AN-770: iCoupler® Isolation in CAN Bus Applications (Rev. 0) PDF

AN-770
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/461-3113 • www.analog.com
iCoupler® Isolation in CAN Bus Applications
by Ronn Kliger and Sean Clark
INTRODUCTION
The Controller Area Network (CAN) bus, a robust protocol
designed for industrial applications, was originally
developed for use in cars. It specifies a maximum cable
length of 40 meters and up to 30 nodes. The flexibility and
advantages of this specification have resulted in increased
use in a wide range of applications.
Because the CAN bus system is typically used to connect
multiple systems and is often run over very long distances,
isolation between the bus and each system connection
is critical. Isolation provides protection from overvoltage
transients between the CAN bus cable network and
the systems connected to it. Isolation also eliminates
ground loops in the network, reduces signal distortion
and errors, and provides protection from voltage/ground
mismatches.
The intention of this application note is to give the user a
brief overview of the CAN bus protocol, focusing on the
system physical layer, as well as an understanding of why
isolation is so important to the system. This application
note also details how to implement isolation in a CAN bus
system using Analog Devices’ iCoupler products.
CAN BUS OVERVIEW
The CAN Bus Protocol
The CAN bus protocol standard is defined by the
International Standardization Organization (ISO) as a
serial communications 2-wire bus, with data rates up to
1 Mbps. It uses two layers: a differential signal physical
layer, specified as ISO 11898, which provides excellent
noise immunity, and a data link layer, which defines how
the signals interact and communicate.
The Data Frame
The CAN bus protocol uses asynchronous data transmission design. The transmitted data is sent in a data frame,
which is controlled by start and stop bits at the beginning
and end of each transmission.
The data frame is composed of an arbitration field, a control
field, a data field, a cyclic redundancy check field, and an
acknowledge field. The frame begins with a start-of-frame
dominant bit, and completes with an end-of-frame field
(bit), as shown in Figure 1.
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SOF
ARBITRATION CONTROL
DATA
CRC
ACK
EOF
Figure 1. CAN Bus Data Transmission Frame
CAN Bus Arbitration
The CAN bus protocol also specifies nondestructive bit
arbitration, which ensures that no data is lost. It is one of
the protocol’s most important features.
The CAN bus protocol defines the digital logic states on
the bus with a logic high as the recessive state and a logic
low as the dominant state. It is designed to allow every
node to listen and transmit at the same time.
All nodes transmit a single dominant start of message
(SOM) bit at the beginning of each message. Other
nodes will see bus activity and will not attempt to start a
transmission until the message packet is complete.
After the SOM bit, the arbitration field is transmitted. The
arbitration field is 11 or 29 bits long, depending on which
variation of the CAN bus protocol is used.
The highest priority message has an arbitration field of
the highest number of dominant bits; it will transmit a
dominant bit first, while the other nodes are transmitting
recessive bits.
Also known as the identifier, the arbitration field prioritizes
the messages on the bus. By the time the arbitration field
has been sent, all nodes except the highest priority node
will have stopped transmitting.
If multiple nodes start transmitting at the same time, the
node transmitting the highest number of dominant bits
always takes control of the bus. All nodes monitor the bus
and stop transmitting when a higher priority transmission
is recognized.
The other nodes attempt to transmit again after the
message is completed. In this second attempt, the next
highest value arbitration field will take control of the bus,
and the arbitration process is repeated.
The nondestructive bus arbitration ensures that the highest
priority message always gets through.
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CANL
CANL
CANH
CANH
Tx
Tx
Rx
120
Tx
Tx
Rx
Rx
Rx
120
Figure 2. CAN Bus Network
CAN Bus Types
The first CAN bus standard introduced uses ISO 11519
and is designed for data rates up to 125 kbps. This is
often referred to as low speed CAN. The second CAN bus
standard introduced uses ISO 11898 and is designed for
signaling rates between 125 kbps and 1 Mbps.This version
is referred to as CAN 2.0A. Both of these standards define
an 11-bit arbitration field.
Operation
CAN bus transceivers use a unique open drain design
(Figure 3).
VCC
Tx
The most recent CAN bus standard is Version 2.0B. This
standard is identical to 2.0A, except that it specifies a 29-bit
arbitration field.
STANDBY
CAN Bus Physical Layer
The physical layer is a balanced, or differential, 2-wire serial
interface (Figure 2). Most CAN systems are designed to
use a supply voltage of 5 V, although some 3 V systems
have been designed.
