V24N3 - OCTOBER

October 2014
Volume 24 Number 3
high step-down ratio
Rugged IO-Link Solutions
controller combines digital
Kevin Wrenner and Juan-G. Aranda
I N
T H I S
I S S U E
power system management
with sub-milliohm DCR
sensing 16
increase output voltage/
current with seriesconnected isolated
µModule® converters 24
12V/100A Hot Swap™
design for server farm 26
compensate for wire drop
to a remote load 29
Industrial automation systems are growing more interconnected
and intelligent to accommodate demands for centralized control,
optimized production and reduced cost. IO-Link® is becoming
an increasingly popular interface to smart sensors and actuators,
combining signaling with power-over-cable technology. The interface
electronics must be rugged, power efficient and compact. Two
new parts capably meet these requirements. The LTC®2874 is a
highly integrated IO-Link master-side physical layer interface (PHY)
for four ports. The LT®3669 is a device-side PHY incorporating
a step-down regulator and LDO. To appreciate the numerous
features of these devices, it helps to review the requirements
of IO-Link. This article begins with a brief overview of IO-Link
technology, and follows with LTC2874
and LT3669 functions and features.
IO-LINK: POWER AND COMMUNICATION FOR
SMART DEVICES
Combining a power feed and a data link inside a cable
assembly isn’t new,1 but its presence in the world of industrial automation is. IO-Link2 emerged in 2009 as a communication interface between automation control systems
(masters) and intelligent sensors and actuators (devices). In
2013 it evolved into an international standard for programmable controllers, IEC 61131-9 single-drop digital communication interface for small sensors and actuators (SDCI),
whose purpose “extends the traditional digital input and
digital output interfaces as defined in IEC 61131-2 towards
a point-to-point communication link [enabling] the transfer of parameters to Devices and the delivery of diagnostic
The LTC3882 POL controller with built-in digital power system management
(see page 16)
w w w. li n ea r.com
(continued on page 4)
Linear in the News
In this issue...
LINEAR CELEBRATES TWO ANNIVERSARIES
COVER STORY
Rugged IO-Link Solutions
Kevin Wrenner and Juan-G. Aranda
1
DESIGN FEATURES
High Step-Down Ratio Controller Combines Digital
Power System Management with Sub-Milliohm DCR
Sensing and Accurate PolyPhase® Load Sharing
James A. McKenzie
16
DESIGN IDEAS
What’s New with LTspice IV?
Gabino Alonso
22
Increasing Output Voltage and Current Range Using
Series-Connected Isolated µModule Converters
Jesus Rosales
24
12V/100A Hot Swap Design for Server Farms
Dan Eddleman
26
Compensate for Wire Drop to a Remote Load
Philip Karantzalis
29
new product briefs
31
back page circuits
32
In August, Linear celebrated the 25th anniversary of the opening of its
Singapore test facility, as well as the 20th anniversary of the company’s
Penang, Malaysia package assembly facility. These two facilities play an
important role in Linear’s vertically integrated manufacturing process. These
facilities help ensure seamless communication between wafer manufacturing
and packaging, enabling short and predictable product delivery times.
These state-of-the-art assembly and test operations complement
Linear’s two US-based wafer manufacturing operations in Camas,
Washington and Milpitas, California. Such integrated operations
represent a major competitive advantage for customers.
Singapore Test Operation Reaches 25 Years
Linear’s Singapore test operation, started in 1989, has sophisticated capabilities for high volume testing capabilities of the company’s numerous product
types—for both integrated circuits and µModule® products. The manufacturing
facility includes capability for high volume testing of many package types, tape
and reel, as well as pack and ship capability to customers and distributors. The
Singapore location also includes the Singapore Design Center, as well as the area
sales office supporting Singapore, Malaysia, India and Australia/New Zealand.
Over the years, Linear has continued to expand its Singapore test operations, with expansion of its first building in 1997/1998 and a second
5-story building completed in 2005. A third major expansion is now
underway, planned for completion by the end of next year. With headcount of nearly a thousand employees today, Linear has a highly experienced team, capable of testing the most high performance analog ICs.
Linear’s Singapore test
facility reaches 25-year
milestone.
2 | October 2014 : LT Journal of Analog Innovation
Linear in the news
Penang, Malaysia Package Assembly
Marks 20 Years
Started in 1994, Linear’s package assembly
operation has over 20 years of experience with wafer sort and assembly for
a wide range of package types for both
ICs and µModule products. Linear has
transformed its Penang assembly operation significantly over the years in several
phases, adding a second 6-story operations building to the prior facility. The
facility now assembles nearly 40 different package types, as well as numerous
µModule products. The Penang facility
today employs over 1,500 people.
AWARDS
Best of Microwaves & RF Industry
Award
Microwaves & RF magazine in June
presented Linear with the award for
Best Technical Support, as part of their
annual Best of Microwaves & RF Industry
Awards, highlighting “companies and
engineers that rise to the challenge and
provide cutting edge value to the industry.”
In presenting the award, the publication
cited Linear’s design support and technical
documentation, located conveniently on its
website. They highlighted the many design
support options offered by Linear, including design simulations, quality and reliability information, and technical support.
EE Times China ACE Awards
Linear was recognized by EE Times China
in September with the Product of the
Year Award in the Power Management
category for the LTM®4676. The device is
a dual 13A or single 26A µModule stepdown DC/DC regulator with a serial digital
interface. The interface enables system
designers and remote operators to command and supervise a system’s power
condition and consumption. The ability to
digitally change power supply parameters
reduces time-to-market and down time by
eliminating what would have historically
required physical hardware, circuit or
system bill-of-materials modifications. The
LTM4676 simplifies system characterization, optimization and data mining during
prototyping, deployment and field operation. Target applications include optical
transport systems, datacom and telecom
switches and routers, industrial test
equipment, robotics, RAID and enterprise
systems where the cost of electrical utilities, cooling and maintenance are critical.
EE Times China also selected several other
Linear products as ACE Award finalists:
Power Semiconductor/ Voltage Converter
category: LTC3300-1 high efficiency
bidirectional multicell battery balancer
RF/ Wireless/ Microwave category: LTC5551
300MHz to 3.5GHz ultra-high dynamic
range downconverting mixer
Data Conversion/ Driver/ Clock
category: LTC2378-20 20-bit,
1Msps ,
low power SAR ADC
CONFERENCES & EVENTS
Dust Consortium, Tokyo Conference Center
Electronica 2014, Messe München, Munich,
Germany, November 11-14, Hall A4, Booths 537 &
538—Linear will exhibit its broad range
of analog products, with emphasis on
automotive and industrial applications.
Linear’s Joy Weiss will participate on
the Markt & Technik panel on “Energy
Efficient Semiconductors – How They
Will Change Our Lives – From Energy
Harvesting to IoT, Smart Production,
Smart Buildings, Smart Grids and
Beyond” at 3:00 pm, November 12.
More info at www.electronica.de/
Energy Harvesting & Storage Conference, Santa
Clara Convention Center, Santa Clara, California,
November 19-20, Booth L28—Presenting
Linear’s energy harvesting and Dust
Networks® wireless sensor network
products. Presentations by Joy Weiss on
“Wireless Sensor Network Considerations
for the Industrial Internet of Things
(IoT)” and James Noon on “Energy
Harvesting: Battery Life Extension &
Storage.” More info at www.idtechex.
com/energy-harvesting-usa/eh.asp
Shinagawa, Tokyo, Japan, October 17, 4F-402—
Presenting the newly established Dust
Consortium, a community of experts
in various industries focus on wireless
sensor networks, including press conference, study session and reception. More
info at www.dust-consortium.jp/
Second Annual Analog Guru’s Conference, Tokyo
Conference Center, Shinagawa, Tokyo, Japan,
December 5, 5F in the Large Hall—Presentations
by Linear Technology Co-founder and
Chief Technical Officer Bob Dobkin; Vice
President, Power Management Products,
Steve Pietkiewicz; and Dr. A. Kawamoto.
More info at http://analog-guru.jp/
Linear was recognized
by EE Times China in
September with the
Product of the Year
Award in the Power
Management category
for the LTM4676, a
dual 13A or single 26A
µModule step-down
DC/DC regulator with a
serial digital interface.
October 2014 : LT Journal of Analog Innovation | 3
To solve the problems of inrush current control and fault isolation, the
LTC2874 generates L+ power supply outputs using a Hot Swap controller
and n-type power MOSFETs. The resistance of the power path is kept low
using external components for the MOSFETs and sense resistors, reducing
IC heat dissipation and maximizing power efficiency during operation.
(LTC2874/LT3669, continued from page 1)
information from the Devices to the
automation system.”3 This technology
allows a distributed control system linked
by fieldbus networks to operate actuators
such as valve terminals; to operate, monitor and collect data from sensors; and to
dynamically reconfigure their settings.
While IO-Link is fully described by a
protocol stack that includes data link and
application layers, it’s built upon physical layer interfaces, or PHYs (Figure 1),
normally connected by 3-wire cables up to
20m long and terminated by standard M5,
M8 or M12 connectors. Two wires (L+ and
L−) supply 200m A at 24VDC from master to
device, and a third wire is a point-to-point,
half-duplex data line (CQ) that operates at
up to 230.4kb/s and shares the L− return.
Optionally, a fourth wire can serve as a
24V digital line. In specialized configurations, this wire, along with a fifth, supply additional power for actuators.
Inherent to IO-Link systems is backward compatibility. For example:
•IO-Link tolerates unshielded connections, allowing reuse of standard
industrial wire in existing installations.
•IO-Link devices can operate without
an IO-Link master in a legacy digital
switching mode called Standard I/O
(SIO). Likewise, IO-Link masters can
operate legacy devices using SIO.
A built-in load current on the CQ line at
the master side (ILLM) facilitates operation of older sensors with discrete PNPtype outputs, which only drive high.
4 | October 2014 : LT Journal of Analog Innovation
MASTER
Figure 1. IO-Link physical layer
interface (PHY). The device side
consists of a high side (and
optionally, low side) driver and a
receiver. The master side has a
push-pull driver, receiver, and a
current sink that operates as a
load for high side device outputs.
DEVICE
L+
24V
C/Q
DRV
DRV
ILL
L–
Any overview of IO-Link must introduce
the scheme known as wake-up. Before
IO-Link communication can commence, an
IO-Link master must determine whether a
connected device is compatible, and, if it
is, identify the highest transmission rate
supported: 230.4kb/s (called COM3 mode),
38.4kb/s (COM2), or 4.8kb/s (COM1). This
requirement, combined with another—an
IO-Link device must start up enabled to
operate in SIO mode outside of an IO-Link
system—poses a problem: how to gain
the attention of an IO-Link device that’s
dutifully transmitting its sensor output.
The answer is by shouting. The master gains the attention of the device by
issuing a wake-up request (WURQ), an
80µs, 0.5A current pulse, which is guaranteed to exceed the drive strength of
an IO-Link device so that, upon detecting the pulse, it may stop driving and
participate in a signaling exchange
of data that informs the master of its
maximum communication rate.
Once operating in communication mode, a
master and device exchange data asynchronously in frames consisting of 11 bits
(Figure 2a). Most of these UART frames
are organized into larger units called
M-sequences (Figure 2b), which begin
with a message sent by the master paired
with a reply message from the device.
M-SEQUENCE (MESSAGE SEQUENCE)
MASTER SEQUENCE
STOP BIT (SP)
START BIT (ST)
0
b0
b1
b2
PARITY BIT (EVEN)
b3
b4
b5
b6
b7
P
1
UART
FRAME
UART
FRAME
• • •
UART
FRAME
UART
FRAME
DATA OCTET
(a)
DEVICE MESSAGE
• • •
UART
FRAME
(b)
Figure 2. (a) IO-Link UART frames contain 11 bits of data. (b) Cyclic data is organized into paired exchanges of
UART frames between master and device called M-sequences.
design features
Two new interface parts target the first I/O technology
for communication with sensors and actuators to be
adopted as an international standard.
