage that is higher than the input voltage.
Input Voltage

Output Voltage

Figure 4. Concept of a simple boost step-up converter.

When selecting a boost converter, it is important to note that boost converters
always specify the maximum rated switch current and not the maximum output
current in their data sheets. In a buck converter, the maximum switch current isdirectly related to the maximum achievable output current, independent of voltage ratio between the input voltage and the output voltage. In a boost regulator,
the voltage ratio directly affects the possible maximum output current basedon a fixed maximum switch current. When selecting a suitable boost regulator
IC, you need to not only know the desired output current, but also the input andoutput voltage of the design in development.
A boost converter is very low noise on the input side, since the inductor in linewith the input connection prevents rapid changes in current flow. However, on
the output side this topology is quite noisy. We only see pulsed current flow
through the outside switch, and thus output ripple is more of a concern compared to the buck topology.
The third basic topology, only consisting of the five basic components, is theinverting buck-boost converter. The name is derived from the fact that thisconverter takes a positive input voltage and converts it into a negative output
voltage. Besides this, the input voltage may be higher or lower than the absolute of the inverted output voltage. For example, –12 V output voltage may begenerated out of 5 V or 24 V on the input. This is possible without making any
special circuit modifications. Figure 5 shows the circuit concept of the inverting
buck-boost converter.
Input Voltage

Output Voltage

Figure 5. Concept of a simple inverting buck-boost converter.

In the inverting buck-boost topology, the inductor is connected from the switch
node to ground. Both the input side as well as the output side of the convertersee pulsed current flow, making this topology relatively noisy on both the input
side as well as the output side. In low noise applications, this nature is compensated by adding additional input and output filtering.
One quite positive aspect of the inverting buck-boost topology is the fact thatany buck switching regulator IC may be used for such a converter. It is as simple
as attaching the output voltage of the buck circuit to system ground. The buck
IC circuit ground will become the adjusted negative voltage. This trait yields avery large selection in switching regulator ICs on the market.

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5Specialized Topologies
Besides the three basic nonisolated switch-mode power supply topologies previously discussed, there are many more topologies available. However, they all
require additional power components. This typically makes them higher costwith lower power conversion efficiency. While there are certain exceptions,generally, adding additional components in the power path will add losses.
Some of the most popular topologies are SEPIC, Zeta, Ćuk, and the 4-switch
buck-boost. They each offer features that the three basic topologies do notoffer. The following is a list of the most important features of each topology:
X

X

X

X

SEPIC
The SEPIC can generate a positive output voltage out of a positive input voltagethat may be higher or lower than the output voltage. Boost regulator ICs may be
used to design a SEPIC power supply. The drawback of this topology is the need fora second inductor or one coupled inductor and also a SEPIC capacitor.
Zeta
The Zeta converter is similar to the SEPIC, but it is capable of generating a positiveor negative output voltage. Also, it does not have a right-half-plane zero (RHPZ),
thereby simplifying the regulation loop. A buck converter IC can be used for sucha topology.

Besides the flyback converter, the forward converter is also very popular. It
uses a transformer in a different way than the flyback. During the on-time, while
there is current flow through the primary side winding, there is also current
flow through the secondary winding. Energy should not be stored in the core of
the transformer. After each switching cycle, we have to make sure that all themagnetization of the core is released to zero, so that the transformer will not
saturate after a number of switching cycles. This energy release out of the corecan be achieved with a few different technologies. One popular way is to use an
active clamp with a small additional switch and capacitor.
Figure 7 shows the LTspice simulation environment schematic of a forward
active clamp design using the ADP1074. In the forward converter, there is anadditional inductor in the output path compared to the flyback, as shown in
Figure 6. While this is one additional component with associated space and cost
implications, it helps to generate a lower noise output voltage compared to aflyback converter. Also, the transformer size needed for a forward converter at
the same power level as a flyback may be much smaller.

ĆukThe Ćuk converter offers an inversion of a positive input voltage into a negative
output voltage. It uses two inductors, one on the input side and one on the output
side, making it quite low noise on the input and output sides. The drawback isthat there are not very many switch-mode power conversion ICs supporting this
topology, since a negative voltage feedback pin is required for the regulation loop.
4-Switch Buck-Boost
This converter type became quite popular in recent years. It offers a positive output voltage from a positive input voltage. The input voltage may be higher or lower
than the adjusted output voltage. This converter replaces a lot of SEPIC designs, as
it offers higher power conversion efficiency and only requires one inductor.

Most Common Isolated Topologies
Besides nonisolated topologies, some applications require galvanically isolated
power converters. The reasons may be safety concerns, the need to have floating grounds in larger systems where different circuits are interconnected, or
the prevention of ground current loops in noise sensitive applications. The mostcommon isolated converter topologies are the flyback and forward converters.
The flyback converter is typically used for power levels up to 60 W. The circuit
operates in a way that during the on-time, energy is stored in a transformer.
During the off-time, this energy is released to the secondary side of the converter, powering the output. This converter is simple to build, but it requires
relatively large transformers to store all the energy necessary for proper operation. This aspect limits the topology to lower power levels. Figure 6 shows aflyback converter on the top and a forward converter on the bottom.

Figure 7. A forward active clamp circuit using the ADP1074 for generating an isolated output
voltage, as simulated in LTspice.

Advanced Isolated Topologies
Besides the flyback and the forward topologies, there are very many different
transformer-based galvanically isolated converter concepts. The following list
gives some very basic explanations about the most common converters:
X

X
Input Voltage

Output Voltage

X

Input Voltage
OutputVoltage

Figure 6. A flyback converter (top) and a forward converter (bottom).

6

Push-Pull
The push-pull topology is similar to that of the forward converter. However, insteadof one low-side switch, this topology requires two active low-side switches. Also,
it requires a primary transformer winding with a center tap. The advantage of thepush-pull is an operation with generally lower noise compared to a forward converter, and also a smaller transformer is needed. The hysteresis of the BH curve ofa transformer is utilized in two quadrants rather than just one.
Half-Bridge/Full-Bridge
These two topologies are typically used for higher power designs starting at a
few hundred watts all the way to a few kilowatts. They require high-side switches
besides low-side switches but enable very high power transfer with relatively smalltransformers.
ZVS
This term comes up often when discussing high power isolated converters.
It stands for zero voltage switching. Another term for such converters is LLC(inductor-inductor-capacitor) converters. These architectures aim for very highefficiency conversion. They generate a resonance circuit and switch the powerswitches while the voltage or current across the switches is close to zero. Thus, theswitching losses are minimized. However, such designs may be difficult to design
and the switching frequency is not fixed, sometimes yielding EMI problems.

How the Smart Hardware Engineer Can Easily Design Power Supplies: Mini TutorialSwitched Capacitor Converters
Besides linear regulators and switch-mode power supplies, there is also a thirdgroup of power converters: the switched capacitor converters. They are also
referred to as charge pumps. They use switches and capacitors to multiply
or invert voltages. They offer the big advantage of not needing any inductor.
Typically, such converters are used for low power levels of below 5 W. However,recently major advancements have been made to allow for much higher powerswitched capacitor converters. Figure 8 shows the LTC7820 in a 120 W design at
98.5% efficiency, converting 48 V to 24 V.

Switch Node
Voltage

Based on Switch
Frequency

Switch Time
Transition

Figure 9. Switching transition speed as well as switching frequency of a switch-mode
power supply.

