LINEAR TECHNOLOGY
LINEAR TECHNOLOGY
LINEAR TECHNOLOGY
SEPTEMBER 2009 VOLUME XIX NUMBER 3
V
PWR
V
IN_EN
V
DD33
V
DD33
V
DD25
SDA
SCL
ALERTB
CONTROL0
CONTROL1
WDI/RESET
FAULTB00
FAULTB01
FAULTB10
FAULTB11
SHARE_CLK
V
IN_SNS
V
DACP0
V
SENSEP0
V
DACM0
V
SENSEM0
V
OUT_EN0
ASEL0
ASEL1
PWRGD
REFP
REFM
LTC2978*
PMBus
INTERFACE
TO INTEMEDIATE
BUS CONVERTER
ENABLE
V
IN
4.5V < V
IBUS
< 15V
GNDWP
V
DD33
TO µP
0.1µF
0.1µF
0.1µF
TO/FROM OTHER
CONTROLLERS
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF EIGHT CHANNELS SHOWN
V
IN
V
OUT
R20
R30
R10
RUN/SS
SGND
V
FB
GND
DC/DC
CONVERTER
LOAD
IN THIS ISSUE…
COVER ARTICLE
Power Management IC Digitally
Monitors and Controls Eight Supplies
...........................................................1
Andrew Gardner
Linear in the News… ...........................2
DESIGN FEATURES
PD Controller ICs with Integrated
Flyback or Forward Controllers
Meet Demands of 25.5W PoE+ .............6
Ryan Huff
Surge Stopper IC Simplifies Design of
Intrinsic Safety Barrier for Electronics
Destined for Hazardous Environments
...........................................................9
Murphy Pickard, Hach Co.
Consider New Precision Amplifiers for
Updated Industrial Equipment Designs
.........................................................16
Brian Black
Analog VGA Simplifies Design
and Outperforms Competing
Gain Control Methods ........................19
Walter Strifler
Power Management IC Digitally Monitors and Controls Eight Supplies
by Andrew Gardner
Introduction
Today’s high reliability systems require
complex digital power management
solutions to sequence, supervise,
monitor and margin a large number of
voltage rails. Indeed, it is not unusual
for a single application board to have
dozens of rails, each with its own
unique requirements. Typically the
power management solutions for these
systems require that several discrete
devices controlled by an FPGA or a
microcontroller are sprinkled around
the board in order to sequence, supervise, monitor and margin the power
supply array. In this scheme, signifi-
cant time is required to develop the
necessary firmware, and the tendency
to underestimate the complexity and
duration of that task is well known.
The LTC®2978 octal PMBus power
supply monitor and controller with
EEPROM offers power supply system
designers an integrated, modular
solution that reduces debugging time
and effort over microcontroller solutions. The LTC2978 can sequence
on, sequence off, monitor, supervise,
margin and trim up to eight power
supplies. Multiple LTC2978s can be
continued on page 3
Accurate Silicon Oscillator Reduces
Overall System Power Consumption
.........................................................22
Albert Huntington
Easy Multivoltage Layout with
Complete Dual and Triple Output
Point-of-Load µModule® Regulators
in 15mm × 15mm Packages...............24
Eddie Beville and Alan Chern
DESIGN IDEAS
....................................................27–40
(complete list on page 27)
New Device Cameos ...........................41
Design Tools ......................................43
Sales Offices .....................................44
Figure 1. Octal power supply controller with PMBus communication. One channel is shown.
L
, Linear Express, Linear Technology, LT, LTC, LTM, BodeCAD, Burst Mode, FilterCAD, LTspice, OPTI-LOOP, Over-TheTop, PolyPhase, SwitcherCAD, µModule and the Linear logo are registered trademarks of Linear Technology Corporation.
Adaptive Power, Bat-Track, C-Load, DirectSense, Easy Drive, FilterView, Hot Swap, LTBiCMOS, LTCMOS, LinearView,
Micropower SwitcherCAD, Multimode Dimming, No Latency ∆Σ, No Latency Delta-Sigma, No R
PanelProtect, PowerPath, PowerSOT, SafeSlot, SmartStart, SNEAK-A-BIT, SoftSpan, Stage Shedding, Super Burst, ThinSOT,
TimerBlox, Triple Mode, True Color PWM, UltraFast and VLDO are trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
, Operational Filter,
SENSE
L LINEAR IN THE NEWS
Linear in the News…
On the Road in China
For the past several years, Linear has participated in the
IIC China Conference. Traditionally, this has been an
opportunity for major electronics companies to showcase
their product capabilities in the major Chinese centers in
Beijing, Shenzhen and Shanghai. This year, for the first
time, the IIC is also holding a trade show in the remote
area of Wuhan, since this area is a growing technology
center, and Linear will participate. This follows Linear’s
participation at the IIC Conference in February/March in
Shenzhen, Beijing and Xian.
At the IIC in Wuhan on September 14–15, Linear will
focus on products for the automotive, industrial and telecom
markets. Some of the product highlights include:
q
LED drivers for a range of applications
q
µModule receiver products for cellular basestations
q
DC/DC µModule regulators, providing easy-to-
implement power solutions
q
Battery stack monitors for hybrid and electric
vehicles
At its booth, Linear will run a demo of the LTC6802
Battery Stack Monitor, showing automotive electronics
designers how to use the device to precisely monitor
every cell in long strings of series-connected lithium-ion
batteries.
Linear Debuts Isolated µModule
Transceiver with Power
Leveraging its experience in µModule technology, Linear
has just announced the first product in a new family of
galvanically isolated µModule products aimed for use in
industrial networks. The LTM®2881 is a complete isolated
RS485/RS422 solution and the first transceiver product
to utilize Linear’s isolator µModule technology, integrating
a 2500V
transceiver and all necessary power components into low
profile LGA and BGA packages. No external
components are required, eliminating issues
with sourcing transformers. In addition, the
LTM2881’s 1W DC/DC converter provides
surplus current for powering external ICs and
LEDs via a 5V regulated output. The LTM2881
exhibits high common mode transient immunity, >30kV/µs, allowing the LTM2881 to
continue communicating, rather than merely
holding a data state, through severe transient
events.
The features of the LTM2881 make it suitable for a wide range of applications, including
breaking ground loops, working with large common mode voltages and when using multiple
unterminated line taps. Integrated selectable
termination allows cables to be properly terminated to avoid signal reflections and distorted
galvanic isolation barrier, a high performance
RMS
waveforms, with the flexibility to add or remove termination
anywhere onto the bus via a software switch. Users will
appreciate how the self-powered LTM2881 takes many
precautions to guarantee safe and reliable communications in RS485 or RS422 systems.