M-sequences transmit process data at
predetermined rates in various available
formats based on the type of device. Other
transmission modes support configuration,
maintenance and diagnostic functions.
HOT SWAP CONTROLLER
PROTECTION AND ADVANTAGES
The IO-Link standard has little to say
about the L+ power-over-cable supply,
suggesting only that 200m A and perhaps a power switch are needed. But
potential problems abound when power
is connected to arbitrarily large loads.
Although high inrush current shouldn’t
damage the sturdy connectors used
for IO-Link, it can still cause connector
sparks and supply droop that can lead
to system resets. Although the powerover-cable (POC) requirement of IO-Link
(4W minimum) is modest compared to
alternative technologies such as Power
over Ethernet, anyone who has experienced faults at 24VDC knows they can
be disruptive or catastrophic, leading to
the question “is something burning?”
To solve the problems of inrush current
control and fault isolation, the LTC2874
generates L+ power supply outputs using
a Hot Swap controller and n-type power
MOSFETs. The resistance of the power path
is kept low using external components for
the MOSFETs and sense resistors, reducing IC heat dissipation and maximizing
power efficiency during operation. This
arrangement gives users flexibility in
MOSFET selection. Because this application requires the MOSFET to operate in
linear mode during current limiting, older
planar process MOSFETs such as Fairchild’s
FQT7N10 are recommended in order to
avoid damage-causing hot spots that
some newer versions and especially trench
transistors can develop in this mode.4 The
controller provides SPI-operated on/off
control, current limiting, and a programmable, timed circuit breaker function.
CG = 22nF
RG = 10Ω
LOAD = 10µF
LOAD = 100µF
L+1
FLDBK_MODE = 1
10V/DIV
Because IO-Link devices usually require
cable-supplied power to operate and
communicate, there’s normally no way
for them to notify their master that power
is absent. In such scenarios, master-side
diagnostic capabilities are especially
valuable. The LTC2874 reports changes
to output supply “power good” status—along with a host of other conditions including overtemperature, input
L+1
L+2
L+2
L+3
L+3
L+4
The LTC2874 adds flexibility to inrush
current control by raising output supplies in a controlled manner determined
either by current limiting (Figure 3a) or,
for load independence, by an external
RC network (Figure 3b). When enabled
by a SPI register bit, the LTC2874 applies
foldback behavior to the current limit
in order to minimize power dissipation
in the MOSFET during start-up and overcurrent conditions. An optional cablesensing mode keeps the L+ power disabled
until a cable is connected to the port.
10V/DIV
L+
5V/DIV
L+4
LT3669
VOUT
2V/DIV
FLDBK_MODE = 0
4ms/DIV
20ms/DIV
(a)
(b)
400µs/DIV
(c)
Figure 3. L+ power supply output start-up (a) in current limit, (b) defined by a GATE resistor-capacitor network, and (c) for LT3669 application circuit configured for 4V
buck output.
October 2014 : LT Journal of Analog Innovation | 5
In IO-Link applications, the LTC2874 and LT3669 simplify wake-up request (WURQ)
handling for their respective microcontrollers. On the master side, the LTC2874 generates
WURQs of the correct polarity and timing automatically when a SPI register pushbutton
bit is set. An interrupt request (IRQ) provides a handshake to the microcontroller. On the
device side, the LT3669 pulls the WAKE output flag low under certain conditions.
20V TO 34V
22V TO 34V
SENSE+
LTC2874
D1: S100
Q1: FQT7N10
2XPTC = 0×0 (DEFAULT)
0.2Ω
SENSE−
D1
GATE
Q1
SENSE+
LTC2874
SENSE−
D1
200mA
GUARANTEED
CABLE
<6Ω
GATE
L+
DEVICE
D1: S100
Q1: FQT7N10
2XPTC = 0×1 (DISABLED)
0.08Ω
Q1
500mA
GUARANTEED
CABLE
<6Ω
>18V
L+
DEVICE
L–
>18V
L–
(a)
(b)
Figure 4. (a) LTC2874 configured for IO-Link-compliant
200mA device supply. Optional D1 provides supply isolation.
(b) Alternative configuration for 500mA.
supply voltage level, and output supply
overcurrent—to the microcontroller via
its SPI port and interrupt request line.
These monitoring capabilities enable the
software to guide operators toward making faster repairs with less down time.
While L+ outputs must normally supply 200m A, the IO-Link standard requires
a boosted current pulse capability at
start-up, guaranteeing 400m A for 50ms
upon reaching 18V. This requirement
can be met indirectly by configuring the
sense resistors for higher current and
constraining the input supply (Figure 4b).
A better approach (Figure 4a) uses the
LTC2874’s optional SPI-controlled current
Figure 5. (a) Self-timed 80µs 500mA wake-up request for an unloaded CQ line. (b) LTC2874-generated WURQ
overdriving an LT3669 device PHY. Upon notifying its microcontroller that a WURQ pulse was detected, the
LT3669 releases CQ1.
(a)
(b)
LT3669 DEVICE
RELEASES CQ1
16th PULSE
SCK
5V/DIV
CQ1
10V/DIV
CQ1
10V/DIV
WURQ
WURQ
WAKE
2V/DIV
IRQ
5V/DIV
20µs/DIV
6 | October 2014 : LT Journal of Analog Innovation
WAKE-UP
DETECTED
40µs/DIV
pulse function to meet the start-up
requirement while preserving DC operating margin relative to the safe operating area (SOA) of the MOSFET. In both
cases, the optional current-limit foldback
helps protect the operating margin at
lower output voltages where power
dissipation in the MOSFET is highest.
EASY WAKE-UP GENERATION AND
DETECTION
In IO-Link applications, the LTC2874 and
LT3669 simplify wake-up request (WURQ)
handling for their respective microcontrollers. On the master side, the LTC2874
generates WURQs of the correct polarity
and timing automatically when a SPI register pushbutton bit is set (Figure 5a).
An interrupt request (IRQ) provides a
handshake to the microcontroller.
design features
While sensors built with digital IO-Link interface are likely less susceptible to noise than
older analog-output models, their wide-swing (24V), single-ended signaling through
unshielded wire can produce electromagnetic interference (EMI). The CQ line drivers of
both the LTC2874 and LT3669 use slew-rate limiting circuitry to reduce the high frequency
content of signaling emissions. Both products also offer a slow edge rate mode.
TXD1
10V/DIV
CQ1
20V/DIV
20
AMPLITUDE (dBV)
CQ2
20V/DIV
TXD1
10V/DIV
CQ3
20V/DIV
CQ4
20V/DIV
5µs/DIV
(a)
(b)
Figure 6. (a) LTC2874 CQ outputs operating with slow edge rate slew control active on two ports. Ports 1 and
3 are shown operating at COM2, or 38.4kb/s, while ports 2 and 4 operate at COM3, or 230.4kb/s. (b) LT3669
CQ1 output with slow and fast edge rate slew control applied for COM2 and COM3 operation, respectively.
•CQ1 is higher than VL+ − 2.95V while
the driver is disabled (TXEN1 low).
The device side microcontroller can then
respond by disabling the driver as needed,
handshaking with the LT3669 (by toggling TXD1 while TXEN1 is low) to reset
its WAKE state, and listening for a start-up
protocol to be initiated by the master.
Decision-making and response is left to
the microcontroller, which, based on
mode and context, must discern between
valid WURQ signaling and invalid cases.
The LT3669’s straightforward approach
to detection maximizes flexibility.
−40
20
0
SLOW CQ1/Q2 Edge Rate
(SR = L)
−40
10µs/DIV
•CQ1 does not approach its targeted rail within 2.95V while the
driver is enabled (TXEN1 high);
−20
−20
CQ1
(SR = H)
10V/DIV
On the device side, the LT3669 pulls
the WAKE output flag low (Figure 5b)
when either of two conditions persists for more than 75µs:
FAST CQ1/Q2 EDGE RATE
(SR = H)
0
CQ1
(SR = L)
10V/DIV
0.0
0.4
0.8
1.2
FREQUENCY (MHz)
1.6
2.0
Figure 7. High frequency EMI reduction of LT3669
operating at 38.4kb/s with slow edge rate control
(bottom) compared to fast edge rate control (top).
CONTROLLED EDGE RATES REDUCE
EMISSIONS
RUGGED INTERFACES TOLERATE
ABUSE
While sensors built with digital IO-Link
interface are likely less susceptible to noise
than older analog-output models, their
wide-swing (24V), single-ended signaling through unshielded wire can produce
electromagnetic interference (EMI). The
CQ line drivers of both the LTC2874 and
LT3669 use slew-rate limiting circuitry
to reduce the high frequency content of
signaling emissions. Both products also
offer a slow edge rate mode (Figure 6)
that can be selected at lower data rates,
suppressing the HF content further. The
improvement achieved by the LT3669
slew rate control is shown in Figure 7.
Any cable interface risks exposing sensitive electronics to uncontrolled harsh
conditions. IO-Link requirements compound the problem, demanding a combination of operating voltage (up to 30V)
and guaranteed current (200m A for each
L+ output, 100m A DC for each CQ driver
output, and 500m A for wake-up request
pulses) that, in the event of an overload or
shorted output, can result in high power
dissipation in the driving MOSFET or IC.
Consequently, the LTC2874 and LT3669
are designed to withstand a wide range
of operating conditions, abuse and
fault modes on their cable interfaces.
The LTC2874 tolerates cable voltages well
outside its operating range (for example,
50V above GND for L+ and 50V from
opposite rails for CQ) and has multiple
ways to protect against an overload.
First, current-limiting responds quickly
October 2014 : LT Journal of Analog Innovation | 7
Table 1. Typical line interface electromagnetic compatibility (EMC) results when data sheet recommendations are followed.
LTC2874
LT3669
CONDITIONS/NOTES
HUMAN BODY MODEL (ESD)
±8kV
±4kV
Without TVS Clamps
IEC 61000-4-2
(ESD)
±8kV (Level 4)
±6kV (Level 3)
Contact discharge
DC1880A/DC1733A demo boards
C PIN = 470pF
±4kV (Level 4)
±4kV (Level 4)
5kHz/15ms
±4kV (Level 4)
±4kV (Level 4)
100kHz/0.75ms
IEC 61000-4-5
(Surge)
±2kV (Level 2)
±2kV (Level 2)
1.2/50µs–8/20µs
TVS CLAMP
SM6T36A
SM6T39A
IEC 61000-4-4
(EFT/Burst)
to prevent damage and reduce power dissipation in the IC or (in the case of the L+
output) MOSFET. The current limit is fixed
for CQ outputs and resistor-configurable
for L+ outputs. If the overcurrent condition persists after a predefined timeout
(mode-specific for CQ, programmable for
L+), a circuit breaker function disables the
output. After allowing a programmable
time for cooling, the LTC2874 auto-retry
function optionally re-enables the output.
The pattern repeats, pulsing the output
at a safely low duty cycle until either
the overload is removed or a controller
intervenes. Additionally, the IC has built
in protection against overtemperature
and supply overvoltage conditions.
The LT3669 is similarly well protected. It
is reverse polarity protected and tolerates up to ±60V between any combination of L+, CQ1, Q2 and GND pins. This
high voltage protection allows the use
of standard TVS diodes for additional
surge protection while still enabling
operation with L+ voltages of up to 36V.
This feature is especially attractive for
devices operating in SIO mode above the
operating voltage range of IO-Link.
The CQ1 and Q2 drivers are current-limited
to a value defined by an external resistor. In the case of heavy loads or short
circuits, additional high speed current
limit clamping and a pulsing scheme
keep power dissipation at a safe level.
8 | October 2014 : LT Journal of Analog Innovation
During pulsing, the on-time depends on
the voltage level of the active outputs
and the off-time is fixed (2.2ms typical), resulting in a duty cycle that adjusts
downward as the output dissipates more
power, keeping the IC safe and optimizing the time to drive heavy loads fully.
Like its master-side counterpart, the
LT3669 has precise built in thermal
shutdown and supply overvoltage protection. For junction temperatures above
140°C (typical), both line drivers are
disabled while the LDO and VOUT outputs continue to operate. Short-circuit
flags SC1 and SC2 report a thermal shutdown event to the microcontroller.