In Figure 9 we can see that a switch-mode power supply does not only generate
noise coming from the adjusted switching frequency, but also from the switching transition speed, which is much higher in frequency. While the switchingfrequency usually runs between 500 kHz and 3 MHz, the switching transition
time may be a few nanoseconds long. At 1 ns switching transition time, we willsee 1 GHz corresponding frequency in the spectrum. At least both of those frequencies will be seen as radiated and conducted emissions. Other frequencies
may also show up coming from oscillations of the regulation loop or interactions between the power supply and filters.
There are two reasons why EMI should be reduced. The first reason is to protect the functionality of the electronic system that a specific power supply is
powering. For example, a 16-bit ADC that is used in the signal path of the systemshould not pick up switching noise coming from the power supply. The second
reason is to fulfill certain EMI regulations that are put in place by governments
all over the world to protect reliable functionality of different electronic systems simultaneously.
Figure 8. An LTC7820 fixed ratio high power charge pump DC-to-DC controller.

Digital Power Supplies
All the power supplies discussed in this article can be implemented as analog
or digital power supplies. What are digital power supplies, really? Power must
always run through an analog power stage with switches, inductors, transformers, and capacitors. The digital aspect is introduced by two digital building
blocks. The first one is digital interfacing, which allows an electronic system to“talk” and “listen” to a power supply. Different parameters may be set on-the-fly
to optimize the power supply for different operating conditions. Also, the power
supply can communicate with a main processor and raise warning or fault flags.
For example, load current, crossing a preset threshold, or excessive temperature of a power supply may easily be monitored by a system.
The second digital building block replaces an analog regulation loop with a digital loop. This can work successfully, but for most applications, the optimum isa standard analog feedback loop with some digital influence on some parameters, such as adjusting the gain of the error amplifier on-the-fly or dynamicallysetting the loop compensation parameters to enable a stable but fast feedback
loop. An example of a device with a purely digital control loop is the ADP1046A
from Analog Devices. One example of a digitally interfaced buck regulator withan analog control loop, optimized by digital influences, is the LTC3883.

EMI Considerations
Electromagnetic interference (EMI) is always a topic to pay attention to when
designing switch-mode power supplies. The reason is that switch-mode power
supplies switch high current flow on and off within very short periods of time.
The faster the switching, the better for total system efficiency. Faster switcting transitions reduce the time during which the switch is partially turned
on. During this partial turn-on time, most switching losses are generated.
Figure 9 shows the waveform of the switch node of a switch-mode power supply. Let’s imagine a buck regulator. High voltage is defined by current flow
through the high-side switch, and low voltage is defined by lack of current
flow through the high-side switch.

EMI comes in two forms, radiated EMI and conducted EMI. The most effective
ways of reducing radiated EMI is to optimize the PCB layout and to use technologies such as the Silent Switcher® technology from Analog Devices. Certainly, it isalso effective to put the circuit into a shielded metal box. However, this may not
be practical and in most cases is very costly.
Conducted EMI is typically attenuated by additional filtering. The next section
will discuss additional filtering to reduce conducted emissions.

Filtering
RC filters are basic low-pass filters. However, in power supply design, every
filter is nothing but an LC filter. Often, just adding some inductance in series isenough, since it will form an LC or CLC filter together with the input or output
capacitors of a switch-mode power supply. Sometimes only capacitors are usedas filters, but, considering the parasitic inductance on power cables or traces,
together with a capacitor we form an LC filter as well. The inductor L may be
an inductor with a core, or it may be a ferrite bead. The purpose of the LC filter
really is a low-pass effect, so that DC power can run through and higher frequency disturbances are attenuated to a large degree. An LC filter has a doublepole, so we get a high frequency attenuation of 40 dB per decade. This filter hasa relatively sharp drop-off. Designing a filter is not rocket science; however,since parasitic components of the circuit, such as trace inductance, have aneffect, modeling a filter also requires modeling the major parasitic effects. Thiscan make simulating a filter quite time consuming. Many designers with filter
design experience know which filters have worked before, and they may iteratively optimize a certain filter for a new design.
In all filter design, one needs to not only consider the small signal behavior,such as a transfer function of a filter in a Bode plot, but one also needs to beaware of the large signal effect. In any LC filter, power runs through the inductor. If that power is not needed at the output anymore, due to a sudden loadtransient, the energy stored in the inductor needs to go somewhere. It charges
up the capacitance of the filter. If the filter is not designed for such worst-caseconditions, that stored power may cause voltage overshoots that can possibly
harm circuitry.
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7

Figure 10. Designing an input filter for a buck regulator with LTpowerCAD.

Finally, filters have a certain impedance. That impedance interacts with the
impedances of the power converters that are attached to the filter. This
interaction may lead to instabilities and oscillations. Simulation tools such as
LTspice and LTpowerCAD from Analog Devices can be a big help in answering allthese questions and designing a perfect filter. Figure 10 shows the graphicaluser interface of the filter designer within the LTpowerCAD design environment.
With this tool, filter design is very simple.

Silent Switchers
Radiated emissions are difficult to block. A special shielding with some metalmaterial is needed. This can be very costly. For a long time, engineers werelooking for ways to reduce the radiated emissions that switch-mode power supplies generated. A few years ago, a great breakthrough was made with Silent
Switcher technology. By reducing the parasitic inductances in the hot loops
of a switch-mode power supply, and by splitting the hot loops into two andsetting them up in a very symmetrical way, radiated emissions mostly cancel
each other out. Today, many Silent Switcher devices are available offering much
lower radiated emissions than heritage products. Reducing the radiated emissions allows for switching transition speeds to increase without a serious EMI
penalty. Making the switching transitions faster reduces switching losses andthus allows for much higher switching frequencies. One example of this innovation is the LTC3310S, which can operate at 5 MHz switching frequency, enabling
extremely compact designs with very low cost external components.

Power Management Is a Necessity but Can Be
Enjoyable Too
In this tutorial we looked at many aspects of power supply design, including thedifferent power supply topologies and their advantages and disadvantages. For
power supply engineers, this information can be very basic, but for experts andnon-experts alike, it is helpful to have software tools such as LTpowerCAD and
LTspice to aid in the design process. With these tools, power converters can be
designed and optimized in very little time. Hopefully this tutorial inspired you tolook forward to your next power supply design challenge.

About the Author
Frederik Dostal studied microelectronics at the University of Erlangen inGermany. Starting work in the power management business in 2001, hehas been active in various applications positions including four years in
Phoenix, Arizona, where he worked on switch-mode power supplies. He
joined Analog Devices in 2009 and works as a field applications engineer
for power management at Analog Devices in München.

Figure 11. LTC3310S Silent Switcher design for the lowest radiated emissions.

8

How the Smart Hardware Engineer Can Easily Design Power Supplies: Mini Tutorial

Article Spotlight

Improving Power Supply Design Using

Semi-Automation—Five Steps to Quickand Efficient Design
Designing the correct power source is essential and complex, since there
is no one typical application. While total automation of power supply
design is yet to be achieved, a comprehensive range of semi-automatedtools are available today. This article details the use of semi-automateddesign tools through five critical steps of the power supply design process. These tools can be valuable to both the novice and expert power
supply design engineer.

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increases in raw power requirements. Designers are well aware of thethermal management problems of high power processors but may notconsider the thermal management issues of power supplies. Not unlike
the transistor-packed processors themselves, worst-case thermal
problems are inevitable when low core voltages require high current—
the overall trend for the power supplies for all data processing systems.