Solar Power Battery Charger
Improves Panel Efficiency
For a given amount of light energy, a solar panel has a
certain output voltage for peak output power production.
Bypass diodes inside a panel can create complex power
versus current characteristics that are not easily optimized
when partial shading exists on the panel. However, virtually all of the 12V system solar panels currently on the
market that are specified with maximum output power
less than 25W are constructed from a simple series cell
arrangement with no bypass diodes. This type of arrangement yields peak output power within a narrow band of
panel output voltages, regardless of lighting conditions.
Peak power in excess of 95% may be produced from panel
voltages of 12.5V–18.5V, depending on the characteristics
of the panel.
ger, the LT®3652, designed to provide an elegant electrical
operating characteristic while extracting the maximum
available power from the solar panel. The LT3652 employs
a simple but innovative input voltage regulation loop,
which controls charge current to hold the input voltage at
a programmed level. This input regulation loop maintains
the panel at the output voltage corresponding to the peak
output power point for the particular solar panel used. The
specific desired peak-power voltage is programmed via a
resistor divider. This method yields charging efficiencies
virtually the same as more costly maximum peak power
tracking (MPPT) solar charging techniques.
Linear has just announced a solar power battery char-
L
2
2
Linear Technology Magazine • September 2009
DESIGN FEATURES L
LTC2978 #1
SHARE_CLK
FAULT
LTC2978 #2
SHARE_CLK
FAULT
LTC2978 #N
SHARE_CLK
FAULT
•
•
•
POWER SUPPLY
ARRAY
V
OUTn
500mV/DIV
200ms/DIVSTART UP
8 SUPPLIES
IN ANY ORDER
SHUT DOWN
8 SUPPLIES
IN ANY ORDER
INDIVIDUAL MARGINING
FOR 8 SUPPLIES
LTC2978, continued from page 1
easily cascaded using the 1-wire shareclock bus and one or more bidirectional
fault pins (Figure 1 shows a typical
application).
In addition, the LTC2978 uses a
protected block of nonvolatile memory
to record system voltage and fault
information in the event of a critical
system failure. Preserving critical
system data in nonvolatile memory
allows users to identify a failing voltage rail and isolate the cause of board
failures during system development,
test debug or failure analysis.
A free, downloadable graphical
PC interface is available to facilitate
interaction with the part in design
and testing. The LTC2978 utilizes the
industry standard PMBus command
protocol in order to simplify firmware
development. The LTC2978’s most
important feature, though, is that its
precision integrated reference and 15bit ∆Σ ADC delivers ±0.25% absolute
accuracy when measuring or adjusting
power supply voltages.
Improve Manufacturing Yields
with Precision Margin Testing
Margin testing of system voltages is
an effective means of weeding out
premature failures in high reliability
systems. Typically, voltages are margined at least ±5% in order to guarantee
Linear Technology Magazine • September 2009
Figure 3. Multiple LTC2978s can be cascaded using only two connections.
Figure 2. The LTC2978 offers flexible
sequencing and precision margining.
that the system under test is robust
enough to operate reliably in the field.
Depending on system tolerances,
however, this approach can lead to
excessive test fallout. Many of these
test rejects might have been avoided
if the tolerances of the supply voltages
in question were tighter.
With its precision reference, multiplexed 15-bit ∆Σ ADC, eight margin
DACs and integrated servo algorithm,
the LTC2978 offers a relatively easyto-use, yet powerful, solution to this
problem (see Figure 4 for the LTC2978
bock diagram). By simply writing an
I2C command to either trim or margin
to a specific voltage, the LTC2978
adjusts the DC/DC point-of-load converter within the prescribed software
and hardware limits to deliver the
commanded output voltage to ±0.25%
absolute accuracy.
The margin DAC outputs are connected to the feedback nodes or trim
inputs of the DC/DC POL converters
via a resistor. The value of this resistor
sets a limit on the range over which
the output voltage can be margined,
an important limitation for power supplies under software control. Another
significant benefit of the 10-bit margin
DACs is that they enable very fine
resolution when margining voltages.
This makes it possible to extract useful
data from failure testing, as opposed
to a trashcan full of failed, but not well
understood, boards.
Flexible Power Sequencing
and Fault Management
Many traditional power sequencing
solutions rely on comparators and
daisy-chained PCB connections. While
relatively easy to implement for a handful of supplies, this approach quickly
becomes complicated as the number
of voltage rails grows, and is relatively
inflexible in the face of specification
changes. It’s also extremely difficult
to implement turn-off sequencing with
this type of approach.
The LTC2978 makes sequencing
easy for any number of supplies. By
using a time-based algorithm, users
can dynamically sequence on and
sequence off in any order (see Figure 2). Sequencing across multiple
LTC2978s is also possible using the
1-wire share-clock bus and one or
more of the bidirectional fault pins
(see Figure 3). This approach greatly
simplifies system design because
channels can be sequenced in any
order, regardless of which LTC2978
provides control. Additional LTC2978s
can also be added later without having to worry about system constraints
such as a limited supply of daughter
card connector pins.
On sequencing can be triggered in
response to a variety of conditions. For
example, the LTC2978s can auto-sequence when the downstream DC/DC
POL converters’ intermediate bus
voltage exceeds a particular turn-on
voltage. Alternatively, on sequencing
3
L DESIGN FEATURES
15
3V REGULATOR
INTERNAL
TEMP
SENSOR
REFERENCE
1.232V
(TYP)
MASKING
CLOCK
GENERATION
OSCILLATOR
UVLO
V
DD
OPEN-DRAIN
OUTPUT
EEPROM
NONVOLATILE MEMORY
RAM
ADC_RESULTS
MONITOR LIMITS
SERVO TARGETS
PMBus
INTERFACE
(400kHz I
2
C
COMPATIBLE)
CONTROLLER
PMBus ALGORITHM
FAULT PROCESSOR
WATCHDOG
SEQUENCER
V
IN
V
DD
V
SENSEP0
V
SENSEM0
V
OUT
V
PWR
17
2.5V REGULATOR
V
IN
V
OUT
V
DD33
36
V
SENSEP0
37
V
SENSEM0
V
SENSEP1
V
SENSEM1
V
SENSEP2
V
SENSEM2
V
SENSEP3
V
SENSEM3
V
SENSEP4
V
SENSEM4
V
SENSEP5
V
SENSEM5
V
SENSEP6
V
SENSEM6
2
V
SENSEP7
3
V
SENSEM7
V
DACP0
V
DACP1
V
DACP2
V
DACP3
V
DACP4
V
DACP5
V
DACP6
V
DACP7
V
DACM0
V
DACM1
V
DACM2
V
DACM3
V
DACM4
V
DACM5
V
DACM6
V
DACM7
4
V
OUT_EN0
5
V
OUT_EN1
6
V
OUT_EN2
7
V
OUT_EN3
8
V
OUT_EN4
9
V
OUT_EN5
10
V
OUT_EN6
11
V
OUT_EN7
12
V
IN_EN
18
V
DD25
65
GND
28
SCL
27
SDA
29
ALERTB
32
ASEL0
33
ASEL1
30
CONTROL0
19
WP
31
CONTROL1
WDI/RESET
22
23
FAULTB00
24
FAULTB01
25
FAULTB10
26
FAULTB11
20
21
SHARE_CLK
PWRGD
16
V
DD33
14
V
IN_SNS
REFP
REFM
3R
R
V
SENSEP1
V
SENSEM1
V
SENSEP2
V
SENSEM2
V
SENSEP3
V
SENSEM3
V
SENSEP4
V
SENSEM4
V
SENSEP5
V
SENSEM5
V
SENSEP6
V
SENSEM6
V
SENSEP7
V
SENSEM7
15-BIT
∆∑ ADC
ADC
CLOCKS
V
DD
+
–
+
–
MUX
34
35
10-BIT
VDAC
+
–
+
+
–
–
+
–
SC
CMP0
CMP0
VBUF0
IDAC0
10 BITS
42
43
46
47
48
49
52
53
62
63
64
1
39
40
44
50
55
56
60
38
41
45
51
54
57
58
61
59
4
Figure 4. Block diagram of the LTC2978
Linear Technology Magazine • September 2009
DESIGN FEATURES L
can initiate in response to the rising- or
falling-edge of the control pin input.