The cable interfaces of both the LTC2874
and LT3669 have built-in protection
against electrostatic discharge and are
easy to protect against a high level
of electromagnetic interference (EMI)
using standard TVS clamps (Table 1).
DRIVING HEAVY LOADS
While IO-Link drivers normally see
capacitive loading of at most 4nF when
connected by cable to another IO-Link
PHY, the LTC2874 and LT3669 can drive
more than 100m A (up to 250m A for the
LT3669) for compatibility with legacy
sensors and a variety of industrial loads.
For example, this drive strength is sufficient to operate miniature incandescent
lamps used in 12V and 24VDC systems.5
Turning on an incandescent lamp is nontrivial for an IC driver. Common tungsten
Figure 8. (a) LT3669 lighting a 12V 5W lamp and (b) driving a 470µF load. Short-circuit flags SC1 and SC2
are active if the driver’s voltage is within 2.95V from the rail opposite the targeted one while the drivers are
externally enabled.
(a)
(b)
RILIM = 42.2kΩ
12V/5W BULB
VL+ = 12V
RILIM = 42.2kΩ
CQ1/Q2
5V/DIV
CQ1
1V/DIV
SC1
5V/DIV
0V
SC1
5V/DIV
SC2
5V/DIV
50ms/DIV
5ms/DIV
design features
While IO-Link drivers normally see capacitive loading of at most 4nF when connected
by cable to another IO-Link PHY, the LTC2874 and LT3669 can drive more
than 100mA (up to 250mA for the LT3669) for compatibility with legacy sensors
and a variety of industrial loads. For example, this drive strength is sufficient to
operate miniature incandescent lamps used in 12V and 24VDC systems.
24V
CVDD2
1µF
VDD
CVDD1
100µF
SENSE+
LTC2874
VL
2.9V TO 5.5V
4.7k
IRQ
VCC
IRQ
2V/DIV
0.2Ω
4
0.2Ω
0.2Ω
SENSE–1
SENSE–2
SENSE–3
SENSE–4
1µF
0.2Ω
GATE1
RXDn
10Ω
Q1
L+1
µC
SPI
CQ1
GATE2
10Ω
Q2
L+2
CQ
10V/DIV
CQ2
GATE3
10Ω
Q3
L+3
100ms/DIV
CQ3
(a)
GATE4
TXENn
(b)
10Ω
Q4
L+4
TXDn
CQ4
GND
GND
Figure 9. (a) LTC2874 turning on a 24V 2W lamp (b) octal lamp driver
Q1 TO Q4: FQT7N10
filaments are about 15 times more conductive when cold compared to when glowing
hot. Consequently, while lighting a bulb,
the driver must cope with a near shortcircuit condition without overheating.
The LT3669 protects itself under such conditions by pulsing the output to limit the
driver power dissipation. Figure 8a shows
a 12V 5W bulb being turned on by the
combined output of both LT3669 drivers
Figure 10. Four dotted LTC2874 CQ outputs driving a heavy load
LTC2874
TXD1
CQ1
TXD2
CQ2
TXD3
CQ3
TXD4
CQ4
GND
DZ1
CQ
10V/DIV
DZ1: SM6T36A
D2: 1N4004
20m
CABLE
D2
EQUIVALENT
1.2H
47Ω
−I(CQ)
200mA/DIV
200ms/DIV
(up to 500m A combined driving capability), and illustrates the variable load
during this process. As the filament heats
up, an increasing portion of the voltage is
transferred to the lamp. Diagnostic flags
SC1 and SC2—which pull low to indicate
short-circuit conditions on the CQ1 and
Q2 drivers, respectively—track the progress toward fully driving the light bulb.
The case of driving a large capacitor
(Figure 8b) similarly flags a short-circuit
condition at the start of the charging
phase but only while the driver’s voltage
is less than 2.95V from the rail opposite the targeted one. Proper processing
of these short-circuit flags allows the
October 2014 : LT Journal of Analog Innovation | 9
The cable interface of the LTC2874 and LT3669 can drive a variety of 12V and 24V relays.
The CQ outputs can operate either high side or low side. In the case of the LTC2874, using
the L+ power supply outputs as high side relay drivers, the CQ pins can sense the state of
each relay, providing a handshake to the microcontroller via either the RXD outputs or the
SPI bus. The LTC2874 can operate eight relays when driving with both CQ and L+ pins.
microcontroller to distinguish between
real short circuits and heavy loads.
Figure 12. SPI-operated quad “ice cube” relay driver demonstrating both low side and high side operation.
24V
The LTC2874, too, can drive large loads
without damage from overheating.
Protective pulsing defined by the built-in
circuit breaker and auto-retry timers will
2.9V TO 5.5V
successfully turn on 1W miniature lamps.
Larger lamps can be driven using more
aggressive microcontroller-defined tim1µF
ing (Figure 9a) by connecting CQ drivers in parallel, or even by operating the
L+ power supply outputs (configured
for sufficient current) as high side drivers. Relying on all individual outputs,
the LTC2874 can operate eight lamps
(Figure 9b). Higher DC current is available
when outputs are combined (Figure 10).
DZ5
1µF
VDD
TXD1
100µF
SENSE+
0.2Ω
TXD2
VL
SENSE–1
4.7k
SENSE–2
IRQ
VCC
DS1
DS2
LTC2874
4
GATE1
RXDn
10Ω
D1–D4: 1N4004
DS1, DS2: FAIRCHILD S100
DZ1–DZ5: SM6T36A
K1–K4: RELAYS
Q1–Q4: FQT7N10
Q1
A
L+1
SPI
D1
CQ1
µC
GATE2
K1
B
10Ω
Q2
A
L+2
D2
CQ2
Driving unterminated, sometimes inductive, cable-connected industrial loads
commonly produces ringing. The receivers of both parts contain programmable
(LTC2874) or mode-specific (LT3669)
noise suppression filters to ensure that
K2
B
A
CQ3
D3
K3
B
TXENn
TXD4
A
CQ4
TXD3
GND
D4
GND
CQ3
20V/DIV
HIGH SIDE
DRIVER
CQ4
20V/DIV
CQ1
20V/DIV
CQ2
20V/DIV
LOW SIDE
DRIVER
100ms/DIV
K4
B
DZ1–DZ4
Figure 11. Each CQ output guarantees twice
the current required to operate a Potter and
Brumfield (Tyco) KRPA-11DG-24.
10 | October 2014 : LT Journal of Analog Innovation
0.2Ω
design features
The LTC2874 and LT3669 are designed to withstand
a wide range of operating conditions, abuse
and fault modes on their cable interfaces.
Figure 13. When L+ outputs
operate relays, the CQ lines can
sense the state of each relay.
24V
DZ5
100µF
VDD
L+1
20V/DIV
+
SENSE
0.2Ω
L+2
20V/DIV
SENSE–1
SPI
4
LTC2874
(1 OF 4 PORTS)
GATE1
RXD1
DS1
10Ω
Q1
DS1: FAIRCHILD S100
DZ1, DZ5: SM6T36A
K1: RELAY
Q1–Q4: FQT7N10
RXD1
5V/DIV
L+1
CQ1
RXD2
5V/DIV
A
TXEN1
TXD1
DRIVING RELAYS
The cable interface of the LTC2874 and
LT3669 can drive a variety of 12V and
24V relays (Figure 11). The CQ outputs
can operate either high side or low side
(Figure 12). In the case of the LTC2874,
using the L+ power supply outputs as high
side relay drivers, the CQ pins can sense
the state of each relay (Figure 13), providing a handshake to the microcontroller
via either the RXD outputs or the SPI bus.
The LTC2874 can operate eight relays
when driving with both CQ and L+ pins.
K1
DZ1
GND
microcontrollers see clean transitions,
whether switching in SIO mode or communicating at the fastest IO-Link rate (COM3).
RELAY SENSE
100ms/DIV
B
EFFICIENT AND FLEXIBLE POWER
CONVERSION KEEPS TINY SENSORS
COOL
Sensors typically incorporate a transducer
that converts a physical parameter to an
electrical signal, a microcontroller that
performs analog-to-digital conversion and
signal processing, and a PHY interface that
level shifts to the high voltage at the cable
interface. Typically, transducers operate
from 3.3V to 15V and microcontrollers
operate from 1.8V to 5V. Given the IO-Link
L+ typical operating voltage of 24V, it’s
clear that some sort of power conversion is required for proper operation
of these lower voltage sensor parts.
While a simple linear regulator is
capable of this task, internal power dissipation limits its application to smaller
loads. For example, for an LDO generating 5V from 24V, at 10m A the pass
transistor dissipates 190mW, which is
Figure 14. Compact device-side
IO-Link PHY and dual power
supply solution using the LT3669
October 2014 : LT Journal of Analog Innovation | 11
Sensors offer a wide breadth of physical measurement capabilities, and
with that just as many varied power requirements. It is impossible to
meet this range of requirements with just a switching regulator or LDO.
Both are built into the LT3669 and LT3669-2, allowing these devices
to meet most power requirements without additional converters.
12 | October 2014 : LT Journal of Analog Innovation
SW
D1
LT3669-2
53.6k
10.2k
COUT
22µF
DA
VL+
L+
DIO
LDOIN
RT
FBLDO
4.7µF
LDO
14k
4.42k
38.3k
1µF
(a)
LOGIC I/O
GND
33µH
BD
FBOUT
BST
0.22µF
SW
D1
LT3669-2
41.2k
VL+
4V
10.2k
COUT
22µF
DA
L+
DIO
LDOIN
RT
FBLDO
4.7µF
LDO
14k
4.42k
38.3k
1µF
LOGIC I/O
GND
BD
FBOUT
BST
0.22µF
SW
D1
LT3669-2
31.6k
10.2k
COUT
22µF
DA
VL+
L+
DIO
LDOIN
RT
FBLDO
4.7µF
LDO
14k
4.42k
38.3k
1µF
3.3V
µC
AGND
(c)
GND
3.3V
3.3V
µC
AGND
(b)
5V
3.3V
µC
AGND
33µH
Sensors offer a wide breadth of physical measurement capabilities, and with
that just as many varied power requirements. It is impossible to meet this range
of requirements with just a switching
regulator or LDO. By having both built
into the LT3669 and LT3669-2, these
devices can meet most power requirements
without additional converters, saving
significant space, design time and cost.
BD
FBOUT
BST
0.22µF
TRANSDUCER
33µH
TRANSDUCER
At these power levels, a switching regulator offers a clear advantage: by reducing
the internal power dissipation, the sensor
can operate reliably at much higher ambient temperatures. Both the LT3669 and
LT3669-2 integrate a step-down switching regulator in addition to an LDO. The
LT3669-2 targets applications requiring
medium to high power levels for the sensor’s low voltage circuitry. With this in
mind, it does not incorporate the catch
diode, keeping that external. With an
external catch diode, it typically achieves
78% efficiency for 24V-to-5V conversion
at its rated load current of 300m A, corresponding to 423mW of internal power
dissipation. Although efficiency falls to
69% at 100m A, the internal power dissipation is still only 225mW, 8 times lower
than the linear regulator equivalent.
For less power demanding circuitry, the
LT3669 (Figure 14) reduces cost and area
by integrating the catch diode, attaining slightly lower efficiency of 64% at
its maximum load current of 100m A.
Figure 15. Various power supply
configurations for the LT3669-2,
with pin LDOIN connected to
the buck regulator output in (a)
and (b) for best efficiency and
to pin DIO in (c).
LOGIC I/O
3.3V
TRANSDUCER
tolerable, but at 100m A the wasted
power increases to 1.9W, which would
significantly raise the die temperature.
design features
The LDO delivers up to 150m A of load current, depending on the setup. With a dedicated input pin LDOIN, it can be configured
to take power from any power source
from 2.25V to 40V. The LDO can operate
either from the switching regulator output,
or separately. Figure 15 shows a number of
possible supply configurations. Connecting
the LDO input pin to the output of the
switching regulator (Figures 15a and 15b)
yields the highest efficiency. If this isn’t
possible, the LDO’s input pin can take
power from L+ indirectly by connecting it
to DIO (an internal diode connects between
L+ and DIO) to preserve reverse polarity
protection, as shown in Figure 15c. In this
case the LDO’s maximum load current is
reduced due to current limit foldback.