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Technical Article

4-Switch Buck-Boost
Controller Layout for Low
Emissions—Single Hot
Loop vs. Dual Hot Loop
Yonghwan Cho and Keith Szolusha
Automotive application circuits must meet strict EMI standards to avoid interfering with broadcast and mobile service frequency bands. In many cases, Silent
Switcher® and Silent Switcher 2 solutions can make a significant difference in theability to meet these standards. Nevertheless, in all cases, careful layout is imperative. In this article we look specifically at two possible solutions for a 4-switch
buck-boost controller and compare EMI chamber results.
A 4-switch buck-boost combines a buck and boost controller into a single IC,
where the converter operates as a buck when the output is lower than the input,
and as a boost when the output is higher than the input. In the region where theoutput and input are similar, all four switches may operate.
Using ADI’s in-house EMI chamber at Santa Clara, CA, the Power Products researchteam launched an investigation into the effectiveness of the original dual hot loop
synchronous layout and if an alternative layout could be used for lower EMI noise
to pass the EMI standard.
The dual hot loop layout involves the symmetrical placement of hot loop ceramic
capacitors around power MOSFETs to contain EMI noise. ADI’s unique sense
resistor location—alongside the inductor and outside of the hot loops—allows
these loops to be very small, and thus minimize the antenna effect of the hotloops. To achieve this symmetry and enable the switch node(s) to reach the nearby
inductor, switching node vias are required, which may compromise the hot loop
area. Using the CISPR 25 compliant EMI chamber, the research team found that
exposure of the switching node and large hot loop area results in unwanted
conducted EMI especially at >30 MHz (the FM radio band), which is the mostchallenging frequency range to attenuate.
The original buck-boost layout, which has a single hot loop, can improve its smallest hot loop by rearranging the power MOSFETs and hot loop capacitors. This layout is known as a single hot loop as a counterpart of the dual hot loop. The benefit
of using a single hot loop is not only smaller switching loss but also that >30 MHz
conducted emissions (CE) are attenuated due to the minimized hot loop area andthe exposure of the switching node. Its effectiveness is verified by comparing the
EMI noise of the new layout to a dual hot loop using the same controller IC andsame power components. A 4-switch buck-boost controller, LT8392, and its two
versions of demo circuit (DC2626A rev.2 and rev.3) were used for the experiment.

Layout Comparison
Figure 1 shows the layout and assembled board pictures of a dual hot loop andsingle hot loop. Each board has four layers: a top layer (Layer 1), Layer 2, Layer 3,
and a bottom layer (Layer 4). However, only the top and bottom layers are shown.
As shown in Figure 1(a), hot loop capacitors are placed at the left and right side of
the center MOSFETs and form identical hot loops. Switching node vias are used toconnect the switching nodes, SW1 and SW2, to the main power inductor throughthe bottom layer (shown in Figure 1(c)) and Layer 3. The SW1 and SW2 top layer
copper nodes are laid out with large area to dissipate the heat of the inductor and
MOSFETs. But at the same time, the largely exposed SW1 and SW2 copper nodes
are a source of EMI emission. If the board is mounted near chassis ground, parasitic capacitance is formed between the chassis and the switching node copper.
It makes high frequency noise flow from the switching node to chassis ground andaffects other circuits in the system. In the CISPR 25 compliant EMI chamber, thehigh frequency noise flows through the ground table of the EMI setup and LISN.
The exposed switching node also acts as an antenna and, thus, causes radiatedEMI noise.
However, a single hot loop does not have the exposed switching node copper atthe bottom layer, as shown in Figure 1(d). At the top layer, shown in Figure 1(b),the hot loop capacitors are placed at only one side of the MOSFETs, which makes
it possible for the switching node to be connected to the inductor without using
switching node vias.
In the single hot loop layout, the top and bottom MOSFETs are not aligned, but oneof them is 90° rotated to make the hot loop as small as possible. The size of the
hot loop of the dual hot loop and the single hot loop are compared in Figure 1(e)and Figure 1(f) with the yellow highlighted box. These boxes show that the hot loopof a single hot loop is half the size of the dual hot loop.
It should be noted that the two 0402 hot loop capacitors of the dual hot loop
shown in Figure 1(a) are not used and the 1210 hot loop capacitors are squeezed to
the MOSFETs to make the smallest hot loop.
A solder mask near the 0402 capacitor pads is peeled off for good connectivity of1210 capacitors. Also, the solder mask near the inductor pad is removed to use thesame inductor in the single hot loop circuit. A smaller hot loop means that the total
VISIT ANALOG.COM/POWER

11

SW1

SW2

SW1

SW2

(a) Dual Hot Loop Top Layer(b) Single Hot Loop Top Layer(c) Dual Hot Loop Bottom Layer(d) Single Hot Loop Bottom Layer(e) Dual Hot Loop Top Component(f) Single Hot Loop Component

Figure 1. Layout and photograph of a dual hot loop and single hot loop.

inductance of the loop is smaller. Thus, switching loss is reduced and LC ringing
of the switching node and switching current is attenuated. Also, the smaller loop
contributes to lower conducted EMI above 30 MHz as radiated emissions affectconducted EMI in that range.
ADI’s 4-switch buck-boost controller can form the smallest hot loop due to the
proprietary peak buck/peak boost current-mode control scheme. The current
sense resistor is connected in series with the main inductor. In contrast, competitors’ controller parts use a valley buck/peak boost current-mode control scheme
where the current sense resistor should be put between the source of the bottom
MOSFETs and ground. Figure 2 shows the recommended buck-boost layout of oneof these parts. As shown with the yellow box, the hot loop is larger than the dual
hot loop or single hot loop. Moreover, the parasitic inductance of the sense resistor increases the total inductance of the hot loop.

L1

SW1

SW2
VOUT

VIN
QL1

QL2

QH1
CIN

QH2

CIN

RSENSE

GND

LM5176

RISNS
COUT

COUT

GND

Figure 2. The recommended buck-boost layout of competitor part LM5176.

12

EMI Comparison
The EMI of the dual hot loop and single hot loop is measured in the CISPR 25
compliant EMI chamber and shown in Figure 3 with a CISPR 25 Class 5 standards
limit. The EMI results are plotted in the same graph to compare the difference,with a dual hot loop marked with a yellow line and a single hot loop marked with a
red line. The gray line is the noise floor that is measured at ambient condition. As
shown in Figure 4, the exposed switching nodes of the bottom layers of a dual hot
loop were ground shielded with copper tape to show how effective the smaller hot
loop is. The emission of a dual hot loop without the copper shield is much higher
than the result in Figure 3. The output is 12 V, 8 A and the input voltage was set to
13 V to make the circuit operate in 4-switch switching mode.
Figure 3(a) shows the peak and average of voltage method conducted emissions,
respectively. A single hot loop has 5 dBµV lower CE above 30 MHz and it satisfies
the CISPR 25 Class 5 standard for both peak and average CE while dual hot loops
have overshoot in average at FM and VHF band (68 MHz to ~108 MHz), as shown inthe yellow highlighted box.
Note that reducing 5 dBµV in that frequency range is really challenging. A single
hot loop is effective not only at the high frequency range of 30 MHz, which is
the most challenging region to attenuate, but also at low frequency (<2 MHz)
that includes AM band (0.53 MHz to ~1.8 MHz). It is always better to have lower
emissions, especially if they are CE, since they affect all of an electrically connected system.
The current probe method is another measurement method that CISPR 25 Class
5 specifies. It measures common-mode conducted emissions at two different
positions, 50 mm and 750 mm from DUT, while the voltage method measures
mixed conducted emissions of both common mode and differential mode. Figure3(b) and 3(c) compare the current probe method conducted emissions of thedual hot loop and single hot loop. They show that the single hot loop has lower

4-Switch Buck-Boost Controller Layout for Low Emissions—Single Hot Loop vs. Dual Hot Loop

80

60

Single Hot Loop
Dual Hot Loop

70

Average Conducted EMI (dBµV)
Max

Peak Conducted EMI (dBµV)
Max

60
50
40
30
20
10
0

–20
0.1

1

Frequency (MHz)

60

10

100

20
10
0

–20
0.1

1

Frequency (MHz)Average Conducted EMI (dBµA)
50 mm

30
20
10
0
–10

–30

10

100

Single Hot Loop
Dual Hot Loop

30

–20

20
10
0
–10
–20
–30
–40
–50

–40
0.1

1

10
Frequency (MHz)

60

100

(b)