Sequencing can also be initiated by a
simple I2C command. The LTC2978
supports any combination of these
conditions.
The bidirectional fault pins can
be used for various fault response
dependencies between channels.
For instance, on sequencing can be
aborted for one or more channels in the
event of short-circuit. Once a rail has
powered-up, the undervoltage supervisor function is enabled (the overvoltage
function is always enabled). The
overvoltage and undervoltage thresholds and response times of the voltage
supervisors are all programmable. In
addition, input voltage and temperature are also monitored. If any of these
quantities exceed their over- or undervalue limits, the customer can select
from a rich variety of fault responses.
Examples include immediate latchoff,
deglitched latchoff, and latchoff with
retry.
An integrated watchdog timer is
available for supervising external
microcontrollers. Two timeout intervals are available: the first watchdog
interval and subsequent intervals.
This makes it possible to specify a
longer timeout interval for the micro
just after the assertion of the power
good signal. In the event of a watchdog
fault, the LTC2978 can be configured
to reset the micro for a predetermined
amount of time before reasserting the
power good output.
Multifaceted Telemetry
The LTC2978 serves up a variety of
telemetry data in its registers. The
multiplexed, 15-bit ∆Σ ADC monitors
input and output voltages and on-chip
temperature, storing minimum and
maximum values for all voltages and
temperature readings. In addition, the
ADC inputs for odd-numbered output
channels can be reconfigured to measure sense resistor voltages. In this
mode, the ∆Σ ADC can resolve voltages
down to 15.3µV, which is invaluable
when attempting to measure current
with inductor DCR circuits.
Although the LTC2978 can be
directly powered from a 3.0V to 3.6V
supply, the ADC is capable of accepting
input voltages of up to 6V—no need
to worry about body diodes or exotic
standby supply voltages. The LTC2978
can also run off of a 4.5V to 15V input
supply using its internal regulator.
A separate high voltage (15V max)
sense input is provided for measuring the input supply voltage for the
DC/DC POL converters controlled by
the LTC2978.
Black Box Data Recorder
In the event a channel is disabled in
response to a fault, the LTC2978’s
data log can be dumped into protected
EEPROM. This 255-byte block of data
is held in NVM until it is cleared with
an I2C command. The data block
contains output and input voltages
and temperature data for the 500ms
preceding the fault as well as the corresponding minimum and maximum
values. Status register values and total
up time since the last system reset are
also stored in the log.
Figure 5 shows the data log contents viewed in the PC-based LTC2978
interface. In this way, the LTC2978
provides a complete snapshot of the
state of the power system immediately
preceding the critical fault, thus making it possible to isolate the source of
the fault well after the fact. This is an
invaluable feature for debugging both
prerelease characterization or in-field
failures in high reliability systems.
Figure 5. The LTC2978 comes with free software that allows easy data monitoring and
cofiguration. The data log shows monitor readings just before a failure for debugging analysis.
Linear Technology Magazine • September 2009
Graphical User Interface
and PMBus
Linear T echnology’s easy-to-use
PC-based graphical user interface
(GUI) allows users to configure the
LTC2978 via a USB interface and a
dongle card. The GUI, which is free
and downloadable, takes much of the
coding out of the development process
and improves time-to-market by allowing the designer to configure all
device parameters within an intuitive
framework. Once the device configura-
continued on page 18
5
L DESIGN FEATURES
100k
20Ω
3.01k
1%
27.4k
1%
10µF
39k
1µF
BAS21
1.2k
38.3k
33pF
1.5nF
10µF
100V
0.1µF
t
ON
12k
PGDLYV
NEG
SYNC
R
CLASS
SHDN
V
CMP
R
CMP
ENDLY OSC SFST
LTC4269-1
GND
UVLO
V
PORTP
V
CC
T2P
T2P
FB
C
CMP
+
+
V
PORTN
383k
1%
3.01k
1%
10k
1nF
3.3nF
•
•
•
•
33mΩ
1%
FDS2582
10k
15Ω
150Ω
PE-68386
BAT54
100Ω
2200pF
MMBT3906 MMBT3904
1µF
1µF
16V
T1
PA2369NL
SENSE
–
SENSE
+
SG
PG
L1
0.18µH
22pF
100µF
5.1Ω
•
FDS8880
47µF
5V
5A
+
SMAJ58A
30.9Ω
24k
107k
10k
S1B
B1100 s 8 PLCS
2.2µF
100V
10µH
DO1608C-103
0.1µF
100V
36V
PDZ36B
BSS63LT1
V
PORTP
48V
AUXILIARY
POWER
–54V FROM
DATA PAIR
–54V FROM
SPARE PAIR
+
–
PD Controller ICs with Integrated Flyback or Forward Controllers Meet Demands of 25.5W PoE+
Introduction
The IEEE 802.3af Power over Ethernet (PoE) standard allows a powered
device (PD), such as an internet
protocol (IP) telephone, to draw up to
12.95W from an Ethernet cable. When
the 802.3af standard was drafted,
12.95W appeared sufficient to cover
the immediately imaginable range of
PD products (primarily IP phones).