CANH
CANL
DRIVER
WAKE UP
MODE
CONTROL
GND
RECEIVER
Rx
LOW POWER
RECEIVER
(SLEEP MODE)
Non-return-to-zero (NRZ) encoding is used for data
communication on the differential 2-wire bus. Using NRZ
encoding ensures compact messages with a minimum
number of transitions and high noise immunity.
Figure 3. Typical CAN Transceiver, Including Low
Power Standby Mode Circuit
The driver uses a pair of open drain devices to create a
differential signal consisting of CANH (high) and CANL
(low) on the bus. Combined, these signals produce the
dominant signal level on the bus. The dominant signal
level represents a logic low. If no transmitter is driving,
pull-up resistors are used to set the bus voltage level to
VCC/2. The VCC/2 level is the recessive signal bus level and
represents a logic high (Figure 4).
The CAN bus specification defines several data rates from
10 kbps to 1 Mbps. However, all system modules must
support 20 kbps.
The ISO 11898 standard specification defines a maximum
bus length of 40 meters, a maximum stub length of
0.3 meters, and a maximum of 30 nodes. However, the
robust design of the CAN bus physical layer allows the
use of much longer cable lengths. With careful design, a
bus cable length of 1,000 meters is possible. As bus length
increases, a corresponding decrease in maximum data
rate will be experienced.
VOLTAGE
VCC – 0.9V (TYP)
System maximum speed depends on the bus cable
length. Maximum cable length for 1 Mbps is 40 meters.
The worst-case transmission time of an 8-byte frame with
an 11-bit identifier is 134 bit times or 134 microseconds at
the maximum baud rate of 1 Mbps.
VCC/2 (TYP)
CANH
1.5V (TYP)
CANL
RECESSIVE
DOMINANT
RECESSIVE
TIME
Figure 4. CAN Bus Signals
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During dominant state the active driver configures the
CANH line to a high level and the CANL line to a low
voltage level.These differential signal levels are typically
VCC – 0.9 V for CANH and 1.5 V above ground for CANL.
The operation of equipment switching large currents,
such as electric motors, causes rapid changes in the
ground potential. These changes can generate a
current flow through any nearby lines to equalize
the ground potential.
External pull-up resistors can be used to configure the
bus for the recessive state. Many CAN bus transceivers
have the driver input and receiver output pins passively
pulled high internally.
Other induction surge sources include electrostatic
discharge (ESD) and lightning strikes. These induced
surges can result in hundreds or even thousands of volts of
potential on the line, and manifest themselves as transient
current and voltage surges.
The nonbus side of the CAN bus transceivers connect to
a CAN controller or a processor. The signals on this side
of the transceiver are standard 0 V to 5 V or 0 V to 3 V
logic levels.
Thus, the cable end node may receive a switching signal
superimposed on a high voltage level with respect to its
local ground. These uncontrolled voltages and currents
can corrupt the signal, and can be catastrophic to the
local transceiver device and system, causing damage or
destruction of the components connected to the bus, and
resulting in system failure. Because CAN bus systems run
over cables of 40 meters or more, and typically interconnect
multiple systems, they are susceptible to these events.
Many transceivers also include a standby control input on
the processor side that allows the controller to place the
transceiver into a low power use standby mode, reducing
system power used. A low power receiver remains active
during standby mode, monitoring the bus for state changes.
The receiver signals the controller to activate the local CAN
node when bus activity is detected.
To protect against this potentially destructive energy, all
devices on the bus, and on the systems connected to the
bus, must each be referenced to only one ground. That
is, the systems connected to the CAN bus, and each CAN
bus transceiver, have a separate and isolated ground.
Referencing the CAN bus system to only one ground
eliminates ground loops, thereby preventing ground loops
and electrical surges from destroying circuits.
Termination
Termination resistors are required at each end of the
cable. The standard termination is 120  between the
differential cables, with termination at each cable end.
This layout results in the nominal 60  bus load, as
required by ISO 11898.
Fault Tolerance
The CAN bus standard recommends, but does not require,
transceivers to survive several fault scenarios.These fault
types include bus wires shorted together, shorted to the
power supply, or shorted to the ground.Typical transceiver
protection for these conditions is between –4 V and
+16 V. However, fault tolerance for all transceivers cannot
be assumed. It is recommended that close attention be
given to data sheet specifications.
Isolation also allows the CAN bus circuit reference voltage
levels to rise and fall with any surges that appear on the
cable line. Allowing the circuit voltage reference to move
with surges, rather than being clamped to a fixed ground,
prevents devices from being damaged or destroyed.