BUILDING LARGER MULTIPORT
MASTERS
The dense integration of a quad IO-Link
master PHY into QFN (Figure 16a) and
TSSOP packages makes the LTC2874 ideal
for building larger multiport masters.
For example, a 12-port master is shown
in Figure 16b. Four ports connect to the
microcontroller’s built-in UARTs; the rest
are serviced via SPI port expanders (U1,
U2), where their UARTs are implemented
using dedicated ARM microcontrollers
running optimized code. This system is
extendable, limited only by the bandwidth
and capabilities of the primary microcontroller. Linear Technology’s demonstration circuit DC2228A (Figure 17a),
a multiport master built in this way,
supports connections to eight IO-Link
devices such as the DC2227A (Figure 17b).
Figure 16. (a) Dense integration enables compact multiport masters to be built
using the 4-port LTC2874. (b) Master power and communications PHY for 12 ports.
(a)
(b)
FIELDBUS
LTC2874
SPI
TXEN1–4
L+1–4
TXD1–4
CQ1–4
RXD1–4
CS
GND
IRQ
12
TXEN1–4
TXD1–4
RXD1–4
SS0_0
IRQ0
4 PORTS
µC
FIELDBUS
PHY
SPI0
SS1_0
SS2_0
3
U1
SPI
CS
IRQ
TXEN1–4
TXD1–4
RXD1–4
12
TXEN1–4
TXD1–4
RXD1–4
12
IRQ1
SPI1
SS0_1
SS1_1
IRQ2
3
U2
SPI
CS
IRQ
LTC2874
SPI
TXEN1–4
L+1–4
TXD1–4
CQ1–4
RXD1–4
CS
GND
IRQ
LTC2874
SPI
TXEN1–4
L+1–4
TXD1–4
CQ1–4
RXD1–4
CS
GND
IRQ
4 PORTS
4 PORTS
U1, U2: PORT EXPANDER USING SAM4N (OR SIMILAR)
NOTE: SHARED INTERRUPTS MIGHT LIMIT PERFORMANCE
October 2014 : LT Journal of Analog Innovation | 13
The dense integration of a quad IO-Link master
PHY into QFN and TSSOP packages makes the
LTC2874 ideal for building larger multiport masters.
(b)
(a)
Figure 17. IO-Link application demonstration circuits (a) DC2228A, an 8-port master built with LTC2874 and powered optionally by 90W Power over Ethernet
(LTPoE++™), and (b) DC2227A, a device-side sensor application built with LT3669-2, a high precision temperature sensor, a photoelectric sensor, and a 28V 100mA
incandescent lamp
IEC 61131-2 SUPPORT
The cable interfaces of both parts are, as
part of the IO-Link definition, loosely compatible with IEC 61131-2, an older standard
specifying digital I/O in programmable
logic controller (PLC) applications.6 This
compatibility includes the optional second
driver Q2 on the LT3669. Additionally, the
LTC2874’s built-in current-sinking loads
have a setting that meets the requirements for Type-1 inputs while keeping
power dissipation to a minimum.
14 | October 2014 : LT Journal of Analog Innovation
COMPLETE IO-LINK COMPATIBLE
POWER AND SIGNALING INTERFACE
Both sides of an IO-Link application,
each with its own microcontroller,
are shown in Figure 18. The masterside LTC2874 supports four such ports.
The device-side LT3669 guarantees
100m A at the 5V switched output, the
3.3V LDO output, and both driver outputs. Connector pin 2 is optional,
supported only at the device side.
CONCLUSION
The LTC2874 and LT3669 offer unmatched
integration and flexibility for building
IO-Link systems. The LTC2874 includes
power, signaling, control and diagnostic
capabilities for four ports, simplifying
the design of larger multiport masters.
The LT3669 includes a spare driver (Q2),
LDO, and a step-down regulator that helps
minimize temperature rise in compact
sensor assemblies. The wide operating
ranges of these devices (8V to 34V for the
LTC2874, and 7.5V to 40V for the LT3669),
allow them to drive a variety of industrial
loads. Both parts are ruggedized for the
harsh environment of 24V automation. n
design features
The LTC2874 and LT3669 offer unmatched integration and flexibility for building
IO-Link systems. The LTC2874 includes power, signaling, control and diagnostic
capabilities for four ports, simplifying the design of larger multiport masters.
The LT3669 includes a spare driver (Q2), LDO, and a step-down regulator
that helps minimize temperature rise in compact sensor assemblies.
VOUT, IOUT**
5V, 100mA
2.9V TO 5.5V
53.6k
VL
10.2k 0.1µF
0.1µF
1/4
LTC2874
4.7k
38.3k
24V
VDD
1µF
100µF
SENSE+
1µF
SENSE–1
CS
SDO
GATE1
RXD1
TXEN1
TXD1
14k
VLDO, ILDO**
3.3V, 100mA
20 METERS
0.2Ω
SCK
µC
GND
CPOR
SW
RT
BST
0.1µF
4.42k
SYNC
LDO
RST
FBLDO
SC1
AGND
SC2
DIO
10Ω
200mA
L+1
CQ1
100mA
Q1: FQT7N10
SURGE PROTECTION NOT SHOWN
*ADDITIONAL BYPASS CAP AS NEEDED
WAKE
EN/UVLO
Q1
4
4
L+
100mA
1
5
33
1
2
2
3
100mA
4
4.7µF
**IOUT(MAX), IS 100mA AND ILDO(MAX) IS 100mA
(REMAINING AVAILABLE IOUT IS: 100mA – ILDO)
470pF
470pF
VOUT
OR
VLDO
SR
LDOIN
VOUT
IRQ
82µH
LT3669
ILIM
*
SDI
BD
FBOUT
10µF
RXD1
µC
TXEN1
Q2
TXD1
CQ1
TXEN2
GND
TXD2
fSW = 600kHz
tRST = 12.5ms
Figure 18. Complete 24V 3-wire power and signaling interface to sensor or actuator. One of four available master ports is shown.
Notes
1Tsun-kit
Chin and Dac Tran, “Combine power feed and
data link via cable for remote peripherals,” EE Times,
November 10, 2011.
2 www.io-link.com.
IO-Link is a registered trademark of
PROFIBUS User Organization (PNO).
3 IEC
61131-9 ed.1.0
4Paul
Schimel, “MOSFET Design Basics You Need To
Know,” Parts 1 and 2, Electronic Design, April 4 and
April 21, 2010.
5“Safely
Light Miniature Incandescent Lamps Using LTC2874,” Kevin Wrenner, January 2014.
http://www.linear.com/solutions/4534
6IEC
61131-2, Third edition, 2007-07.
October 2014 : LT Journal of Analog Innovation | 15
High Step-Down Ratio Controller Combines Digital Power
System Management with Sub-Milliohm DCR Sensing and
Accurate PolyPhase Load Sharing
James A. McKenzie
The increasing complexity of electronics, particularly large computing systems, has
exerted pressure on power supplies to improve efficiency, transient response, monitoring
and reporting functionality, and digital control. High efficiency is paramount in distributed
systems, where high step-down ratios from intermediate voltage busses are used to
create local low voltage supplies sourcing high currents. Sensitive low voltage subsystems
require accurate output voltage regulation single-cycle load step response. Such needs are
frequently met with PolyPhase® designs located in close proximity to their point of load.
VIN
7V TO 14V
1Ω
330µF
×2
+
VDD33
10k
4.99k
10k
10k
4.99k
10k
VDD25
2.2µF
4.99k
5
100nF
VCC
1µF
VDD33 VDD25
SDA
ALERT
GPIO0
SYNC
17.4k
PGND
FB0
SHARE_CLK
16.2k
100pF
LTC3882
COMP0
7.32k
ASEL0
FDMF6820A
PLACE Q1,Q2 NEAR L1, L2 RESPECTIVELY
VOUT1_CFG
FREQ_CFG
PHAS_CFG
IAVG0
10nF
Q1
7.32k
Q2
5
220pF
1µF
100pF
VOUT0_CFG
FB1
10nF
137Ω
ISENSE1–
2.2µF
VIN PHASE GH CGND GND BOOT VDRV
ISENSE1+
TG1/PWM1
GND
TG1_PWM1
PGND
BG1/EN1
DrMOS: FAIRCHILD FD6802A
10Ω
VIN
22µF
×2
VSENSE1+
IAVG1
16 | October 2014 : LT Journal of Analog Innovation
VCIN
137Ω
10nF
COMP1
Figure 1. Dual output application
using DrMOS to develop over
90W of output power
220pF
10nF
TSNS0
TSNS1
ASEL1
IAVG_GND
VIN PHASE GH CGND GND BOOT VDRV
VSENSE0–
RUN1
VDD25
10Ω
VSENSE0+
RUN0
WP
22µF
×2
TG0_PWM0
ISENSE0–
GPIO1
RUN
VCC
TG0/PWM0
BG0/EN0
ISENSE0+
2.2µF
VIN
4.7µF
VINSNS
SCL
1µF
INPUT SUPPLY +5V TO +12V
FDMF6820A
VCIN
design features
The LTC3882 satisfies the broad demands placed on the modern power
supply. It is a dual channel DC/DC synchronous step-down PWM controller
with PMBus-compliant serial interface. Each channel can produce independent
output voltages from 0.5V to 5.25V. Up to four LTC3882s can operate interleaved
in parallel, creating single-output rails containing up to eight phases.
Higher system complexity translates
into demand for nontraditional features
from the power subsystem. Host systems can have dozens of local voltage
rails delivering a wide range of power
levels. The power subsystem must be
capable of accurately reporting key
operating parameters and providing
rapid, autonomous fault response.
5VBIAS
+5V INPUT SUPPLY
2k
1µF
N
ARCHITECTURE FOR HIGH
PERFORMANCE LOAD STEP
RESPONSE AND REGULATION
The LTC3882 satisfies the broad demands
placed on the modern power supply. It
is a dual channel DC/DC synchronous
step-down PWM controller with PMBuscompliant serial interface. Each channel
can produce independent output voltages
from 0.5V to 5.25V. Up to four LTC3882s
can operate interleaved in parallel, creating single-output rails containing up to
eight phases. Multiples of 6- or 8-phase
designs can also be developed when power
or reliability dictate higher phase counts.
Once its onboard EEPROM is programmed,
the LTC3882 can operate autonomously
without host support, even during a
fault condition. The LTC3882 is available
in a 40-lead 6mm × 6mm QFN package.
Figure 1 shows a typical solution.
To support high step-down ratios and fast
load transient response, the LTC3882 uses a
constant frequency, leading-edge modulation voltage mode architecture. This architecture is combined with a very low offset,
high bandwidth voltage error amplifier
and proprietary internal feedforward
compensation. The low output impedance of a true voltage amplifier allows
implementation of flexible Type III loop
compensation. Internal feedforward
compensation instantaneously adjusts
duty cycle for changes in input voltage,
significantly reducing output overshoot or
undershoot. It also creates constant modulator gain independent of input voltage,
SMOD
PWM
TG1_PWM0
DISB
RUN
L1
0.22µH
PULSE
VSWH
CGND
1.21k
GL
100µF
×4
+
VOUT0
1.8V, 30A
470µF
×4
Figure 2. Load step response of Figure 1 circuit
GND
0.22µF
VOUT
20mV/DIV
45mV
IOUT
10A/DIV
5VBIAS
15A
2k
50µs/DIV
LOAD STEP = 0A TO 15A TO 0A
di/dt = 15A/µs
1µF
N
0.22µF
SMOD
PWM
TG1_PWM1
DISB
RUN
VSWH
CGND
GL
1.21k
L2
0.22µH
PULSE
100µF
×4
+
VOUT1
1V, 40A
470µF
×4
GND
October 2014 : LT Journal of Analog Innovation | 17
Each LTC3882 PWM channel provides five selectable PWM control
protocols for interfacing to power stage designs that have 3.3V-compatible
control inputs. The user can choose the optimum type of power stage for
the design requirements: discrete FET drivers, DrMOS devices or power
blocks. These can be mixed and matched on a per channel basis.