Of course, application developers
are always far more innovative than
standards committees anticipate, so
new power-hungry applications for
PoE immediately started to appear,
such as dual-radio IEEE 802.11a/g
and 802.11n wireless access points,
security cameras with pan/tilt/zoom
motors, and color LCD IP video
phones. 12.95W was suddenly not
enough. The IEEE committee responded with the 802.3at standard,
which raises the available PD power
to 25.5W. The new “at” standard, commonly referred to as PoE+, also adds
a “handshaking” communications
requirement between PDs and power
sourcing equipment (PSEs), while allowing backward compatibility with
the legacy “af” standard.
New power control ICs are required
to take advantage of these expanded
requirements. The DC/DC conversion
and control schemes used for legacy
“af” PDs are not optimized for the increased power capability and feature
requirements of PoE+. For instance,
in both standards the 37V to 57V PoE
voltage is converted to lower voltages
that digital circuitry can tolerate.
This DC/DC conversion is handled
in the lower power 12.95W standard
with a conventionally rectified (i.e.,
diode rectified) flyback converter. The
higher power 25.5W standard is better
by Ryan Huff
served by a synchronously rectified
(i.e. MOSFET rectified) flyback or a
forward power supply topology.
To meet the new performance
requirements of PoE+, including
handshaking, Linear Technology offers
a new family of PD controller ICs that
integrate a front-end PD controller with
a high performance synchronously
rectified flyback (LTC4269-1) or a
forward (LTC4269-2) power supply
controller.
Features
Both parts combine a PD controller—which includes the handshaking
circuitry, Hot Swap™ FET, and input
protection—with a DC/DC power
supply controller. While the power
supply sections of the two parts are
very different, the PD controller in
both is identical.
6
Figure 1. LTC4269-1-based synchronous flyback converter
Linear Technology Magazine • September 2009
+
T2P
T2P
V
NEG
V
PORTN
SHDN
SMAJ58A
R
CLASS
V
PORTP
PGND GND BLANK DELAY
82k
30.9Ω24k
158k 332k
133Ω
BAS516
BAS516
PA2431NL
BAS516
10k
IRF6217
FDS8880
FDS8880
5.1Ω
158k
22.1k
33k
1.5k
50mΩ
2k
5.1Ω
1.2k
TLV431A
PS2801-1-L
V
CC
11.3k
3.65k
22k
0.22µF
0.1µF
R
OSC
V
REF
FB
COMP
I
SENSE
OC
SS_MAXDC
FDS2582
SD_V
SEC
V
IN
S
OUT
LTC4269-2
0.1µF
18V
PDZ18B
10µF
16V
V
CC
33k
237k
107k
10.0k
S1B
B1100 s 8 PLCS
2.2µF
100V
+
10µF
100V
10µH
DO1608C-103
1mH
DO1608C-105
6.8µH
PG0702.682
10.0k
OUT
4.7nF
1nF
5.1Ω
1nF
0.1µF
100V
4.7nF
250V
10nF
+
220µF
6.3V
PSLVOJ227M(12)A
5V
5A
36V
PDZ36B
BSS63LT1
V
PORTP
–48V
AUXILIARY
POWER
–54V FROM
DATA PAIR
–54V FROM
BC857BF
•
•
•
EFFICIENCY (%)
LOAD CURRENT (A)
50.5
95
65
1 1.5 2 2.5 3 3.5 4 4.5
70
75
85
80
90
VIN = 42V
VIN = 50V
VIN = 57V
In the LTC4269, handshaking circuitry, also known as the “High Power
Available,” “Two Finger Detect,” or
“Ping Pong” indicator, allows the PD
to take full advantage of a new PSE’s
full 25.5W of available power. Both
parts include an integrated Hot Swap
MOSFET for a controlled power up of
the PD. The switch has a low 700mΩ
(typical) resistance and a robust 100V
max rating, thus meeting the needs of
a wide range of applications. Auxiliary
power supplies (“wall warts”) can be
accommodated by interfacing to the
SHDN pin to disable the PoE power
path. Setting a programmable classification current allows different
power leveled PDs to be recognized
by the PSE. Achieving this is as easy
as choosing the proper resistor and
placing it from the R
pin. The ICs are chock-full of protection features, including overvoltage,
undervoltage, and overtemperature to
name a few. Finally, complementary
power good indicators signal that the
CLASS
pin to V
PORTN
PD Hot Swap MOSFET is out of the
inrush limit and ready to draw full
power.
The power supply controllers of the
LTC4269s also share some features.
Both offer programmable switching
frequency, which allows the designer
to optimize the trade-off between efficiency and size, or the designer can
choose a specific frequency to meet
application specific EMI requirements. The power supply soft-start
time is also adjustable to prevent
the PSE from dropping out its power
due to excessive inrush current and
virtually eliminate any power supply
DESIGN FEATURES L
Figure 3. LTC4269-2-based self-driven synchronous forward converter
Figure 2. Efficiency of the circuit in Figure 1
Linear Technology Magazine • September 2009
7
L DESIGN FEATURES
EFFICIENCY (%)
LOAD CURRENT (A)
50.5
95
65
1 1.5 2 2.5 3 3.5 4 4.5
70
75
85
80
90
VIN = 42V
VIN = 50V
VIN = 57V
output voltage overshoot. Both parts
include short circuit protection with
automatic restart.
LTC4269-1 Synchronous
Flyback for Optimized
Combination of Efficiency,
Simplicity, Size and Cost
A synchronous flyback supply utilizing the LTC4269-1 offers the best
combination of efficiency, simplicity,
size and cost. See Figures 1 and 2 for
the schematic and efficiency curves,
respectively, for an LTC4269-1-based
PD power supply capable of a 5V output voltage at 5A.
The flyback parts count is low for a
few reasons. There is no need for the
large output inductor that a forward
converter (see Figure 3) needs, for this
function is rolled into the isolation
transformer (T1). A small, inexpensive
second-stage filter inductor (L1) is
used in the flyback in order to reduce
output voltage ripple, but it should not
be confused with a traditional output
inductor.
In the case of the LTC4269-1, neither a secondary side reference nor an
optocoupler are needed to transmit the
output voltage regulation information
across the isolation boundary. This is
because the IC uses the third (bias)
winding on the transformer, T1, to get
the output voltage information across
the boundary. Finally, the synchronous flyback topology requires half
of the switching MOSFETs (only two)
needed by the forward converter.
Performance, in terms of efficiency, tops out at above 90% for the
Figure 4. Efficiency of the circuit in Figure 3
LTC4269-1 synchronous flyback. As
a contrast, typical PoE efficiencies at
the “af” power level for a conventionally rectified flyback were in the lower
half of the 80%’s. This higher efficiency is due to the IC’s well-controlled
implementation of the synchronous
rectifier’s gate drive. This efficiency
is not attainable with an uncontrolled
self-driven synchronous rectification
scheme that is sometimes used.