–60
0.1

1

10
Frequency (MHz)40Single Hot Loop
Dual Hot Loop

50

Average Conducted EMI (dBµA)
750 mm

30
20
10
0
–10
–20
–30

100

Single Hot Loop
Dual Hot Loop

30

40
Peak Conducted EMI (dBµA)
750 mm30

40

40
Peak Conducted EMI (dBµA)
50 mm

(a)

Single Hot Loop
Dual Hot Loop

50

20
10
0
–10
–20
–30
–40
–50

–40
0.1

1

10
Frequency (MHz)

60

100

(c)

Average Radiated Emissions (dBµV/m)
Vertical Polarization

40
30
20
10
0

1

10
Frequency (MHz)

100

–60
0.1

1

10
Frequency (MHz)50Single Hot Loop
Dual Hot Loop

50
Peak Radiated Emissions (dBµV/m)
Vertical Polarization

40

–10

–10

–10
0.1

Single Hot Loop
Dual Hot Loop

50

1k

(d)

100

Single Hot Loop
Dual Hot Loop

40
30
20
10
0
–10
–20
0.1

1

10
Frequency (MHz)

100

1k

Figure 3. EMI comparison graph of a dual hot loop and single hot loop: (a) voltage method conducted emissions peak and average, (b) current probe method conducted emissions 50 mm
peak and average, (c) current probe method conducted emissions 750 mm peak and average, and (d) radiated emissions vertical peak and average.
VISIT ANALOG.COM/POWER

13

conducted emissions above 30 MHz, and especially at FM band, as shown in theyellow highlighted boxes. Unlike the voltage method conducted emissions, there isno significant benefit of a single hot loop over dual hot loop at the low frequency
around the AM band.

Shielded
SW2

Shielded
SW1

Conclusion
The suggested new buck-boost layout, single hot loop, is recommended for new,high power designs. Due to the minimized exposure of the switching node and the
hot loop area, the single hot loop has significant benefit to reduce both conducted
and radiated emissions without any thermal disadvantages. Notably, it reduces
conducted emissions above 30 MHz, which is the most challenging frequency
region to attenuate. Thanks to the proprietary peak buck/peak boost currentmode control feature of ADI’s 4-switch buck-boost controllers (LT8390/LT8390A,
LT8391/LT8391A, LT8392, LT8393, LT8253, etc.), the hot loop can be made muchsmaller than those with competitors’ parts. The control feature results in higher
efficiency and lower EMI, making ADI’s 4-switch buck-boost controllers the best
choice for automotive applications or any EMI sensitive applications.

About the Authors
Figure 4. Shielded switching nodes of bottom layer of a dual hot loop.

Lastly, Figure 3(d) shows the radiated emissions (RE) of the two different buckboost layouts. The results are almost identical except that the dual hot loop hasa spike around 90 MHz, which is 5 dBµV/m higher than the single hot loop.

Thermal Comparison
A thermal comparison between the dual hot loop and the single hot loop is made in
Figure 5. The thermal images are taken at 9.4 V input voltage with SSFM on. 9.4 V
is the lowest point of the 4-switch operation region before the operating modeis changed to 2-switch pure boost when the output voltage is 12 V. Thus, the test
condition is the harshest. The hottest component of the dual hot loop, the boostside bottom MOSFET, and the single hot loop have almost the same temperature.
Although the single hot loop does not have the switching node vias and copper atthe bottom layer that can dissipate heat, its switching loss is lower than the dual
hot loop due to the smaller hot loop. Also, by not using the switching node vias, thesingle hot loop has better heat dissipation at the top layer because the contact
area of the MOSFET drain pad and the switching node copper is larger than thatof the dual hot loop.

(a)Yonghwan Cho is a senior applications engineer with Analog Devices
in Santa Clara, California. He works on DC-to-DC switching regulators,
including 4-switch buck-boost voltage regulators and LED drivers forautomotive applications. Yonghwan received his Ph.D. degree in electrical engineering in 2017 from North Carolina State University in Raleigh,North Carolina.
Keith Szolusha is an applications director with Analog Devices in Santa
Clara, California. Keith has worked in the BBI Power Products Group since
2000, focusing on boost, buck-boost, and LED driver products, while also
managing the power products EMI chamber. He received his B.S.E.E. in 1997
and M.S.E.E. in 1998 from MIT in Cambridge, Massachusetts, with a concentration in technical writing.

90.0

90.0

84
80
76
72
68
64
60
56
52
48
44
40
36
32
28
24
20.0
°C

84
80
76
72
68
64
60
56
52
48
44
40
36
32
28
24
20.0
°C

(b)

Figure 5. (a) Thermal image of a dual hot loop, and (b) thermal image of a single hot loop.

144-Switch Buck-Boost Controller Layout for Low Emissions—Single Hot Loop vs. Dual Hot Loop

ADM2867E Isolated Power Transceiver

Isolated Power
RS-485 Transceivers
Reduce Design Time
ADM2867E
X

Low radiated emissions meet EMC compliance​

X

Simplified PCB layout and small SOIC form factor​

X

Includes smart features that reduce end systeminstall and debug time

Learn More at analog.com/ADM2867E

Technical Article

High Power Density in a
Small Form Factor
Steve Knoth
Background
Sophisticated high power density digital integrated circuits (ICs), such as graphics
processor units (GPUs) and field programmable gate arrays (FPGAs), can be found ina broad range of feature-rich electronic environments, including:X

Automotive

X

MedicalX

Telecom

X

DatacomX

Industrial

X

Communications

X

GamingX

Consumer audio/video

With this level of market penetration, it is no surprise that the global demand for highcurrent, low voltage digital ICs is exploding. The current global market is assessedat more than US $1.8B, and this is expected to rise annually by 10.87% to reach
US $3.7B over the 2018 to 2025 period. As one of the biggest slices of this market,
FPGAs account for a projected US $1.53B by the end of 2025. The rest of the digital
IC market is represented by GPUs, microcontrollers and microprocessors, programmable logic devices (PLDs), digital signal processors (DSPs), and application-specificintegrated circuits (ASICs).
High power density digital ICs have penetrated virtually every embedded system.
FPGAs enable cutting-edge applications in the market segments listed above. For
example, in automotive applications, advanced driver assistance systems (ADASs)
and collision avoidance systems prevent catastrophe due to human error. Likewise,
government-mandated safety features such as antilock brake systems, stability control, and electronically controlled independent suspension systems require
FPGAs to function.

16

High Power Density in a Small Form Factor

In consumer electronics, the demand for Internet of Things (IoT) functionality,
sophisticated graphic engine functionality, and machine-to-machine (M2M) functionality call for advanced digital ICs. Massive data storage and cloud computing
centers and expansive networks of optical networking modules drive the need for
FPGAs and digital ICs.
These digital ICs are powerful, yet temperamental, especially regarding power
requirements. Traditionally, efficient switching regulator controllers that drive highpower MOSFETs have been used to power FPGAs and ASICs, but these controller-based
power schemes have potential noise interference issues, relatively slow transient
response, and layout limitations. In recent years, small and quiet low dropout (LDO)
regulators that minimize heat have been used as an alternative, but not without theirown set of limitations. Recent power conversion innovations have introduced highpower monolithic switching regulators that are able to efficiently power digital ICs
with low noise and high efficiency while minimizing space requirements.

Switching Regulators vs. Charge Pumps
vs. LDO Regulators
Low voltage, high current step-down conversion and regulation can be achieved
via a variety of methods, each with its own performance and design trade-offs.
Switching regulator controllers feature high efficiency at high load currents overa wide range of voltages, but they require several external components such as
inductors, capacitors, and FETs to operate; and they can be a source of high andlow frequency noise. Inductorless charge pumps (or switched capacitor voltage
converters) can also be used to produce low voltages, but are limited in output current capability, suffer from poor transient performance, and require several external
components. For these reasons, charge pumps are not commonly found in digital IC
power applications. Linear regulators—especially LDO regulators—are simple in thatthey only require two external capacitors to operate. However, they may be power
limited depending on the size of the input-to-output voltage differential across the
IC and how much current is demanded by the load, plus the thermal resistance characteristics of the package. This certainly limits their ability to power digital ICs.