Figure 3. Transient response with AVP enabled
Figure 4. Continuous conduction mode start-up with
output prebiased
IOUT
10A/DIV
VOUT
0.5V/DIV
IL1, IL2
10A/DIV
VOUT
50mV/DIV
200µs/DIV
affording more aggressive loop compensation with improved transient response.
Both channels feature remote output voltage sense. Channel 0 has a corresponding
negative sense input for ground offsets.
The remote negative sense for channel 1 is the package ground paddle. A
separate control loop yields exceptional
DC and dynamic PolyPhase load sharing.
This high performance architecture can
deliver excellent load transient response.
Figure 2 shows a typical output transient for an LTC3882 power supply.
Leading-edge modulation affords fast,
single-cycle response to output load steps
and does not restrict minimum duty cycle.
PWM output control pulses can become
vanishingly small with this scheme, and
minimum on-time is normally limited by
the power stage design, not the controller. This, plus feedforward compensation,
facilitates robust operation at high stepdown ratios. The LTC3882 operates with
18 | October 2014 : LT Journal of Analog Innovation
input power bus voltages from 3V to 38V.
Stable operation with no pulse-skipping at
step-down ratios approaching 25:1 is possible, even at higher switching frequencies.
For compact solutions, stable operation is
possible using only ceramic output capacitors, and the LTC3882 features programmable active voltage positioning (AVP),
allowing further optimization of ESR and
reduction in output capacitor size. Figure 3
shows a typical example of AVP operation.
Depending on the needs of the application, peak efficiency or solution size
can be prioritized by choosing an optimal operating frequency. The LTC3882’s
250kHz to 1.25MHz programmable
switching frequency supports optimization of inductor size and output current
ripple. The LTC3882 can also serve as a
shared PWM clock master or accept an
external clock input for synchronization
to another system time base. For very
small magnetic-component footprints,
VIN = 12V
1ms/DIV
a higher frequency version is available.
Contact Linear Technology for details.
As shown in Figure 4, the LTC3882 has
the ability to start into a prebiased
output without perturbing it, regardless of its soft-start parameter programming or inductor current operating
mode (continuous or discontinuous).
LOW DCR SENSING FOR HIGH
POWER
At relatively high output currents, conversion efficiency must be maximized
to limit heat production and minimize
related cooling costs due to conduction
losses. Some conduction losses occur
in the current sensing element used for
detecting output overload and other
functions. In a step-down topology, the
sense element is key to efficiency because
it continuously sees the full DC load
current plus additional current ripple.
design features
The LTC3882 monitors critical supply parameters with an internal 16-bit
ADC. Digital readback via PMBus is available for input and output
voltages, output currents, duty cycles and temperatures. The LTC3882
tracks, maintains and provides peak values for these parameters.
Figure 5. 2-channel efficiency and loss using
FDMF5820A DrMOS power stage
Figure 6. Typical slave ISENSE offset to ideal (master)
92
13
3500
8593 UNITS
FROM 3 LOTS
3000 T = 38°C
J
CHO MASTER
2500
91
11
90
9
87
7
86
85
84
5
83
VIN = 12V
VOUT = 1V
SYNC = 500kHz
82
81
80
0
10
20
30 40 50 60
LOAD CURRENT (A)
70
80
NUMBER OF ICs
88
POWERLOSS (W)
EFFICIENCY (%)
89
2000
1500
1000
3
500
1
0
The LTC3882 supports conventional
sense resistor topologies as well as low
DCR sense schemes that can produce a
full-scale voltage of only a few tens of
millivolts. Trimmed internal auto-zeroed
gain amplifiers maintain fast and accurate
supervisor detection of output overcurrent conditions. The classical fixed ramp
voltage mode PWM architecture allows
large signal control of the duty cycle and
eliminates noise concerns that could be
created by low DCR designs using currentbased control schemes. Typical efficiency
and loss for a LTC3882 power supply built
with Fairchild FDMF5820A DrMOS devices
is shown in Figure 5.
DIGITAL ENHANCEMENTS IMPROVE
OUTPUT ACCURACY
The LTC3882 contains an optional digital
output servo function. When enabled,
the 16-bit ADC output for channel voltage is used to servo to the desired average
output value. In this case, the converter
–400 –300–200–100 0 100 200 300 400
CH1 ISENSE OFFSET TO IDEAL (µV)
has an impressive typical output error
of only ±0.2% and a worse case error
over temperature of ±0.5%. These tolerances are guaranteed over an output voltage range of 600mV to 5V.
ACCURATE LOAD CURRENT
SHARING
For PolyPhase operation, the LTC3882
features a separate current sharing loop
that provides accurate load balancing, an
improvement over conventional voltage
mode converters. Channels are designated as control masters or as slaves by
pin strapping. The IAVG pin on the master
channel provides a voltage analog of its
instantaneous output current. A filter
capacitor of 100pf to 200pf is added
to this line, which is then routed to all
slave phases. The slaves use this information and the primary COMP control
voltage from the master to match their
own output current to that of the master.
Figure 6 shows the typical cumulative
current sense offset of a slave phase. For
low DCR sensing, this translates into typical DC current matching of better than
2% at full output power. Figure 7 shows
that this matching is maintained dynamically through high speed load steps.
WIDE SELECTION OF POWER
STAGES
Each LTC3882 PWM channel provides five
selectable PWM control protocols for
interfacing to power stage designs that
have 3.3V-compatible control inputs. The
user can choose the optimum type of
power stage for the design requirements:
discrete FET drivers, DrMOS devices or
power blocks. These can be mixed and
matched on a per channel basis, allowing optimization of power subsystem
partitioning, size and cost, according to
the power delivery needs of each rail.
October 2014 : LT Journal of Analog Innovation | 19
There are many reasons to consider use of a power systems management (PSM)
controller. PMBus commands can be issued to the LTC3882 to set output voltage, margin
voltages, switching frequency, output on/off sequencing and other operating parameters.
The LTC3882 supports over 100 PMBus commands, both standard and custom.
ACCURATE OPERATING PARAMETER
TELEMETRY
The LTC3882 monitors critical supply
parameters with an internal 16-bit ADC.
Digital readback via PMBus is available
for input and output voltages, output
currents, duty cycles and temperatures.
The LTC3882 tracks, maintains and provides peak values for these parameters.
Beyond basic supply parameter telemetry,
the LTC3882 can report a wide range of
internal and external status information
to the system host over the PMBus.
FAST, PROGRAMMABLE FAULT
RESPONSE
Faults can be detected and communicated
using a shared fault bus between LTC3882s
as well as other Linear Tehnology
PSM family members, such as the LTC3880.
The LTC3882 provides a standard opendrain ALERT output with compliant
ARA response for notification of a wide
range of fault conditions to the bus host.
The LTC3882 implements high speed, low
level hardware responses to critical faults
to protect the power stage and downstream system load. PMBus commands
can then be used to configure higher-level
responses, mask faults to the system, and
determine which faults are propagated to
the shared fault bus. This provides flexibility in dynamically managing fault handling
at the system level, even after hardware
has been designed and fabricated.
The LTC3882 includes extensive logging capability that records the state of
converter operating conditions immediately prior to a fault. This log can be
20 | October 2014 : LT Journal of Analog Innovation
Figure 7. Dynamic load balancing during an output
transient
IOUT
20A/DIV
VOUT
20mV/DIV
IL1, IL2
10A/DIV
VOUT = 1V
VIN = 12V
SYNC = 500kHz
L = 320nH
5µs/DIV
enabled and stored to internal EEPROM to
provide a black box recorder function
for in-system diagnosis or subsequent
remote debugging of abnormal events.
USING DIGITAL PROGRAMMABILITY
TO ADVANTAGE
There are many reasons to consider use
of a power systems management (PSM)
controller. PMBus commands can be
issued to the LTC3882 to set output voltage, margin voltages, switching frequency,
output on/off sequencing and other
operating parameters. In total, the LTC3882
supports over 100 PMBus commands,
both standard and custom. A principal
benefit of this programmability is reduced
design cost and faster time to market.
Once a fundamental hardware macro
design is complete, many variations can
quickly be created, brought to operation,
and verified as needed by simply adjusting digitally programmable parameters
inside the LTC3882 controller. Adjustments
can continue beyond production release
as needed, including fully synchronized
resequencing/retiming of power rails.
Combined with optional external resistor
programming of key supply parameters,
this kind of flexibility can avoid risky,
costly PCB spins or hand-wired modifications due to last-minute changes in
requirements or evolving system use.
Final configurations can be stored to
internal EEPROM using a variety of means,
including custom factory programming. Once a configuration is stored,
the controller powers up autonomously
to that state without burdening the host
for additional programming. However,
even after a final EEPROM configuration is
loaded, optional external programming
resistors can be used to modify a few
key operating parameters: output voltage, frequency, phase and bus address.
Once designed, the multiple addressing schemes supported by the LTC3882
allow the system to communicate with
devices globally or selectively at the
rail, device or individual channel level,
depending on control and monitoring
requirements. PMBus then facilitates
sophisticated high level system operations,
such as energy-efficient application load
balancing, local phase shedding, fault
containment/redundancy or interactive
preventive maintenance. These functions
would simply not be cost-effective or
even possible with conventional power
supply components in large systems.
design features
Once designed, the multiple addressing schemes supported by the LTC3882 allow
the system to communicate with devices globally or selectively at the rail, device
or individual channel level, depending on control and monitoring requirements.
DESIGN DEVELOPMENT SUPPORT
Linear Technology offers an array of
free software tools to assist with design,
development and debug of LTC3882-based
power supplies. LTpowerCAD™ provides
recommendations for component values and performance estimates specific
to the target application. This PC-based
tool guides the user through the entire
PWM design process, reducing development
effort and reducing cycle time. It shows
real-time results of feedback loop stability, and the design can be exported to
LTspice® for additional design verification.
PCB layout examples can also be provided.
LTpowerPlay™ is a PC-based tool with a
GUI that supports a wide range and combination of Linear Technology PSM products.
The LTC3882 PMBus command and feature
set is consistent with other devices in
the Linear PSM family. It operates seamlessly with these devices for flexibility
and system-level optimization of power
management design. LTpowerPlay provides a comprehensive, cohesive PMBus
development environment with full configuration, internal EEPROM programming,
fault logging and real-time telemetry data/
graphing. This can be especially helpful to power supply designers needing to
quickly bring up a large, complex power
subsystem. The tool can communicate
with custom designs and standard demo
circuits, such as the DC1936A. Both of
these tools and other design reference
materials are available at www.linear.com.
Proven firmware examples for use with the
LTC3882 are available to qualified customers. Contact Linear Technology for details.
CONCLUSION
The LTC3882 is a high performance
PSM voltage mode buck controller capable
of very accurate output voltage regulation,
supporting up to eight phases with well
balanced current sharing. It can be used
with discrete FET drivers, DrMOS devices
or power blocks. An onboard 16-bit
ADC provides accurate telemetry of all
critical operating parameters. It features
sophisticated fault management, reporting, sharing and storage. With its internal
EEPROM for settings and optional external resistor configuration, the LTC3882
can operate independently or under
PMBus bus control in complex, managed
power subsystems. Applications include
high current distributed power systems,
servers, network storage, intelligent high
efficiency power regulation and industrial systems such as ATE and telecom. n
October 2014 : LT Journal of Analog Innovation | 21
What’s New with LTspice IV?
Gabino Alonso
New Blog Article: “Modeling Safe
Operating Area Behavior of N-Channel
MOSFETs” by Dan Eddleman
www.linear.com/solutions/5239
BLOG BY ENGINEERS, FOR
ENGINEERS
Check out the LTspice blog
(www.linear.com/solutions/LTspice)
for tech news, insider tips and interesting points of view regarding LTspice.