Regulation over the full PoE+ input
voltage range and 0A to 5A output current range for the LTC4269-1 is better
than ±1%. Output voltage ripple for
the fundamental switching frequency
is less than 30mV peak-to-peak.
LTC4269-2
Synchronous Forward
to Maximize Efficiency
If the efficiency of a PoE+ power supply
is paramount, an LTC4269-2-based
synchronous forward supply is the answer at 92.5% efficiency. The increased
efficiency comes with the trade-off of
increased circuit size and complexity.
Figure 3 shows a complete PD power
supply. Figure 4 shows efficiency,
and Figure 5 compares the physical
size of the flyback (LTC4269-1) versus
the forward (LTC4269-2). The forward
supplies 5V at 5A.
The increase in the forward’s efficiency comes about in part from
decreased RMS currents in the secondary side MOSFETs and in part from
separating the transformer and output
inductor. Both of these changes from
the flyback reduce resistive losses.
The forward supply uses twice the
number of MOSFETs as a flyback so
each switch handles just a portion
of the current that the switches in
the flyback do, thus reducing the I2R
power losses. By separating the isolation transformer and output inductor,
instead of using the transformer for
both as in the flyback, the same power
is processed through two components
instead of one. The net effect is more
copper, thus less resistance and lower
resistive losses.
The cost of the circuit obviously
increases with the addition of larger
and more expensive power path
components. Complexity also goes
up with the need to control twice as
many MOSFETs. Also, the forward
topology does not lend itself to the
third winding feedback method. This
means extra complexity in the design
and compensation of a secondary side
reference and opto-coupler circuitry.
Other than the ultra high efficiency
of the LTC4269-2’s synchronous
forward, the solution has similar performance to the flyback. The output
ripple of the fundamental switching
frequency is about 40mV peak-topeak. The regulation over the entire
input voltage and load current range
is well under ±1%.
8
Figure 5. LTC4269-1 and -2 solutions
Conclusion
Two new highly integrated PD controller ICs are fully compliant with, and
take full advantage of, the upcoming
IEEE 802.3at PoE+ standard. The
LTC4269 family of parts support the
preferred high performance power
supply topologies for use in the new
standard.
Linear Technology Magazine • September 2009
L
DESIGN FEATURES L
IGNITION SOURCE
THERMAL OR
ELECTRIC
OXIDIZER
AIR OR
OXYGEN
FUEL
GAS, VAPOR
OR POWDER
COMBUSTION
Surge Stopper IC Simplifies Design of
Intrinsic Safety Barrier for Electronics
Destined for Hazardous Environments
by Murphy Pickard, Hach Co.
Introduction
As applications for electronic instrumentation proliferate, an increasing
number of applications require equipment safe enough to operate in
hazardous environments. Chemical
plants, refineries, oil/gas wells, coal,
and textile operations are all examples
of potentially explosive environments
that use electronic instrumentation.
In order to operate safely in such environments, instrumentation must be
made explosion proof.
Companies that supply apparatus to these markets must integrate
protection into the design. It falls to
the electronic designer to consider
available safety measures and implement them with minimum cost and
impact on proper circuit operation.
This is a daunting task from a design
standpoint, made even more difficult
by the number of hazardous environment standards that must be met to
satisfy global or domestic markets.
Although the various standards are
moving slowly to harmonization, in
some cases they still contradict themselves and each other.
This article discusses the essential
requirements of safety standards, and
methodologies for meeting these re-
Table 1. Established protection techniques
‘Ex’ Designation Technique Description Application
‘p’ Separation: Gas Pressurization Equipment Rooms
‘o’ Separation: Liquid Oil Fill Transformers
‘q’ Separation: Semi-Solid Sand Fill Instrumentation
‘m’ Separation: Solid Encapsulation Instrumentation
‘n’ Construction Nonincendive Switchgear
‘e’ Construction Increased Safety Lighting, Motors
‘d’ Containment Flameproof Pumps
‘i’ Electrical Design Intrinsic Safety Instrumentation
Figure 1. The ignition triangle
LT4356 series surge stopper
IC can be used to design
an active barrier with
parameters that can be
easily altered to quickly
produce custom barriers.
Since the fundamental
circuit topology won’t
be changing much, once
such an active design is
approved, it will be more
readily approved when only
component value changes
are made.
About the Author
Murphy Pickard is an Electronic
Engineer in the Flow & Sampling
Business Unit of Hach Company
(www.hach.com) of Loveland, CO.
If you have questions about this
article or intrinsic safety bar rier design, feel free to contact
the author at 800-227-4224 or
mpickard@hach.com.
quirements. In particular, the LT4356
series of overvoltage/overcurrent protection devices offers an efficient and
elegant means of creating protection
barriers in electronic apparatus. To
fully understand the requirements
and solutions, one must become moderately acquainted with the standards
themselves, and the agencies that
enforce them.
Intrinsic Safety and the
Classification of Hazardous
Environments
Simply put, in a hazardous environment, the designer’s task is to prevent
an ignition source from meeting an
explosive atmosphere. There are several techniques for achieving this end,
and this article focuses on a design
discipline referred to as intrinsically
safe (IS) design. Figure 1 depicts the
ignition triangle, illustrating that a
fuel, an oxidizer and an ignition source
must all be present for an explosion to
occur. Several techniques simply prevent an existing ignition source from
contacting an explosive atmosphere,
while Intrinsically Safe design actually
eliminates the ignition source. The
principal protection techniques are
listed in Table 1.
Separation techniques are well
suited for many applications but
require special sealing methods and
Linear Technology Magazine • September 2009
9
L DESIGN FEATURES
substances, often creating a permanent barrier, making repair or service
impossible. Construction techniques
are mechanical approaches, and again
require special materials.
Only the Intrinsic Safety technique
allows normal instrument fabrication
methods and materials and requires
no exotic construction or packaging. Additionally, IS circuits may be
serviced with power present, and are
generally the lowest cost approach
to gaining certification. Further, only
IS certified equipment is allowed in
ATEX Zone 0 areas (Directive 94/9/
EC ATEX “Atmosphères Explosibles”).
This is true because the instrument
design ensures that there is not
enough electrical (spark) or thermal
energy present to serve as an ignition
source. Specifically, an Intrinsically
Safe circuit is one in which any spark
or any thermal effect produced in the
conditions specified in the principal
Standard (IEC 60079-2006), which
includes normal operation and specified fault conditions, is not capable of
causing ignition of a given explosive
gas atmosphere.