Monolithic Buck Converter Design Challenges
Moore’s Law has proven to be visionary and valid since its debut in 1965. Wafer
fabrication technology line widths are continually being reduced, pushing digital IC
voltages lower. Smaller geometry processes allow higher integration of more powerhungry features in the end product. For example, modern computer servers andoptical communication routing systems demand higher bandwidth to process morecomputing data and internet traffic; these systems also generate a lot of heat, andtherefore highly efficient ICs are required. Cars have more on-board electronics forentertainment, navigation, self-driving features, and even engine control. As a result,
there is an increase in both the system’s current consumption and the associatedtotal power required. Therefore, state-of-the-art packaging and innovative, internal
power-stage design are required to drive the heat out of the power IC while delivering unprecedented power.
High power supply rejection ratio (PSRR) and low output voltage noise, or ripple, areimportant considerations. A device with high supply rejection can filter and rejectnoise at the input, resulting in a clean and stable output. Furthermore, power solutions with low output voltage noise across a wide bandwidth or low output ripple
are desirable—modern digital systems have several rails where noise sensitivity isa major design consideration. As speed requirements for high end FPGAs increase,
supply noise tolerance decreases in order to minimize bit errors. Noise-induced
digital faults drastically reduce the effective data throughput speeds for these highspeed PLDs. Input supply noise at high current is one of the more demanding specifications placed on power supplies.
Higher transceiver speeds—in FPGAs, for example—dictate high current levels due
to high power consumption from fine geometry circuit switching. These ICs are fast.
They may cycle load current from near-zero to several amps within tens to hundredsof nanoseconds, requiring a regulator with ultrafast transient response.
With board area reserved for the power regulators ever-decreasing, manysystem designers turn to monolithic switching regulators operating at
fast switching frequencies to reduce the size of external components andtotal solution size—accepting the trade-off of some efficiency loss due
to switching losses at higher frequencies. This trade-off is eliminated bya new generation of monolithic switching regulators. These new regulators featuresynchronous operation with integrated high-side and low-side switches, allowing
for tight control of switch gate voltages, greatly reducing dead time, and resulting inhigher efficiencies even at high frequencies.
One of the biggest challenges with high current monolithic switchers is their ability
to dissipate heat that results from power loss in the IC. This challenge can be met
by using multiple power and ground pins, plus thermally enhanced laminate-based
packages with copper (Cu) pillars where the heat can be easily transferred from the
IC into the board. The large copper planes on the board connected to these power
pins allow for the heat to spread more evenly.

New Family of Silent Switcher Buck Converters
It’s clear that buck converter solutions for high performance digital ICs must havethe following attributes:
X

Fast switching frequency to minimize the size of external components

X

Zero dead-time design to maximize efficiency at high frequency

X

Monolithic chip on-board power device for smaller solution size

X

X

Multiphase operation enables parallel operation for high output currents andreduced ripple

X

Synchronous operation for high efficiency and minimal power loss

X

Easy design simplifies design cycle, qualification, and testing

X

Very low output ripple

X

Fast transient response timeX

Operation over a wide input/output voltage range

X

High output current capability

X

Excellent thermal performance

X

Compact footprint

These features can be found in Analog Devices’ Power by Linear™ LTC33xx familyof monolithic high, medium, and low current buck regulators. The highest current
member is the LTC3310S, a 5 V, 10 A, high power density, low EMI, Silent Switcher®
2, monolithic, synchronous buck converter in a 9 mm2 package (power density =1.11 A/mm2). The device’s fixed frequency peak current-mode architecture is ideal
for applications that demand fast transient response. The LTC3310S uses the Silent
Switcher 2 architecture with integrated hot loop bypass capacitors to deliver ahighly efficient, small footprint solution at frequencies up to 5 MHz with excellent EMI
performance. Multiphase operation enables direct paralleling of up to four devices
for higher current up to 40 A.
The LTC3310S’ 2.25 V to 5.5 V input range supports a wide variety of applications,
including most intermediate bus voltages. Integrated low on-resistance MOSFETs
deliver continuous load currents as high as 10 A with minimal thermal derating.
Output voltages ranging from 0.5 V to VIN are ideal for point-of-load applications
such as high current, low voltage DSP/FPGA/GPU/ASIC designs. Other key
applications include optical networking, telecom/datacom, automotive systems,
distributed power architectures, or any medium-to-high power density systems.
Figure 1 shows the simplicity of a typical design, while Figure 2 shows how easy
it is to produce a 4-phase, 40 A configuration.
VIN
2.25 V to 5 V

22 µF

22 µF
EN

0.1 µF
10 kΩ

VIN

SW
MODE/SYNC
LTC3310S
FB
PGOOD
SSTT
AGND
ITH
RT
PGND

100 nH

VOUT
1.2 V
10 A
15 pF 140 kΩ
22 µF
×3
100 kΩ

274 kΩ

220 pFL = XEL4030-101ME, COILCRAFT

Figure 1. LTC3310S typical application.

The LTC3310S’ low 35 ns minimum on-time enables a large step-down ratio at highfrequency, and 100% duty-cycle operation delivers low dropout performance when
the input and output voltages are close in value. The operating frequency can besynchronized to an external clock. The LTC3310S total reference voltage accuracy
is better than ±1% over the –40°C to +125°C operating junction temperature range.
Additional features include a power good signal when the output is in regulation,
precision enable threshold, output overvoltage protection, thermal shutdown, dietemperature monitor, programmable soft start, tracking, clock synchronization,mode selection, and output short-circuit protection.
The LTC3310S is available in a thermally enhanced 18-lead, 3 mm × 3 mm × 0.94 mm
LQFN package. The E- and I-grades are specified from the –40°C to +125°C operating
junction temperature range, while the J- and H-grades are specified from the –40°C
to +150°C operating junction temperature range.

Low EMI to meet low system noise requirements

VISIT ANALOG.COM/POWER

17

VIN
2.25 V to 5.5 V
1 µF

47 µF

0.22 µF

0.22 µF
VIN

PGOOD

MODE/SYNC

SW

EN

100 nH6.8 pF

LTC3310S
ITH
10 kΩ

FB
RT

SSTT

390 pF

AGND PGND

47 µF

140 kΩ

100 kΩ

22 µF
×2
10 µF
×2

VOUT
1.2 V
40 A

274 kΩ

0.1 µF

VIN
1 µF

47 µF

0.22 µF

0.22 µF

VIN

PGOOD

MODE/SYNC

SW

EN

LTC3310S

100 nH
22 µF
×2
10 µF
×2

VIN

FB

ITH

301 kΩ
AGND PGND

47 µF

90°

RT
100 kΩ

VIN
1 µF

47 µF

0.22 µF

VIN

EN

0.22 µF
PGOOD

MODE/SYNC

100 nHSW

LTC3310S

22 µF
×2
10 µF
×2

VIN

FB

ITH
AGND PGND

47 µF

180°

RT

VIN
1 µF

47 µF

0.22 µF

0.22 µF
VIN

EN

PGOOD

MODE/SYNC

SW

LTC3310S

100 nH
VIN

FB

ITH

100 kΩ
AGND PGND

47 µF

22 µF
×2
10 µF
×2

270°

RT

301 kΩ

L = COILCRAFT, XEL4030-101ME

Figure 2. Four LTC3310S monolithic regulators in parallel, forming a 4-phase, 40 A step-down regulator.