Modeling Safe Operating Area Behavior
of N-Channel MOSFETs by Dan Eddleman
—Follow @LTspice at www.twitter.com/LTspice
—Like us at facebook.com/LTspice
influence the electrical behavior of the
circuit simulation. Even though using
the SOAtherm-NMOS symbol/models is
as easy as placing the symbol on top of
the NMOS in an LTspice schematic and
editing the component attributes, this
blog provides a step-by-step tutorial and
highlights some design considerations.
www.linear.com/solutions/5239—Verifying
SELECTED DEMO CIRCUITS
that a Hot Swap design does not exceed
MOSFET’s safe operating area (SOA) is challenging. Fortunately, thermal behavior
and SOA can now be modeled in LTspice.
For a complete list of example simulations utilizing Linear Technology’s devices,
please visit www.linear.com/democircuits.
Buck Switching Regulators
The new SOAtherm-NMOS symbol included
in LTspice contains a collection of
MOSFET thermal models that can be used
to verify that the maximum die temperature is not exceeded. The SOAtherm
provides MOSFETs’ junction and case
temperatures in °C (represented as voltage in waveform viewer) and does not
What is LTspice IV?
LTspice® IV is a high performance SPICE
simulator, schematic capture and waveform
viewer designed to speed the process of power
supply design. LTspice IV adds enhancements
and models to SPICE, significantly reducing
simulation time compared to typical SPICE
simulators, allowing one to view waveforms for
most switching regulators in minutes compared
to hours for other SPICE simulators.
LTspice IV is available free from Linear
Technology at www.linear.com/LTspice. Included
in the download is a complete working version of
LTspice IV, macro models for Linear Technology’s
power products, over 200 op amp models, as
well as models for resistors, transistors and
MOSFETs.
22 | October 2014 : LT Journal of Analog Innovation
• LT3840: High efficiency synchronous buck
converter (4.5V–60V to 3.3V at 20A)
www.linear.com/LT3840
• LT8620: 5V 2MHz buck converter
(5.5V–65V to 5V at 2A)
www.linear.com/LT8620
• LTC3607: Dual monolithic
synchronous buck regulator
(4.5V–15V to 1.8V, 600m A & 3.3V,
600m A) www.linear.com/LTC3607
• LTC3622: Dual monolithic synchronous
buck regulator (5V–17V to 3.3V, 1A & 5V,
1A) www.linear.com/LTC3622
• LTC3875 & LTC3874: High efficiency,
4-phase buck supply with sub-milliohm
DCR sensing (4.5V–14V to 1V, 120A)
www.linear.com/LTC3875
• LTM4630: High efficiency 6-phase
80A buck regulator (11V–13V to 0.95V,
80A) www.linear.com/LTM4630
Easily model the thermal behavior of MOSFETs in
LTspice using the SOAtherm NMOS symbol.
Boost Switching Regulators
• LTC3769: High voltage 60V synchronous
boost controller (6V–55V to 48V, 1A)
www.linear.com/LTC3769
Flyback, Forward and Isolated
Controllers
• LTM8058: 2kV isolated flyback
converter with LDO post regulator
(4.3V–29V to 5.7V, 120m A & 5V, 120m A)
www.linear.com/LTM8058
LED Drivers
• LT3796-1 & LTC1541: SEPIC LED driver with
analog dimming (8V–20V to
35V string, 1A) www.linear.com/LT3796
100:1
• LT3797: Triple LED boost controller
(2.7V–40V to 3x 50V LED strings,
1A) www.linear.com/LT3797
Wireless Power
• LTC4120: Wireless power receiver
with 800m A buck battery charger
www.linear.com/LTC4120
design ideas
VOUT1
VIN
VIN
4.3V TO 29V
•
RUN
LOW
NOISE
LDO
•
2.2µF
SELECTED MODELS
To search the LTspice library for
a particular device model, choose
Component from the Edit menu or press
F2. LTspice is updated often with new
models, so be sure to keep your installation of LTspice current by choosing
Sync Release from the Tools menu.
• LTC3807: Low IQ, synchronous step-down
controller with 24V output voltage
capability www.linear.com/LTC3807
• LTM4634: Triple output
5A /5A /4A step-down DC/DC µModule
regulator www.linear.com/LTM4634
ADJ2
VOUT2
5V
22µF
BYP
162k
VOUT–
GND
Demonstration circuit is now
available for LTM8058 2kV
isolated flyback converter
with LDO post regulator
VOUT1
5.7V
VOUT2
10µF
BIAS
4.7µF
6.19k
SS
ADJ1
LTM8058
2kVAC ISOLATION
Buck-Boost Switching Regulators
Current Sense Amplifiers
• LTC3114-1: 40V, 1A synchronous
• LT6119: Current sense amplifier,
buck-boost DC/DC converter with
programmable output current
www.linear.com/LTC3114-1
Flyback, Forward and Isolated
Controllers
• LT8310: 100V input forward converter
reference and comparators with
POR www.linear.com/LT6119-1
Hot Swap Controllers
• LTC4231: Micropower Hot Swap
controller www.linear.com/LTC4231 n
controller www.linear.com/LT8310
Power User Tip
CONNECTING THE DOTS
Sometimes the simplest things elude us. Typically, after placing components in an
LTspice schematic you select draw wires (F3), left click to start a wire, left click again
to change direction or join, repeat until your circuit is complete and then right click
to cancel. But did you know you can draw wires through components like resistors
and the wire will automatically be cut so that the components are in series with the
wire? Drawing a wire straight through several components is an easy way to connect
components in series.
COPY AND PASTE BETWEEN SCHEMATICS
Another feature not commonly understood is how to copy and paste between
schematics using the duplicate command. To copy objects from one schematic to
another, in the source schematic, invoke the duplicate command (F6 or Ctrl + C)—the
crosshair pointer changes to the duplicate symbol, . Left-click to select the object
you want to duplicate, or select a group of objects by dragging a box around them.
Once the object or section is copied, simply click in the target schematic window (or
the tab) and click again to paste. In Windows, both schematics must be in the same
invocation of LTspice.
Happy simulations!
October 2014 : LT Journal of Analog Innovation | 23
Increasing Output Voltage and Current Range Using
Series-Connected Isolated µModule Converters
Jesus Rosales
The LTM®8057 and LTM8058 UL60950recognized 2kV AC isolated µModule
converters are used here to demonstrate
this design approach, which can also
be applied to the LTM8046, LTM8047
and LTM8048. Let’s assume an output of 10V at 300m A is desired from a
20V input. Reviewing the maximum
output current curve from Figure 1, we
VIN
5V TO 28V
GND
+
CIN1
10µF
35V
C4
4.7µF
50V
C6
0.1µF
25V
R2
6.98k
1%
Figure 2. Two LTM8057 µModule
regulators with outputs connected
in series, supporting a 10V, 300mA
output application from a 20V input
24 | October 2014 : LT Journal of Analog Innovation
However, upon noticing that a single
LTM8057 can deliver 300m A at 5V from
a 20V input, a solution becomes apparent. Since the output voltage is isolated
from the input, the outputs of two
VIN
RUN
C1
10µF
50V
C3
10µF
50V
notice that a single LTM8057 is insufficient to meet the output current requirement under these conditions.
C8
4.7µF
50V
C7
0.1µF
25V
R4
6.98k
1%
VOUT
VOUT–
BIAS
VOUT
C210V
22µF
16V
450
400
OUTPUT CURRENT (mA)
Linear Technology’s isolated µModule® converters
are compact solutions for breaking ground loops.
These converters employ a flyback architecture whose
maximum output current varies with input voltage and
output voltage. Although their output voltage range
is limited to a maximum of 12V, one can increase the
output voltage or the output current range. The solution
simply involves connecting the secondary side of two
or more isolated µModule converters in series.
350
300
250
200
150
100
50
0
2.5V
3.3V
5V
7.5V
10V
12V
4 6 8 10 12 14 16 18 20 22 23 24 28 30
VIN (V)
Figure 1. Typical maximum output current vs input
voltage
LTM8057s set at 5V can be connected
in series to achieve a 10V output at
300m A (Figure 2).
The same circuit in Figure 2 can also
be used to increase the output voltage
range when more than 12V is needed. By
adjusting the feedback resistors to provide a 7.5V nominal output voltage, the
combined output voltage has increased
to 15V. The output current capability for
U1
LTM8057EY
SS
ADJ
GND
VOUT
VIN
RUN
BIAS
VOUT
–
C5
22µF
16V
RTN
U2
LTM8057
SS
ADJ
GND
Figure 3. Two LTM8057 µModule regulators with outputs connected in
series deliver more than 160mA at 15V output, 12V input.
design ideas
The output capabilities of isolated μModule converters can be increased
by adding one or more isolated µModule converters with the outputs
tied in series while preserving the output noise characteristics.
the 15V is the same as that of the individual 7.5V µModule regulator (Figure 3).
VIN
5V TO 28V
GND
The circuit shown in Figure 2 supports a
third option: providing positive and negative outputs with a common return. The
return node for both outputs is the common connection in the middle of the output stack. With this approach the circuit in
Figure 2 would have 5V and –5V outputs.
Each output can be of different magnitude, since the output voltages for each
converter are determined independently.
C1
10µF
50V
1210
C6
0.1µF
25V
R5
5.9k
1%
VOUT1
VIN
RUN
C14
4.7µF
50V
C9
0.1µF
25V
R7
5.9k
1%
C2
22µF
16V
VOUT–
BIAS
U1
LTM8058
VOUT2
SS
C5
0.01µF
25V
BYP
ADJ1
GND
ADJ2
C3
10µF
16V
C8
22µF
16V
VOUT–
BIAS
VOUT2
10V
R4
162k
1%
VOUT1
VIN
RUN
C7
10µF
50V
U1
LTM8058
SS
VOUT2
C15
0.01µF
25V
BYP
ADJ1
GND
ADJ2
R10
162k
1%
C16
10µF
16V
RTN
Figure 4. Two LTM8058 µModule regulators
connected with VOUT2 in series for 10V output
0
0
–20
–20
–40
–40
INTENSITY (dBm)
INTENSITY (dBm)
Linear Technology’s isolated μModule
converters provide a simple and compact solution for isolated power at
regulated output voltages. The LTM8057
and LTM8058 solutions shown here successfully demonstrate that the output
capabilities of isolated μModule converters can be increased by adding one or
more isolated μModule converters with
the outputs tied in series while preserving the output noise characteristics. n
CIN1
10µF
35V
C4
4.7µF
50V
LOW OUTPUT NOISE SERIESCONNECTED CONVERTERS
The low output spectrum noise advantage of the LTM8058 with its integrated
LDO post regulator can be maintained with
series-connected outputs. Figure 4 shows
the schematic for two LTM8058s with
VOUT2, the output of the LDO connected
in series for 10V output. Figures 5 and 6,
respectively, show the output noise spectrum of the LTM8058 under a 100m A load
at 10V with the LDO outputs connected
in series (Figure 4 schematic) and the
flyback outputs connected in series.
+
–60
–80
–80
–100
–100
–120
–60
0 0.5 1
1.5 2 2.5 3 3.5 4
FREQUENCY (MHz)
4.5
5
Figure 5. Noise spectrum for two LTM8058s with the
LDO outputs connected in series under a 100mA,
10V output load
–120
0 0.5 1
1.5 2 2.5 3 3.5 4
FREQUENCY (MHz)
4.5
5
Figure 6. Noise spectrum for two LTM8058s with the
flyback outputs connected in series under a 100mA,
10V output load
October 2014 : LT Journal of Analog Innovation | 25
12V/100A Hot Swap Design for Server Farms
Dan Eddleman
As data centers servicing the cloud grow in speed and capacity, backplane supplies
are called on to deliver currents that push the performance boundaries of Hot Swap
components. Hot Swap solutions allow boards to be inserted and removed from a live
backplane without disturbing the power distributed to other boards. A typical Hot Swap
solution uses a series MOSFET to manage the flow of power between the backplane and
the board—preventing glitches and faults from disrupting power to the rest of the system.