Several bodies oversee compliance
to standards and issue certifications
to manufacturers. In North America
FM, UL and CSA govern IEC-79 series
standard certification, while ATEX
standard compliance in the European
Union is certified principally by DEMKO. The level of protection required
depends on the environment in which
the instrument will operate. International Standards and Codes of Practice
classify environments according to
the risk of explosion. The type and
the volatility of the gas/vapor/dust
present and the likelihood of its presence determine such risk. Depending
on the jurisdiction, the classification
system is by Class/Division (North
America) or Zone (EU). These systems
are generally compatible, and for the
purposes of this article, we concentrate
on the Class/Division system as many
countries have adopted IEC79 series
Standards, the most fully utilized and
harmonized of all standards extant.
When electrical equipment and
flammable materials are present simultaneously, both the equipment and
Table 2. Hazardous environment classification systems
Class Hazard
I Gas/Vapor
II Dust
III Particles/Fibers/Filings
Division
(North America)
1 Likely
2 Unlikely 2 Unlikely
Gas Group Industry
I Underground
II Surface
Apparatus Group Representative Gas
IIA Propane
IIB Ethylene
IIC Hydrogen
Temperature Code Maximum Surface Temperature °C (40°C Ambient)
T1 450
T2 300
T3 200
T4 135
T5 100
T6 85
Presence
explosive atmospheres must be classified. The level of protection provided
must be the same or better than that
required by the standards for use in
such environment. The environment,
or “plant,” is classified according to the
type (Class and Group) and probability
of presence (Division) of the explosive
atmosphere. The equipment is classified according to the maximum surface
temperature (Temperature Code) of
any component of the equipment exposed to the hazardous atmosphere,
and by the maximum amount of energy
(Apparatus Group) it can produce or
release in a spark event. It is important
to understand that there is no relationship between the surface temperature
and the spark ignition energy necessary to ignite a given gas. These limits
Zone
(Europe)
0 Continually
1 Likely
Presence
The Role of Electronic Design
in Intrinsic Safety
An IS circuit is defined in Standard
IEC79-11 as:
“A circuit in which any spark or
thermal effect produced in the condition specified in this International
Standard, which include normal operation and specified fault conditions,
is not capable of causing ignition in a
given explosive gas atmosphere.”
Thus, a circuit must contain safety
components that prevent spark or heat
energy of a sufficient level to cause an
explosion under fault conditions. It
is the responsibility of the circuit designer to incorporate these protective
components into the design while still
maintaining proper circuit operation.
This is seldom an easy task.
are summarized in Table 2.
10
Linear Technology Magazine • September 2009
Any device designed for use in
R
V
OC
I
SC
INTRINSICALLY
SAFE
EQUIPMENT
HAZARDOUS AREA NON-HAZARDOUS AREA
APPROVED APPROVED
INTRINSIC
SAFETY
BARRIER
CONTROL
EQUIPMENT
ROOM
hazardous environments may be
categorized as either a simple or nonsimple apparatus. Without going into
detail, a simple apparatus requires no
agency certification if it contains passive components, does not generate or
store significant energy greater than
1.5V, 100mA, and 25mW. Examples of
simple apparatus are resistors, diodes,
LEDs, photocells, thermocouples,
switches, terminal blocks and the like.
For obvious reasons we will not dwell
on this class of equipment.
A non-simple IS apparatus, with
which electronic instrument designers are concerned, are categorized as
either “Ex ib,” which may have one
countable fault, and “Ex ia,” which may
have two countable faults. Countable
faults refer to arbitrary faults imposed
by the examiner to analyze efficacy of
protection against thermal and spark
ignition faults. A non-countable fault
occurs not from component failures,
but from circuit spacing issues such
as creepage/clearance, improper
component voltage/current/power
rating or component construction. It
is the designer’s job to ensure that
his component selection and circuit
layout do not contain any non-countable faults or he may fail certification
from these alone.
During the compliance examination
the assessor is allowed to fail one (Ex
ib) or two (Ex ia) protective components and explore the implications for
safety of these failures. If these failures
do not degrade the circuit’s safety
features, the apparatus is awarded
a hazardous location certification.
Referring to Table 2, a certification
to Class I, Division 1, Group IIC, T6
allows operation in any hazardous
environment, including ATEX Zone 0
Linear Technology Magazine • September 2009
Figure 2. Isolation/protective barrier location
areas. Clearly, Ex ia is the most difficult certification to obtain, and the
manufacturer should determine that
he must have this level of protection
before incurring the cost of doing so.
Most applications require only Class
I/Div 1 or 2 (Zone 1) certification.
The Barrier Concept
A barrier that limits power/voltage/
current to safe levels for the particular environment must moderate
any power or signaling flow between
a hazardous location and a nonhazardous location. Such a barrier
is termed an Associated Apparatus
in the Standards. It is important to
realize that an IS barrier, containing
protective components, resides in the
non-hazardous area and supplies
power to the IS certified apparatus in
the hazardous area, including Simple
Apparatus. Both pieces of equipment
must comply with IS rules. That is
to say that for an Ex ia certification,
both units must be approved to suffer
double faults while maintaining safety
from ignition as Figure 2 illustrates.
Proper or merchantable operation
of the apparatus is irrelevant to the
examiner, as long as it is safe.
The concept of a barrier is a powerful
tool in gaining compliance. It is clear
that the non-hazardous area barrier
in Figure 2 must limit the total power
available to the IS apparatus in the
hazardous area. However, multiple
barriers may also exist within the
Figure 3. Simple passive component barrier
DESIGN FEATURES L
hazardous area apparatus. Internal
barriers may be used to further limit
power to sub-circuits within the equipment to prevent application of multiple
countable faults.
In the broadest terms, protective
components are either series type or
shunt type. A current-limiting resistor
is the most common series protective
device, while a voltage-limiting Zener
diode is the most common shunt
protective device. When used in combinations to limit power, protective
devices are referred to as barriers.
Barriers in which true galvanic isolation is maintained are referred to as
“isolators.” Examples of isolators are
transformers, capacitive couplers and
optical couplers. Isolators however will
not provide DC power or transfer DC
signals and are not germane to this
discussion. We will not delve into the
use of resistors or diodes to isolate
energy-storing components to provide
spark ignition protection, but this is
provided for in the Standards and
is a different concept from galvanic
isolators.
Safety Components
and Barrier Design
Barriers can be categorized as either
passive or active according to the
components used to design them.
Passive barriers have the advantage
of conceptual simplicity, ease of
design and ready availability in the
market. However, the protected field
apparatus must suffer the voltage
burden imposed by the barrier and
still function properly. Passive barriers
are energy inefficient and bulky. If any
significant power must be transferred
to the field device beyond a few milliwatts, the safety components become
very large.