High Efficiency, Low EMI,
and Fast Transient Response
Silent Switcher buck regulator designs offer high efficiency at high switchingfrequencies (>2 MHz) with ultralow electromagnetic interference (EMI) emissions,offering very compact and quiet step-down solutions. The Silent Switcher family
uses special design and packaging techniques to enable >92% efficiency at 2 MHz
while easily passing the CISPR 25 Class 5 peak EMI limits. The next-generation Silent
Switcher 2 technology internal construction uses copper pillars in lieu of bondwires, adds internal bypass capacitors, and an integrated substrate ground plane
to further improve EMI, which is not sensitive to PCB layout, simplifying designs andreducing performance risks.

18

High Power Density in a Small Form Factor

The “S” in the LTC3310S part number indicates its second-generation Silent
Switcher technology. The IC has integrated VIN ceramic capacitors to keep all
fast ac current loops small, improving the EMI performance. This technology
allows fast switching edges for high efficiency at high switching frequencies,
while simultaneously achieving good EMI performance (see Figure 3, Figure 4,
and Figure 5). Furthermore, it allows for faster, cleaner, low overshootswitching edges, greatly improving efficiency at high switching frequencies.
The graph in Figure 6 shows the LTC3310S’s high efficiency performance.

50

30

Efficiency (%)

Amplitude (dBµV/m)

40

20
10

2.7

80

2.4

70

2.1

60

1.8

50

1.5

40

1.2

20

–10

Peak Limit
Peak
0

10

20

30

40

50

60

70

80

90

100

110

1

2

3

4

5

6

7

0.9

Efficiency
Power Loss

0.3

8

9

0.6

0
10

Figure 6. LTC3310S efficiency performance.

DC2629A Demo Board(with EMI Filter Installed)
3.3 V Input to 1.2 V Output at 7.5 A, fSW = 2 MHz

Figure 3. CISPR 25 conducted EMI emissions with Class 5 peak limits (voltage method).
50
Horizontal Polarization
45 Peak Detector

The LTC3310S’s fixed frequency peak-current mode architecture eases compensation and allows the IC to rapidly respond to transient steps. External compensation
components allow the control loop to be optimized for the highest bandwidth andfastest transient response.

6 A, 4 A, and 3 A Silent Switcher Bucks in a
2 mm × 2 mm Package

40
35
30
25
20
15
10
Peak Limit
Peak

5
0

100

200

300

400

500

600

700

800

900 1 GHz

Frequency (MHz)DC2629A Demo Board(with EMI Filter Installed)
3.3 V Input to 1.2 V Output at 7.5 A, fSW = 2 MHz

Figure 4. Radiated emissions for horizontal polarization.
Vertical Polarization
45 Peak Detector
40
35
30
25
20
15
10
Peak Limit
Peak

5
0

100

200

300

400

500

600

700

Frequency (MHz)DC2629A Demo Board(with EMI Filter Installed)
3.3 V Input to 1.2 V Output at 7.5 A, fSW = 2 MHz

Figure 5. Radiated emissions for vertical polarization.

For increased power density, first-generation Silent Switcher architecture is a goodsolution. Silent Switcher topology is like Silent Switcher 2 topology except the VIN
bypass capacitors are external instead of within the plastic encapsulation flip-chiplaminate style package. For full Silent Switcher, low EMI performance, external VIN
bypass capacitors are placed symmetrically, external to the package. This split cap,
symmetrical arrangement minimizes the effective hot loop area, thereby reducing
EMI and allowing for a smaller package footprint size.
LTC3309A, LTC3308A, and LTC3307A are 5 V input regulators that can support 6 A, 4 A,
and 3 A (respectively), for high power density, low EMI monolithic synchronous buckconversion. They all operate at up to 3 MHz in a 4 mm2 footprint package (LTC3309A
power density = 1.5 A/mm2).
Figure 7 shows a typical LTC3309A application. The fixed frequency peak currentmode architecture is ideal for fast transient response, including fast transient
response during Burst Mode® operation (see Figure 8). The LTC3309A features Silent
Switcher architecture, utilizing external hot loop bypass capacitors. This design
enables highly efficient, small footprint solutions at high operating frequencies withexcellent EMI performance.

50

0

0

VIN = 3.3 V
VOUT = 1.2 V
fOSC = 2 MHz

Load Current (A)Frequency (MHz)

0

10
0

0

Amplitude (dBµV/m)

3.0

90

30
0

Amplitude (dBµV/m)

100

Power Loss (W)

60

800

900 1 GHz

The family’s 2.25 V to 5.5 V input voltage range supports a wide variety of applications, including most intermediate bus voltages, and is compatible with lithium andnickel-based battery types. Integrated low on-resistance MOSFETs deliver continuous load currents as high as 6 A. Output voltages, ranging from 0.5 V to VIN, are ideal
for point-of-load applications such as high current/low voltage DSP/FPGA/GPU/ASIC
reference designs. Other key applications include telecom/datacom and automotive
systems, distributed power architectures, and general-purpose power systems.
The LTC3309A, LTC3308A, and LTC3307A operate in forced continuous or pulse skip
modes for low noise, or low ripple, low IQ Burst Mode operation for high efficiency atlight loads, ideal for battery-powered systems. Low 22 ns minimum on-time enableshigh step-down ratios even as the power supply operates at high frequency, and100% duty cycle operation delivers low dropout performance when input and output

VISIT ANALOG.COM/POWER

19

voltages are the same. The operating frequency can be synchronized to an external
clock. The total reference voltage accuracy is better than ±1% over the –55°C to+150°C operating junction temperature range. The device safely tolerates inductor
saturation in overload. Additional features include a power good signal when the
output is in regulation, internal soft start, precision enable threshold, output overvoltage and short-circuit protection, thermal shutdown, and clock synchronization.

ILOAD
2.5 A/divIL
2 A/div

The LTC3309A, LTC3308A, and LTC3307A are all pin-compatible devices available in athermally enhanced, compact, and low profile 12-lead, 2 mm × 2 mm × 0.74 mm LQFN
package. The E- and I-grades are specified from a –40°C to +125°C operating junction temperature range. The J- and H-grades are specified from a –40°C to +150°C
operating junction temperature range, and the MP-grade is specified from a –55°C
to +150°C operating junction temperature range.

VOUT
50 mV/div

10 µs/div
VIN = 2.25 V to 5.5 V
10 µF

0.1 µF0201
220 nH0.1 µF
0201
EN

VIN

VOUT
1.2 V
6A

SW
SW

VIN

6.8 pF
LTC3309A

3.3 VIN to 1.2 VOUT, 2 MHz Typical Application
COUT = 66 µF, L = 220 nH
Load Step: 0.1 A to 4.5 A in 1 µs

10 µF

140 kΩ

FB
100 kΩ

33 µF
×2Figure 8. LTC3309A transient response in Burst Mode operation.

Table 1 compares the features of the members of the LTC33xx Silent Switcher andSilent Switcher 2 family.

MODE/SYNC
AGND
RT
PGND PGOOD
fOSC = 2 MHz

Figure 7. LTC3309A typical application circuit.

Table 1. Fault Mode and Supported Range
Vendor

ADI

ADI

ADI

ADI

Part #

LTC3307A

LTC3308A

LTC3309A

LTC3310S

Topology

Single synchronous
monolithic, Silent Switcher

Single synchronous
monolithic, Silent Switcher

Single synchronous
monolithic, Silent Switcher

Single synchronous
monolithic, Silent Switcher 2

VIN Range

2.25 V to 5.5 V

2.25 V to 5.5 V

2.25 V to 5.5 V

2.25 V to 5.5 V

VOUT Range

0.5 V to VIN

0.5 V to VIN

0.5 V to VIN

0.5 V to VIN

3A

4A

6A

10 A

92%
(3.3 VIN/1.2 VOUT/2 A)

92%(3.3 VIN/1.2 VOUT/2 A)

92%(3.3 VIN/1.2 VOUT/2 A)

92%(3.3 VIN/1.2 VOUT/3 A)

1 MHz to 3 MHz

1 MHz to 3 MHz

1 MHz to 3 MHz

500 kHz to 5 MHz

Constant-frequency,
peak current mode

Constant-frequency,
peak current mode

Constant-frequency,
peak current mode

Constant-frequency,
peak current mode

VREF Accuracy Room/Temp±0.2%/±1%±0.2%/±1%±0.2%/±1%±1%

Output Current
Efficiency
Switching Frequency
Control Mode

Current Limit Accuracy

±15%

±15%

±15%

±9%

Minimum On-Time

22 ns

22 ns

22 ns

35 ns

Directly Parallelable x Phase?