The challenges of designing robust Hot
Swap solutions multiply with increasing
current demands. With load currents at
100A, simply determining power dissipation requirements is no longer sufficient.
Designers must pay careful attention to
the MOSFET safe operating area (SOA) and
understand Kelvin current sensing techniques for multiple sense resistors. This
article shows how to address these issues
using the example of a 12V/100A solution
based on the LTC4218 Hot Swap controller.
resistance is low during start-up (such as
might occur during an output short-circuit
fault), the LTC4218 detects this condition and turns off the series MOSFET.
During start-up, the circuit’s current
limit threshold is reduced by pulling
the LTC4218’s ISET pin low through R4
until the PG signal transitions high. R4’s
RSENSE RESISTOR ARRAY
8x 1mΩ PARALLEL SENSE RESISTORS
8x PAIRS OF 1Ω RESISTORS
RSENSE8
1mΩ
12V/100A HOT SWAP DESIGN
Figure 1 shows the LTC4218 Hot
Swap controller managing power
to a board that contains up to
1000µ F of bypass capacitance, draws
up to 100A of load current and is hot
plugged into a 12V backplane supply.
Supporting the 100A load current without excessive power dissipation in the
MOSFETs M1 and M2 requires that the
PG (power good) signal disable the
load until the output is fully powered.
Typically, this is implemented by controlling the RESET signal of downstream
circuitry with the Hot Swap controller’s
PG signal. In the circuit of Figure 1, if the
effective load resistance is greater than
10Ω during start-up (while PG is low), the
output powers up normally. If the output
26 | October 2014 : LT Journal of Analog Innovation
3k resistance lowers the current limit
threshold to roughly 13% of the normal
operating current limit. Any fault conditions that sink extra current beyond that
level during start-up cause the TIMER to
activate and shut off the MOSFET. (The
relatively small components M3, M4, R6,
R7, and C4 work together to effectively
M2
IN
RSP8
1Ω
OUT
RSM8
1Ω
RSENSE1
1mΩ
LOAD
M1
ENABLE/RESET
RSP1
1Ω
10Ω
RSM1
1Ω
10Ω
C6
1000µF
150k
R8
1k
100k
SENSE–
C1
12nF
GATE SOURCE
SENSE+
FB
1µF
VDD
M1, M2: IPT004N03L
M3, M4, M5: 2N7002
R1
187k
TIMER
UV
FLT
R2
3.65k
IMON
R5
20k
C2
0.1µF
PG
ISET
R4
3k
C4
1nF
LTC4218GN
OV
R3
20k
Figure 1. 12V/100A Hot
Swap solution
20k
M4
GND
INTVCC
R6
20k
C4
1nF
M3
R7
20k
ENABLE/RESET
M5
design ideas
IN
PROPER KELVIN SENSING WITH
MULTIPLE SENSE RESISTORS
M1
RSENSE
OUT
SENSE−
SENSE+
R1
LTC4218
C
Figure 2. Kelvin sensing with single
sense resistor
connect R4’s 3k resistance between ISET and
ground when the PG pin is pulling low.)
The output ramp rate during start-up is set
by the LTC4218’s 24µ A pull-up current into
C1 and the gates of MOSFETs M1 and M2.
The result is an output ramp rate of 2V/ms.
Because the load circuitry is disabled by
the PG signal, the current at start-up is
dedicated to charging the capacitance
downstream of the Hot Swap circuit,
represented by C6 in Figure 1. Ramping
the 1000µ F of capacitance at 2V/ms
requires 1000µ F • (2V/ms)=2A of current.
This is far below the start-up current
limit threshold set by R4 at 16A or 13% of
the normal operating current limit. This
allows plenty of margin for inaccuracies
in the current sensing. Exceeding this current limit threshold for even a short time
during start-up indicates a fault condition
at the output, and the LTC4218 responds
by turning off MOSFETs M1 and M2.
MOSFET SAFE OPERATING AREA
In this application, the entire SOA can be
satisfied by M1 or M2 alone. It is unwise
to assume that current and SOA share
equally among MOSFETs during start-up or
output overload faults that cause significant drain to source voltages across the
MOSFETs. Either MOSFET should be able to
support the entire SOA of the application.
On the other hand, when the MOSFET is
fully enhanced during normal operation,
its behavior is similar to a resistor and it
is safe to assume that current shares more
equally. In this application, two MOSFETs
are used to reduce the power dissipation
during normal operation, not to satisfy
transient safe operation area requirements.
At 100A, the power dissipated by a single
1mΩ MOSFET is I2R = (100A)2 • 1mΩ = 10W.
If the current shares equally at 50A,
the power in each MOSFET is a more
reasonable I2R = (50A)2 • 1mΩ = 2.5W.
At these current levels, properly monitoring the voltage across the sense
resistance can be challenging. With the
LTC4218’s 15mV current sense threshold, a 100A current limit requires
less than 0.15mΩ of sense resistance,
usually achieved using parallel resistors in a Kelvin sensing scheme.
When a single sense resistor is used in Hot
Swap (or other current sense) applications,
it is common practice to use separate low
current Kelvin traces between the sense
pins of the IC and the sense resistor. An
example layout of Kelvin connections
to a current sense resistor is shown in
Figure 2. The low current Kelvin sense
paths directly between the sense resistor and the LTC4218 SENSE+ and SENSE−
pins eliminate errors due to the voltage
drops that occur when high currents
pass through the resistive PCB copper.
In this 100A application, though, it is
necessary to implement the sense resistance with multiple parallel sense resistors. Eight 1mΩ resistors in parallel is a
reasonable choice, as it results in a typical
Figure 3. Kelvin sensing layout for eight parallel resistors uses top and bottom of board
(top)
(bottom)
IN
IN
RSENSE8
RSENSE4
RSP4
SENSE
+
RSM4
SENSE−
M2
RSM3
RSP7
RSM2
RSENSE1
RSP1
RSM1
RSM8
SENSE+
RSM7
RSENSE6
RSENSE2
RSP2
−
RSENSE7
RSENSE3
RSP3
RSP8
SENSE
M1
RSP6
RSM6
RSENSE5
RSP5
RSM5
October 2014 : LT Journal of Analog Innovation | 27
Modern servers utilize load currents that bring new challenges to Hot Swap
design. Two areas of concern are MOSFET safe operating area, and the Kelvin
sensing techniques for multiple sense resistors. The 12V/100A LTC4218 Hot Swap
controller solution shown here specifically addresses these design points.
current limit of 8 • (15mV/1mΩ) = 120A,
providing a comfortable margin above
the 100A delivered to the load.
Nevertheless, multiplying the number
of sense resistors multiplies the layout
challenges; the straightforward layout
shown for a single resistor in Figure 2
no longer suffices. Current rarely shares
equally among the sense resistors—it is
not unusual to see a 50% difference in
current between several low value sense
resistors in high current applications. The
resistors placed more closely to MOSFETs
M1 and M2 conduct a greater proportion
of the load current than the sense resistors placed farther away, due to the finite
resistance of the PC board copper planes
showing up in series with the sense resistors. If possible, the preferred layout is
to place an equal number of sense resistors on the top and the bottom of the
PC board. This minimizes the parasitic
voltage drops caused by the lateral current
flow through the copper planes required
to reach the farthest sense resistor.
Even with an optimal PC board layout,
it is necessary to use a resistor network
to average the voltages sensed across
the individual 1mΩ resistors. In this
12V/100A application, the SENSE+ and
SENSE− pins of the LTC4218 are joined
to the eight 1mΩ sense resistors with
an array of 1Ω resistors as shown in
Figure 1. The resulting voltage between
the SENSE+ and SENSE− pins is the average of all of the voltages across the
1mΩ sense resistors, effectively Kelvin
sensing the eight 1mΩ resistors. An
example layout is shown in Figure 3.
28 | October 2014 : LT Journal of Analog Innovation
LAB RESULTS
Of course, calculations and circuit simulations are no substitute for benchtop
testing, especially when working with
high current Hot Swap solutions. Figure 4
shows an oscilloscope waveform of this
design starting up into a 100Ω resistor followed by a 100A load step after
the ENABLE/RESET signal transitions
high. Note that the ENABLE/RESET in this
setup drives the 4V ON signal of an electronic load box rather than the 12V level
from M5 and R10 shown in Figure 1.
The waveform in Figure 4 is typical
of proper operation when no faults
are present. The 12V input supply
ramps up first. Then, the LTC4218
charges the 1000µ F output capacitor at
2V/ms. Finally, the 100A load turns on
when the ENABLE/RESET output transitions high, signaling that the MOSFETs
M1 and M2 are fully enhanced.
VIN
5V/DIV
VOUT
5V/DIV
VIN
VOUT
Figure 5 shows the LTC4218 turning off
MOSFETs M1 and M2 when a short circuit
occurs on the output. 100ms after the
input voltage rises, the circuit begins to
charge the output node. The LTC4218 limits the charging current to the 16A start-up
current limit threshold and quickly detects
the short-circuit. This solution responds
properly and shuts off power to the load
to avoid any disruption (and damage)
to other components in the system.
CONCLUSION
Over the years, the designers of Hot
Swap solutions have had to continually
address fresh challenges posed by ever
increasing supply currents. Some issues
are not new, such as the power dissipation requirements that result from high
current, but today’s current levels have
pushed some new design issues to the fore,
such as MOSFET safe operating area, and
the Kelvin sensing techniques for multiple
sense resistors. The 12V/100A LTC4218
Hot Swap controller solution shown here
specifically addresses these design points. n
VIN
5V/DIV
VOUT
5V/DIV
PG
5V/DIV
PG
5V/DIV
IIN
50A/DIV
IIN
10A/DIV
20µs/DIV
Figure 4. Normal start-up
VIN
VOUT
20µs/DIV
Figure 5. Start-up into a short circuit
design ideas
Compensate for Wire Drop to a Remote Load
Philip Karantzalis
A common problem in power distribution systems is degradation of regulation due to the
wire voltage drop between the regulator and the load. Any increase in wire resistance,
cable length or load current increases the voltage drop over the distribution wire, increasing
the difference between voltage at the load and the voltage programmed by the regulator.
Remote sensing requires routing additional wires to the load. No extra wiring is required
with the LT6110 cable/wire drop compensator. This article shows how the LT6110 can
improve regulation by compensating for a wide range of regulator-to-load voltage drops.
THE LT6110 CABLE/WIRE
COMPENSATOR
Figure 1 shows a 1-wire compensation
block diagram. If the remote load circuit
does not share the regulator’s ground,
two wires are required, one to the load
and one ground return wire. The LT6110
high side amplifier senses the load current by measuring the voltage, VSENSE ,
across the sense resistor, RSENSE , and sinks
a current, IIOUT, proportional to the load
current, ILOAD. IIOUT scale factor is programmable with the RIN resistor from
10µ A to 1m A. Wire voltage drop, VDROP,
compensation is accomplished by sinking
IIOUT through the RFA feedback resistor
to increase the regulator’s output by an
amount equal to VDROP. An LT6110 cable/
Figure 1. No extra wires are required to compensate
for wire voltage drop to a remote load
COMPENSATING CABLE VOLTAGE
DROPS FOR A BUCK REGULATOR
wire voltage drop compensation design is
simple: set the IIOUT • RFA product equal
to the maximum cable/wire voltage drop.
The LT6110 includes an internal
20mΩ RSENSE suitable for load currents
up to 3A; an external RSENSE is required
for ILOAD greater than 3A. The external
RSENSE can be a sense resistor, the DC resistance of an inductor or a PCB trace resistor.
In addition to the IIOUT sink current, the
LT6110 IMON pin provides a sourcing current, IMON, to compensate current-referenced linear regulators such as the LT3080.