Active barriers have a tremendous
advantage in efficiency and component
size, but are generally more difficult to
design and may be more expensive to
produce. Additionally, these are typically custom designs that are not easily
reused. The most serious disadvantage
of active barriers is not conceptual,
but bureaucratic. The examiners
who analyze the barrier design are
completely familiar with common pas-
11
L DESIGN FEATURES
SHORT CIRCUIT CURRENT (mA)
OPEN CIRCUIT VOLTAGE (V)
1k10
10k
10
100
100
1k
Grp-A&B: Acetylene,Hydrogen
Grp-C: Ethylene
Grp-D: Propane
Grp-D: Methane
–
+
–
+
V
CC
SHDN
IN+
AMPLIFIER
TIMER
IA
1.25V
50mV
1.25V
SNS
OUTPUT
TO LOAD
SUPPLY
INPUT
TMRGND
GATE
A
OUT
OUT
FLT
EN
FB
+
–
30MA
VA
LT4356-1
sive designs, and may require actual
spark testing (at your expense) before
approving active designs. However, as
we will see, the LT4356 series surge
stopper IC can be used to design an
active barrier whose parameters can
be easily altered to quickly provide
custom barriers. Since the fundamental circuit topology won’t be changing
much, once such an active design is
approved, it will be more readily approved when only component value
changes are made. If the IS instrument
supplier is performing even a few IS
barrier designs, significant savings are
realized in energy efficiency, barrier
size and cost.
A passive design for associated apparatus, the barrier, that supplies DC
power to the field apparatus utilizes
three venerable passive devices to
implement protection: fuses, resistors
and Zener diodes. Safety factors of
1.5 or 1.7 are applied to these device
parameters. Furthermore, for doublefault protection at ‘ia’ protection level,
multiply redundant components are
necessary. Figure 3 shows the most
common type of passive barrier design
as an example.
Only the Zener diodes can limit open
circuit voltage and only the resistor
and fuse can limit current. Fuses are
not considered as a spark-ignition
energy limit device because of its
slow reaction time. In each case, the
devices dissipate power and must be
12
at a worst-case barrier performance,
always erring on the side of safety.
The barrier is assumed to pass a
maximum power of V
OC
• I
SC
= P
MAX
/2
when the field apparatus impedance is
equal to the barrier source impedance,
the point of maximum power transfer.
For this analysis the resistor value is
assumed to be (R – %tolerance) and V
OC
at (Vz + %tolerance). Any component
in the field apparatus must be able
Figure 4. Resistive circuit
spark ignition curves
to tolerate P
lower values by secondary means. If
we assume that the field apparatus is
/2 unless protected at
MAX
nothing more than an LED, the LED
properly rated. The Zeners actually do
sink some reverse leakage current even
though they are not fully on.
The examiner assumes the Zener
voltage knee to occur at the high
end of its tolerance, usually 5%. The
Zener must be rated at 1.5 times the
maximum power of the barrier, the
resistors must be rated at 1.5 times
the maximum power and the fuse is
presumed to pass 1.7 times its rated
current. The resistor is presumed
to be at the low end of its tolerance
range. All active and passive devices
must also have an absolute maximum
breakdown voltage specification that
is 1.5 times the maximum operating
voltage they will encounter in normal
or fault conditions. These presumptions are imposed not to frustrate
the electronic designer, but to arrive
must be able to dissipate P
MAX
/2 without exceeding the apparatus Surface
Temperature code, such as 85°C for
a T6 rated product.
In practical barrier designs, protective component redundancy is
necessary for compliance, especially
for Zener diodes. Two Zeners in parallel
are required for Ex ib rated equipment,
and three parallel Zeners for Ex ia
protection level. Note that the Zener
power dissipation rating depends on
the fuse clearing. If the fuse were not
present, proof must be supplied that
the Zener can dissipate the full barrier
power indefinitely without failing or
exceeding the temperature rating of
the apparatus. In addition, the IEC79
Standard requires that all fuses not
contained in approved holders must
be encapsulated. Further requirements exist for the protective resistor:
it must be “infallible.” If two resistors
are used in series, each resistor must
be of a high enough value as to limit
current if one of them fails short. If
two resistors are used in parallel,
each must be specified to dissipate the
maximum fault power if one resistor
fails open. An infallible resistor is one
of metal film, ceramic glazed wirewound, or thick film SMD type with
a conformal coating, all with suitable
creepage/clearance spacing to avoid
a non-countable fault. The infallible
resistor is considered to fail only to
Figure 5. Simplified block diagram of the LT4356
an open circuit. The examiner may
take this as one countable fault, but
unless it reveals failures downstream
of the resistor, it does not inform the
analysis.
Linear Technology Magazine • September 2009
DESIGN FEATURES L
C
TMR
0.1µF
R
SNS
10mΩ
Q1
IRLR2908
Q2
IRLR2908
V
IN
12V
V
OUT
12V, 3A
CLAMPED
AT 16V
LT4356S
GND TMR
OUTSNS
SHDN
A
OUT
IN
+
V
CC
EN
FLT
FB
D2*
SMAJ58CA
R2
4.99k
R1
59k
GATE
R7
10k
R5
1M
Q3
2N3904
D1
1N4148
R3
10Ω
R4
10Ω
*DIODES INC.
Despite their simplicity, passive
barriers exact a high price in power
loss and size. Maximum power is
transferred to field apparatus only
when its input impedance is equal to
the resistance of the current limiting
resistor in the barrier, and this is only
half of the power supplied to the barrier. If more than a few milliwatts are
required in the field apparatus, the
barrier resistor may become physically
large. Such resistors are understandably expensive, have a limited value
range and are difficult to source and
mount. If a fuse is not included in
the design, the Zener diodes likewise
become bulky and expensive. The fact
that the fuse must be encapsulated
(Paragraph 7.3) usually dictates that
the entire barrier is encapsulated,
making it impossible to service as
well as messy and more expensive to
manufacture.
Determining Maximum Safe
Field Apparatus Power Limits
The actual power that may be transferred to a field apparatus through
the associated apparatus barrier is
determined entirely by the level of
certification the instrument supplier
is seeking. This in turn is determined
entirely by the environment it will
encounter.
The Class and Division rating desired is easily determined. However,
Linear Technology Magazine • September 2009
Figure 6. Redundant pass transistors
the flammable gas/dust type is what
determines the Apparatus Group and
T code. The fact that hydrogen has a
relatively high ignition temperature
(560°C) and very low spark ignition
energy (20µJ) demonstrates that
careful thought must given to these
parameters before seeking certification
testing. Here we confine our discussions to Class I locations, gasses and
vapors in surface operations, Group II.