No

No

No

Yes, 4 phase

IQ Supply Burst Mode/Non-Burst

40 µA BM/1.3 mA

40 µA BM/1.3 mA

40 µA BM/1.3 mA

1.3 mA

Package Theta JA

51°C/W

51°C/W

51°C/W

40°C/W

Solution Size

~20 mm2~20 mm2~20 mm2

47 mm22 mm × 2 mm × 0.74 mm,
12-lead LQFN

2 mm × 2 mm × 0.74 mm,
12-lead LQFN

2 mm × 2 mm × 0.74 mm,
12-lead LQFN

3 mm × 3 mm × 0.94 mm,
18-lead LQFN

Package

20

High Power Density in a Small Form Factor

Conclusion
The trend in high performance digital ICs—such as GPUs, FPGAs, and microprocessors—is rapidly raising current demands coupled with dropping operating voltages,
a result of shrinking line width wafer fabrication technologies. Current and voltage
demands are only part of the power supply picture. Digital IC advancements comewith a host of other requirements, including fast transient response, low EMI, low
noise/ripple, and efficient operation to minimize heat.
Traditionally, digital ICs has been powered by LDO regulators or inductor-based
switching regulator controllers with off-board power devices. With increased power
supply performance and space requirements, in many cases these traditional
approaches are not up to the task. ADI’s new generation of monolithic power supplies are up to the task, including the LTC3310S, LTC3309A, LTC3308A, and LTC3307A,
which support 10 A, 6 A, 4 A, and 3 A, respectively. These high power density Silent
Switcher and Silent Switcher 2 buck regulators are housed in thermally efficient,compact flip-chip laminate packages, and have a variety of feature sets to satisfy
the requirements of a wide range of digital IC power problems.

About the Author
Steve Knoth is a senior product marketing manager in Analog Devices’Power Group. He is responsible for all power management integrated
circuit (PMIC) products, low dropout (LDO) regulators, battery chargers,
charge pumps, charge pump-based LED drivers, supercapacitor chargers, and low voltage monolithic switching regulators. Prior to rejoining
Analog Devices in 2004, Steve held various marketing and product engineering positions since 1990 at Micro Power Systems, Analog Devices, andMicrel Semiconductor. He earned his bachelor’s degree in electrical engineering in 1988 and a master’s degree in physics in 1995, both from San
Jose State University. Steve also received an M.B.A. in technology management from the University of Phoenix in 2000. In addition to enjoying timewith his kids, Steve is an avid music lover and can be found tinkering withpinball and arcade games or muscle cars, and buying, selling, and collecting vintage toys, movie, sports, and automotive memorabilia.

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21

Featured Products

Analog Devices is committed to delivering the most innovative
products that sense, measure, connect, interpret, power, and secure tohelp you solve your toughest design problems and create solutions that
are Ahead of What’s Possible.™ The latest Silent Switcher and power
management products and solutions from some of the best designers
in the world are listed below.

LTC3310S
5 V, 10 A Synchronous Step-Down Silent Switcher 2 Device in 3 mm × 3 mm LQFN
The LTC3310S is a very small, low noise, monolithic step-down DC-to-DC converter capable of providing up to 10 A of output current from a 2.25 V to 5.5 V
input supply. The device employs Silent Switcher 2 architecture with internal
hot loop bypass capacitors to achieve both low EMI and high efficiency at switching frequencies as high as 5 MHz. For systems with higher power requirements,
multiphasing parallel converters are readily implemented.
Visit the Product Page

Buy the Eval Board

Download the Data Sheet

Sample and Buy

LT8652S
Dual-Channel 8.5 A, 18 V Synchronous Step-Down Silent Switcher Device
with 16 μA Quiescent Current
The LT8652S is a dual step-down regulator that delivers up to 8.5 A of continuous
current from both channels and supports loads up to 12 A from each channel. The
LT8652S uses second generation Silent Switcher technology including integrated
bypass capacitors to deliver a high frequency, high efficiency, small solution withexcellent EMI performance.

22

Visit the Product Page

Buy the Eval Board

Download the Data Sheet

Sample and Buy

FEATURED PRODUCTS

Featured Products

LTM8060
Quad 40 VIN, Silent Switcher μModule Regulator with Configurable 3 A Output Array
The LTM8060 is a quad 40 VIN, 3A step-down Silent Switcher μModule regulator.
The Silent Switcher architecture minimizes EMI while delivering high efficiency
at frequencies up to 3 MHz. Included in the package are the controllers, powerswitches, inductors, and support components. Operating over a wide input
voltage range, the LTM8060 supports output voltages from 0.8 V to 8 V, and aswitching frequency range of 200 kHz to 3 MHz, each set by a single resistor.
Visit the Product Page

Buy the Eval Board

Download the Data Sheet

Sample and Buy

LT8645S
65 V, 8 A Synchronous Step-Down Silent Switcher 2 Device
with 2.5 μA Quiescent Current
The LT8645S/LT8646S synchronous step-down regulators feature second generation Silent Switcher architecture designed to minimize EMI emissions while delivering high efficiency at high switching frequencies. This includes the integrationof bypass capacitors to optimize all the fast current loops inside and make it easy
to achieve advertised EMI performance by eliminating layout sensitivity.
Visit the Product Page

Buy the Eval Board

Download the Data Sheet

Sample and Buy

LT860942 V, 3 A Synchronous Step-Down Regulator with 2.5 μA Quiescent Current
The LT8609/LT8609A/LT8609B are compact, high efficiency, high speed synchronous monolithic step-down switching regulators that consume only 1.7 μA of
nonswitching quiescent current. The LT8609/LT8609A/LT8609B can deliver 3 A ofcontinuous current. Burst Mode® operation enables high efficiency down to very
low output currents while keeping the output ripple below 10 mV p-p.
Visit the Product Page

Buy the Eval Board

Download the Data Sheet

Sample and Buy

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23

Technical Article

42 V, 2 A/3 A Peak Synchronous
Step-Down Regulators with
2.5 µA Quiescent Current andUltralow EMI Emission
Dong Wang

The LT8609, LT8609A, LT8609B, and LT8609S are synchronous monolithic step-down
regulators that feature a wide 3 V to 42 V input range. This device family is optimized
for applications requiring low EMI, high efficiency, and a small solution size—suitablefor demanding automotive, industrial, computing, and communications applications.
All regulators in this series have the same 2 A continuous, 3 A transient (<1 second) load
current capability. Their features are summarized in Table 1.
The LT8609, LT8609A, and LT8609S feature 2.5 µA ultralow quiescent current, which
is important for battery-powered systems. With integrated top and bottom N-channel
MOSFETs, the regulators exhibit impressive light load efficiency. The LT8609B operates
in pulse-skipping mode only, with higher quiescent current than the other devices, andit offers lower ripple during light load operation.
All of these devices can pass CISPR 25 Class 5 radiated EMI regulation, the most rigorous EMI standard for automotive equipment. Furthermore, the LT8609, LT8609A,
and LT8609S feature spread spectrum frequency operation to reduce EMI peaks. The
LT8609S displays the most impressive EMI performance in this family, based on its
proprietary Silent Switcher® 2 technology, described below.