VIN
IN
OUT
REGULATOR
FB
The maximum 5A ILOAD through
the 140mΩ wire resistance and
25mΩ RSENSE creates an 825mV voltage
drop. To regulate the load voltage, VLOAD,
for 0A ≤ ILOAD ≤ 5A, IIOUT • RFA must equal
825mV. There are two design options:
select IIOUT and calculate the RFA resistor,
ILOAD
VREG
VFB
Figure 2 shows a complete cable/wire
voltage drop compensation system
consisting of a 3.3V, 5A buck regulator and an LT6110, which regulates the
voltage of a remote load connected
through 20 feet of 18 AWG copper
wire. The buck regulator’s 5A output
requires the use of an external RSENSE .
I+IN
RFA
+
–
+IN V+
RSENSE
20mΩ
RG
IIOUT
IOUT
IMON
LT6110
VLOAD
CLOAD
VSENSE
RIN
RFB
RWIRE
RS
REMOTE
LOAD
–IN
+ –
V–
October 2014 : LT Journal of Analog Innovation | 29
For precise load regulation, an accurate estimate of the resistance between the power
source and load is required. If RWIRE, RSENSE and the resistance of the cable connectors
and PCB traces in series with the wire are accurately estimated, the LT6110 can
compensate for a wide range of voltage drops to a high degree of precision.
VIN
5V TO 40V
10µF
VIN
OUT
EN
BOOST
SS
SW
LT3976
100k
PDS540
2Ω 100µF
VREG
10k
470pF
RT
0.01µF
VISHAY
IHLP4040DZE
6.8µH
0.47µF
FB
SYNC GND
VFB
1.197V
180pF
340k
200k
8
1
+IN
NC
2
7
EN
V+
LT6110
3
6
IMON
RS
4
GND
–IN
5
1.5k
0.1µF
VISHAY
VSL2512R0250F
RWIRE
140mΩ
20 FT, 18AWG
VLOAD
3.3V
220µF LOAD 5A
Figure 2. Example of a high current remote load regulation: a 3.3V, 5A buck regulator with LT6110 cable/wire voltage drop compensation
or design the regulator’s feedback resistors for very low current and calculate
the RIN resistor to set IIOUT. Typically
IIOUT is set to 100µ A (the IIOUT error
is ±1% from 30µ A to 300µ A). In the
Figure 2 circuit the feedback path current is 6µ A (VFB /200k), the RFA resistor
is 10k and the RIN resistor must be calculated to set IIOUT • RFA = 825mV.
IIOUT = VSENSE/RIN
IIOUT • RFA = VDROP
and
RIN = RFA •
RSENSE
RSENSE • R WIRE
so for RFA = 10k, RSENSE = 25mΩ and
RWIRE = 140mΩ, RIN = 1.5k.
Without cable/wire drop compensation
the maximum change in load voltage,
∆VLOAD, is 700mV (5 • 140mΩ), or an error
of 21.2% for a 3.3V output. The LT6110
reduces ∆VLOAD to only 50mV at 25°C, or an
error of 1.5%. This is an order of magnitude improvement in load regulation.
30 | October 2014 : LT Journal of Analog Innovation
PRECISION LOAD REGULATION
CONCLUSION
A modest improvement in load regulation with the LT6110 only requires a
moderately accurate RWIRE estimation.
The load regulation error is the product
of two errors: error due to the wire/cable
resistance and error due to the LT6110
compensation circuit. For example, using
the Figure 2 circuit, even if the RSENSE and
RWIRE calculation error is 25%, the LT6110
still reduces VLOAD error to 6.25%.
The LT6110 cable/wire voltage drop
compensator improves the voltage regulation of remote loads, where high current,
long cable runs and resistance would
otherwise significantly affect regulation.
Accurate regulation can be achieved
without adding sense wires, buying Kelvin
resistors, using more copper or implementing point-of-load regulators—common drawbacks of other solutions. In
contrast, compensator solutions require
little space while minimizing design
complexity and component costs. n
For precise load regulation, an accurate
estimate of the resistance between the
power source and load is required. If
RWIRE ,RSENSE and the resistance of the
cable connectors and PCB traces in series
with the wire are accurately estimated, the
LT6110 can compensate for a wide range of
voltage drops to a high degree of precision.
Using the LT6110, an accurate
RWIRE estimation and a precision RSENSE ,
the ∆VLOAD compensation error can be
reduced to match the regulator’s voltage error over any length of wire.
new product briefs
New Product Briefs
USB µMODULE ISOLATOR WITH
POWER PROTECTS HUBS &
PERIPHERAL PORTS
The LTM2884 is a USB µModule isolator that combines USB data communications and USB power, and guards
against ground-to-ground differentials
and large common mode transients. The
rugged interface and isolation makes the
LTM2884 ideal for systems implementing
USB in harsh industrial or medical environments where protection from ground
differences is required. The LTM2884
separates grounds by isolating a pair
of USB signal transceivers using internal
inductive signal isolation, providing
2500VRMS isolation plus superior common
mode transient rejection of >30kV/µs.
The LTM2884 features an integrated, low
EMI, external or bus powered, DC/DC converter that powers the isolated transceiver
and provides up to 2.5W of isolated power
for USB peripherals or hub/host controllers.
The unique automatic bus speed detection
feature allows the LTM2884 to be used
in hub/host/bus isolation applications.
Integrated downstream facing pull-down
resistors and upstream facing pull-up
resistors are automatically configured
to match the speed of the downstream
device, enabling the LTM2884 to monitor
and report bus speeds to the host. With
2500VRMS of galvanic isolation, integrated
power and USB 2.0 compatible transceivers, the LTM2884 requires no external
components and is a complete µModule
solution for isolated USB communications.
The LTM2884 is suitable for a wide
range of applications, including host,
hub and peripheral device isolation,
as well as USB inline bus isolation. The
±15kV ESD-protected transceivers operate at USB 2.0 full speed (12Mbps) and
low speed (1.5Mbps). A suspend mode
monitors for inactivity and reduces
VBUS current to less than 2.0m A. The
integrated DC/DC converter is capable of
supplying 2.5W when connected to an
external high voltage supply or 1W when
connected to the USB VBUS supply.
The LTM2884 is available in a compact
15mm × 15mm × 5mm surface mount
BGA package; all integrated circuits
and passive components are housed in
this RoHS-compliant µModule package. The LTM2884 is available in commercial, industrial and automotive
versions, supporting operating temperature ranges from 0°C to 70°C, −40°C
to 85°C and −40°C to 105°C respectively.
16-CHANNEL, 16-BIT ±10V SOFTSPAN
DAC DRIVES 10mA & 1000pF LOADS
The LTC2668-16 is a 16-channel, 16-bit
voltage output digital-to-analog converter
(DAC) with SoftSpan™ outputs, each of
which can be independently configured for
one of five selectable unipolar and bipolar
output ranges up to ±10V. Each rail-to-rail
DAC output is capable of sourcing or sinking 10m A with guaranteed load regulation
and is stable driving capacitive loads as
large as 1000pF. This makes the LTC2668
ideal for driving a variety of demanding loads in applications such as optical
modules, programmable logic controllers
(PLCs), MRI and X-ray imaging, automatic
test equipment, laser etch equipment,
spectrum analyzers and oscilloscopes.
The LTC2668 offers many space-saving
features in a compact 6mm × 6mm
QFN package, nearly 50% smaller footprint than alternative 16-channel DACs.
The LTC2668 can be operated from a
single 5V supply, or from dual bipolar
supplies depending on the output voltage
range requirement. The device includes
a precision 2.5V 10ppm/°C max reference to generate the five SoftSpan output
ranges, or it can be driven with an external
reference. A convenient 16:1 high voltage analog multiplexer enables the user
to monitor circuit integrity or perform
in-circuit calibration, saving significant
board real estate. The LTC2668 supports
an A/B toggle function for generating
an AC bias or for applying dither to a
system. Configuration of the LTC2668
is handled via a SPI-compatible serial
interface which can be powered from an
independent 1.8V to 5V digital supply.
The LTC2668 is offered in both 16-bit
and 12-bit versions and is available
in commercial, industrial and automotive (−40°C to 125°C) temperature
grades. The DC2025A evaluation board
for the LTC2668 family is available at
www.linear.com/demo or via a local
Linear Technology sales office. The demo
board is supported by the Linduino™
firmware development system, using the
DC2026A. For more information, visit
www.linear.com/product/LTC2668 and
www.linear.com/solutions/linduino. n
October 2014 : LT Journal of Analog Innovation | 31
highlights from circuits.linear.com
VIN
12V
10µF
16V
VIN
15.3µH
SYNC
255k
SWA
UVLO
•
•
•
•
10µF
16V
OVLO/DC
RDC
10k
LT3999
15.8k
VOUT
12V
0.8A
LT3999 12V TO 12V, 10W LOW NOISE ISOLATED DC/DC CONVERTER
The LT®3999 is a monolithic, high voltage, high frequency DC/DC
transformer driver providing isolated power in a small solution footprint..
www.linear.com/solutions/5377
SWB
RT
ILIM/SS
RBIAS
28k
500kHz
0.1µF
49.9k
GND
LTC2946 BIDIRECTIONAL POWER MONITOR WITH
ENERGY AND CHARGE MONITOR IN FORWARD PATH
The LTC®2946 is a rail-to-rail system monitor that measures
current, voltage, power, charge and energy. It features an
operating range of 2.7V to 100V and includes a shunt regulator
for supplies above 100V. The current measurement common
mode range of 0V to 100V is independent of the input supply. A
12-bit ADC measures load current, input voltage and an auxiliary
external voltage. Load current and internally calculated power
are integrated over an external clock, or crystal or internal
oscillator time base for charge and energy. An accurate time
base allows the LTC2946 to provide measurement accuracy of
better than ±0.6% for charge and ±1% for power and energy.
Minimum and maximum values are stored and an overrange
alert with programmable thresholds minimizes the need for
software polling. Data is reported via a standard I2C interface.
www.linear.com/solutions/5393
RSNS
0.2Ω
VIN
2.7V TO 5.8V
ADIN
SENSE+
R1
2k
SENSE– INTVCC
VDD
ADR1
C2
0.1µF
LTC2946
R2
2k
SCL
SCL
SDAI
SDA
ALERT
GPIO3
3.3V
GND
VDD
µP
INT
GND
R4
2k
GPIO1
GPIO2
CLKIN
R3
2k
SDAO
ADR0
ACCUMULATE
3.3V
VOUT
0.5A
GP OUTPUT
CLKOUT
C3 X1
33pF
X1: ABLS-4.000MHz-B2-T
C4
33pF
POWER FOR REVERSE PATH = CODEADIN × CODEVDD TO BE PERFORMED BY µP
CA[7] = 1, SEE TABLE 3
VIN
12.5V TO 38V
CIN
22µF
INTVCC
100k
VIN
PGOOD
LTC3807
RUN
INTVCC
2.2µF
PGND
ILIM
VOUT
60.4k
4.7nF
15.4k
47pF
D1
EXTVCC
PLLIN/MODE
FREQ
TG
BOOST
MTOP
0.1µF
SW
MBOT
BG
ITH
L1
4.7µH
LTC3807 HIGH EFFICIENCY 12V OUTPUT AT
10A STEP-DOWN CONVERTER
The LTC®3807 is a high performance step-down switching regulator
DC/DC controller that drives an all N-channel synchronous
power MOSFET stage. A constant frequency current mode
architecture allows a phase-lockable frequency of up to 750kHz.
www.linear.com/solutions/5396
RSENSE
5mΩ
+
COUT
180µF
VOUT
12V
10A
10µF
SENSE+
0.1µF
TRACK/SS
SGND
SGND
SENSE–
1nF
1M
VFB
71.5k
MTOP, MBOT: RJK0452
L1 WÜRTH 7443320470
COUT: SANYO 16SVP150M
D1: DFLS1100
L, LT, LTC, LTM, Linear Technology, the Linear logo, Dust Networks, LTspice, PolyPhase and µModule are registered trademarks, and Hot Swap, Linduino, LTPoE++, LTPowerCAD, LTPowerPlay and SoftSpan are trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
© 2014 Linear Technology Corporation/Printed in U.S.A./66.7K
Linear Technology Corporation
1630 McCarthy Boulevard, Milpitas, CA 95035
(408) 432-1900
www.linear.com
Cert no. SW-COC-001530