To determine how much power can be
available at the output of a barrier, and
still be safely faulted open or shorted,
we utilize the empirically determined
gas ignition curves published in the
standards. These curves indicate the
maximum voltage and current allowable for a given gas group.
There are three charts published in
the standards, one for resistive, inductive and capacitive circuits. Figure 4
shows the curve for a simple resistive
circuit. For sake of discussion, we
assume that we are dealing with the
worst environment for spark ignition,
acetylene, Group IIA. Referring to Figure 4, at 20V
it appears that up to
OC
400mA ISC is allowed without danger
of ignition. Additionally, this power
must not permit a corresponding
surface temperature rise high enough
to thermally ignite the gas in normal
or fault conditions.
Some authorities recommend derating the voltage V
by 10% and the
OC
current ISC by 33%. This is stated in the
standards (IEC 60079-11, 10.1.4.2)
under safety factors. The calculated
value of the current limiting series
resistor is simply V
OC
/I
= 20/0.4
SC
= 5Ω. The power the resistor must
dissipate is V
(V
)2/R, whichever is highest dur-
OC
OC
• I
or (ISC)2/R or
SC
ing circuit operation or fault. Simple
calculations show that even small
amounts of power may require rather
physically large current-limiting resistors. A final note: the Standards state
that from empirical and analytical
data, a T4 (135°C) temperature code
is automatically awarded to any circuit
using 1.3 watts or less.
Using the LT4356
Surge Stopper as an
Intrinsic Safety Barrier
The LT4356 series of overvoltage/
overcurrent limiters are excellent
choices for designing active protective
barriers with minimum parts count
and wasted power. Recognizing this
fact, Linear Technology offers the
IC in a 16-lead SO package with pin
spacing sufficient to avoid penalizing
the design with a non-countable fault
when encapsulated. For voltages up
to 10V, some Standards require a
1.5mm (59.1mil) creepage spacing,
and 2.0mm (78.7 mil) for up to 30V.
Before the 2006 79 series Standard,
the IC must be encapsulated to meet
these requirements because of the 50
mil (1.2mm) lead spacing of the 16lead SO package, but encapsulation
has the added advantage of raising
the thermal limits on any associated
components in the circuit.
However, the latest version of the
harmonized Standard, IEC60079-11
(5th edition 2006-07) dramatically
reduces these creepage requirements
on printed circuit boards when the
apparatus is enclosed in such a way
as to meet ingress protection standards. These standards, known as
IP levels, prevent ingress of dust or
moisture, thereby guaranteeing a pollution degree of 2 or less. The idea is
that the cleaner and drier the circuit
board stays, the lower the board’s CTI
(Comparative Tracking Index) and the
less likely leakage current will occur.
13
L DESIGN FEATURES
C
TMR
0.1µF
R
SENSE
3 s 0.5Ω
IN PARALLEL
Q1
IRLR2906
INPUT TEST
VOLTAGE STEP
0V TO 15V
VINV
IN
V
OUT
9.9V
300mA
LT4356CDE
GNDTMR
OUTSNS
SHDNSHDN
A
OUT
IN
+
V
CC
EN
FLT
FB
C
TMR
0.1µF
4.99k
GATE
22µF
25V
100k
10Ω
SMAJ58A*
*DIODES INC.
UNDERVOLTAGE LOCKOUT
LED SUPPLY
C
SNUB
0.1µF
100V
1M
1k
BZT52C16T
BZT52C5V6T
1M
OPT
R
SNUB
10Ω
3.9k
250mW
LED
GREEN
3.9k
3.9k
22k
34.8k
LED
GREEN
LED
RED
LED
GREEN
DRAN
DRAN
+
MMBT5551
Figure 7. Schematic of a modified DC1018A evaluation board
Annex F of 79-11 therefore allows only
0.2mm creepage all the way up to 50V
for Class I environments. Since most
instrumentation is enclosed anyway,
it behooves the designer to use an
enclosure with a high IP rating, such
as IP67 or IP68 to avoid encapsulation
requirements. Unless encapsulation
is necessary to meet thermal limits,
its cost and associated problems are
best avoided.
Figure 5 is a simplified block diagram of the LT4356 IC. The LT4356
monitors both current and voltage
continually and turns off the series
pass MOSFET quickly if a fault occurs. Both current and voltage limits
are set by external components, so
limits may be changed easily. The
current shunt resistor and the voltage
feedback resistors should be made infallible to achieve certification. Usually
the feedback resistors can be made
arbitrarily large so that a MOSFET
fault that shorts input power directly
to the feedback resistors cannot cause
significant power dissipation.
Nevertheless, two cautionary notes
are in order. The first is that active
devices (controllable semiconductors)
can be used in Ex ib situations for
power limitation (thermal ignition)
14
but not for spark ignition protection.
See paragraphs 7.5.2 and 7.5.3 in the
Standards. Some interpretations may
allow active barrier use in Zone 0, but
only in triplicate form. The second
caution is that, as with any IS barrier,
even for Ex ib (single fault) applications, barrier failure usually results
in non-countable thermal fault failure
downstream of the barrier. Therefore,
redundancy is required in case one of
the barriers fails.
The LT4356 provides for two series
pass transistors, typically for reverse
polarity protection. Protection against
polarity reversal is required “where this
could occur.” A single diode is deemed
acceptable to satisfy this requirement,
but two pass transistors offer better
protection from countable faults without a significant voltage drop.
For Ex ib environments, the examiner can use his single countable fault
to internally short all the pins on the
IC to analyze resultant failures. While
properly rated redundant Zeners could
be positioned at the output of the
LT4356 to provide a voltage limit, at
any appreciable power level the cost
and difficulty of specifying these Zeners
makes it more cost effective to simply
duplicate the entire barrier. Note that
for Ex ia applications, either triplicate
barriers, or two barriers with a series
infallible resistor are required to meet
the double-fault analysis rule.
From here on, we assume that
spacing and thermal rise, component
ratings, PCB tack width and redundancy rules are followed and the circuit
cannot be failed with either countable
or non-countable faults. The remaining question is that of spark ignition
energy. For this purpose, the LT4356
may not prove useful, depending on
the application.
The LT4356 reacts to both current
and voltage faults by turning off the
pass transistor(s). However, since it
does not shut down instantaneously,
some amount of energy squirts
through the barrier. In the standards
this is termed the let-through energy,
and is usually assessed using oscilloscope measurements and/or an
actual spark ignition test in a chamber. If this energy is enough to ignite
the subject gas, the barrier has failed
certification. Acceptable let-through
energy is summarized in Table 3.
Bench tests reveal that the LT4356 is
much more than adequate for even Ex
ia thermal ignition applications. Bench
testing was done using a modified
Linear Technology Magazine • September 2009
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