A 5.5 V to 42 V input to 5 V/2 A output power supply is shown in Figure 1. Thissolution features a 16-lead LT8609S regulator with a 2 MHz switching frequency.
Only a few components are required for the complete solution, including inductor
L1 and a few passive components. Figure 2 shows that this solution can achieve
92.9% peak efficiency.

Burst Mode Operation to Improve
Light Load Efficiency
During light load operation and no-load standby mode, high efficiency and low idle
current are important for battery-powered applications. The LT8609, LT8609A, andLT8609S feature a low 2.5 µA quiescent current in Burst Mode® operation. During lightload and no-load conditions, the switching frequency is gradually reduced, greatly
reducing power loss while keeping the output voltage ripple relatively low. Figure 2
shows that light load efficiency remains above 85% while the power loss approaches
zero at minimal loads.
94

5 V, 2 MHz Step-Down
C2
4.7 µF

VIN
SW
EN/UV

R4
100 kΩSYNC
INTVCCC3
1 µF

LT8609S
C1
10 µF

VOUT
5 V/2 A

Power
Good

PG
C5
10 pF

TR/SS

1.4

92

L1
2.2 µH

Efficiency (%)

VIN
5.5 V
to 42 V

5.5 V to 42 V Input, Low EMI, High Efficiency 5 V,
2 A Supply

90

1.0

88

0.8

86

0.6

Load

84
FB

RT
R1
18.2 kΩ

GND

R2
1 MΩ
R3
182 kΩ

fSW = 2 MHz

Figure 1. Ultralow EMI emission LT8609S 12 V to 5 V synchronous step-down converter.

C4
47 µF

1.2

Eﬃciency

0.4
LT8609LT8609A
LT8609S

82
80

0

0.5

1.0
Output Current (A)Loss (W)Introduction

1.5

0.2

2.0

0

Figure 2. Efficiency vs. load current for LT8609/LT8609A/LT8609S-based 12 VIN to 5 VOUT
step-down converter.

24

42 V, 2 A/3 A PEAK SYNCHRONOUS STEP-DOWN REGULATORS WITH 2.5 μA QUIESCENT CURRENT AND ULTRALOW EMI EMISSION

High Switching Frequency with Ultralow EMI
Emission and Improved Thermal Performance
EMI compliance is a concern in a number of environments, including automotive
systems. With integrated MOSFETs, advanced process technology, and up to 2.2 MHz
operation, all of the devices in this family can achieve a small solution size whilesatisfying the most stringent EMI standards. Spread spectrum frequency operation,
which reduces EMI peaks, is available in all but the LT8609B. Furthermore, the LT8609S
incorporates Silent Switcher 2 technology. Silent Switcher 2 devices feature integrated
hot loop and warm loop caps to make EMI performance insensitive to board layout
and the number of board layers. A board with fewer layers can be used to reduce
manufacturing costs without sacrificing EMI and thermal performance.

Figure 4. Thermal performance comparison between 2- and 4-layer boards forthe circuit in Figure 1.

Part

Package

Performance

Operation Mode

Figure 2 shows that the LT8609S features the best peak and full load efficiency of thedevice family. Figure 3 and Figure 4 show a CISPR 25 EMI and thermal performancecomparison of the Figure 1 solution on 2- and 4-layer boards.

LT860910-lead MSE

High efficiency

Burst Mode operation,pulse-skipping mode, spread
spectrum mode, sync mode

Conclusion

LT8609A

10-lead MSE

Optimized for both
efficiency and
EMI performance

Burst Mode operation,pulse-skipping mode, spread
spectrum mode, sync mode

The devices in the LT8609 family are easy to use monolithic step-down regulators withintegrated power MOSFETs and built-in compensation circuits. They are optimized
for applications with wide input voltage range and low EMI noise requirements.
Low, 2.5 µA quiescent current and Burst Mode operation make them excellent batterypowered step-down converter solutions. A 200 kHz to 2.2 MHz switching frequency
range make them suitable for most low power to micropower applications. Integrated
MOSFETs and up to 2.2 MHz switching frequency capability minimize solution size.
CISPR 25 scanning results show excellent radiated EMI performance, compliant withthe most stringent EMI standards. Silent Switcher 2 technology in the LT8609S makes
its performance immune to layout and layer changes, which greatly reduces development and manufacturing costs.

LT8609B

10-lead MSE

High efficiency

Pulse-skipping mode

16-lead LQFN

Silent Switcher 2technology
incorporated withbest efficiency and
EMI performance

Burst Mode operation,pulse-skipping mode,
spread spectrum mode,sync mode

50
45
40
Amplitude (dBμV/m)

35
30
25
20
15

LT8609S

About the Author
Dong Wang is a senior applications engineer for power products at
Analog Devices who began his career at Linear Technology (now part of
ADI) in 2013. He currently provides applications support for nonisolatedmonolithic step-down converters. Dong Wang has broad interests in
power management solutions and analog circuits, including high frequency power conversion, distributed power systems, power factor correction techniques, low voltage, high current conversion techniques, highfrequency magnetic integration, and modeling and control of converters.
Dong Wang graduated from Zhejiang University in Hangzhou, China with aPh.D. in electrical engineering.

1005

2 Layers Broad Peak

0

4 Layers Broad Peak

-05
-10

Class Peak Limit
0

100

200

300

400 500 600
Frequency (MHz)

700

800

900

1000

Figure 3. CISPR 25 radiated EMI performance comparison between 2- and 4-layer boards forthe circuit in Figure 1.

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25

LTC3310S Silent Switcher 2 Converter

5 V, 10 A Synchronous
Step-Down Silent
Switcher® 2 Converterwith Ultralow EMI
Emissions
LTC3310S
XHigh efficiency: 4.5 mΩ NMOS and 16 mΩ PMOSX

Wide bandwidth, fast transient response

X

Thermally enhanced 3 mm × 3 mm LQFN package

Learn More at analog.com/LTC3310s

Design Tools and Videos

Design Note: Monolithic 65 V, 8 A Step-Down Regulators with
Fast Transient Response and Ultralow EMI Emissions
The LT8645S and LT8646S are 65 V synchronous step-down monolithic
regulators capable of supporting 8 A outputs. Their Silent Switcher 2 architecture enables exceptional EMI performance regardless of layout. The LT8646Sfeatures RC external compensation to optimize transient response.

Read More

Video: Silent Switcher 2Monolithic Buck Regulator

LT8336 Video: 4 V, 2.5 A, Low IQ Synchronous
Step-Up Silent Switcher Device

See the EMC capability of
our Silent Switcher 2monolithic buckregulators. Discover how
they can reduce the riskof EMC-related redesigns
due to power, and how
you can eliminate the need for
additional EMI shielding and simplify layouts.

LT8336 is a low IQ,
synchronous step-up
DC-to-DC converter. Itfeatures Silent Switcher
architecture and optional
spread spectrum frequency modulation tominimize EMI emissions while delivering highefficiencies at high switching frequencies.

Watch Now

Watch Now

LTM8060 Video: 40 VIN, Quad 3 A
Silent Switcher μModule Regulator

LTM8051 Video: Silent Switcher
μModule Regulator

Analog Devices’ LTM8060’s
Silent Switcher architecture minimizes EMI
emissions, enabling it topass the CISPR 22 Class B
and CISPR 25 Class 5
standards with no input
filter. This is critical for noise-sensitive applications
such as signal processing in medical imaging and
RF. Furthermore, multi-output capability allows
customers to use the LTM8060 as quad 3 A, dual 6 A,
or single 6 A for a variety of voltage and load current
requirements.

The LTM8051 is a quad
40 VIN, 1.2 A step-down
Silent Switcher μModule
regulator. Silent Switcher
technology minimizes EMI
while delivering highefficiency at frequencies
up to 3 MHz. Included: controllers,
power switches, inductors, andsupport components.

Watch Now

Watch Now

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