To accomplish system isolation, both the CAN bus signal
lines and power supplies must be isolated. Power isolation
is obtained through the use of an isolated dc-to-dc power
supply. Signal isolation can be implemented using optocouplers or with Analog Devices’ innovative iCouplers.
SYSTEM ISOLATION OVERVIEW
Unwanted currents and voltages on a cable bus connecting
two systems have the potential to cause severe problems.
High voltages and currents can destroy components connected to the bus. These unwanted voltages and currents
come primarily from two sources: ground loops and
electrical line surges. These voltages can far exceed the
CAN bus recommended fault protection levels.
ISOLATION IMPLEMENTATION
The implementation of isolation is not overly complex,
however, the designer must consider several important
factors when implementing the isolation circuitry.
The CAN bus requires resistor connections to achieve
the recessive state, which is typically VCC/2, and the
combination of CANH and CANL to achieve the dominant
state. Digital isolators do not support this signal standard.
Therefore, it is not possible to insert a digital isolator
between the CAN bus transceivers and the cable.
Ground loops occur when a bus or system uses multiple
ground paths. It cannot be assumed that two system
grounds connected to the bus and separated by several
meters or more will be at the same potential. Because these
grounds are unlikely to be at the same potential, current
will flow between these points. This unintended current
flow can damage or destroy components.
CAN bus signal path isolation is accomplished by
designing isolators into the digital signal path between
the transceivers and the local CAN bus controller. The
system side of the CAN bus transceivers use digital
logic level signals of 0 V to 5 V or 0 V to 3 V, and typically
connect to a CAN controller or processor. iCoupler isolators contain input and output circuits that are isolated
Electrical surges can be caused by many sources. These
surges are the result of currents coupled onto cable
lines through induction. Long cable lines and systems
in industrial environments are especially susceptible to
electrical surges.
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from each other. Placing an iCoupler in this location
electrically isolates the CAN bus cable signals from each
system connected to it.
The CAN bus specification defines two maximum data rate
speeds: 125 kbps and 1 Mbps. Fortunately, all iCoupler
products operate up to data rates of 1 Mbps. The iCoupler
products portfolio also includes devices that operate up to
data rates of 10 Mbps, 25 Mbps, and 100 Mbps.
To complete the isolation of the CAN bus circuits from
the local system, a dc-to-dc isolated power converter is
required, regardless of whether iCouplers or optocouplers
are used.The isolated power supply is used to supply power
to the local CAN bus transceiver and CAN bus side of the
isolator. The isolated power supply is typically supplied
from the local system.
Device cost typically rises in proportion to data rate
performance. Therefore, a designer should take care
not to specify a device with more performance than is
required. However, low performance device selection can
make future system performance upgrades more costly
and involved, because all devices not compatible with
upgraded system data speeds will require replacement.
The combination of digital isolators and an isolated dc-todc power supply creates an effective protection against
surge damage, and eliminates ground loops. Figure 5
illustrates system isolation design in a typical CAN bus
system configuration using iCoupler integration.
ADI’s iCouplers have a significantly shorter propagation
delay than optocouplers. Shorter propagation delays allow
a faster signal response time between a processor and the
bus.This is especially critical during the arbitration period,
when each node must decide which message receives
priority and controls the bus. Therefore, the propagation
delay time will define the maximum allowable line length
of the bus at the required data rate.
ISOLATION DEVICE SELECTION
System performance requirements will have the most
impact on the selection of an isolation device. Other
considerations include space constraints and cost.
Data Rate Requirements
System data rate requirements are likely to be the single
most important parameter for device selection.
Space Requirements
Maximum dimension requirements are a concern for
virtually all applications, and some implementations can
be severely space-limited. Fortunately, there are now
solutions for these situations.
ISOLATED DC/DC
SUPPLY
LOCAL
VCC
LOCAL
GROUND
LOCAL VCC
ISOLATED
SUPPLY VCC
ADuM1100
VDD1
V1
VDD1
CAN CONTROLLER
GND1
VDD2
GND2
ISOLATED
SUPPLY
GROUND
CANL
VO
Tx
CANH
CAN BUS
TRANSCEIVER
GND2
ADuM1100
VDD1
V1
VDD1
GND1
VDD2
GND2
VO
STB
GND2
ADuM1100
GND2
VO
GND2
VDD2
GND1
VDD1
VI
Rx
VDD1
LOCAL GROUND
Figure 5. CAN Bus Isolation Using Three ADuM1100s
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AN-770
Solutions for systems where space is an issue include
the ADuM1301. The ADuM1301 is a 3-channel isolation
device in a 16-lead SOIC package, taking the place of three
optocouplers and associated circuitry (Figure 6).
in a less complex board layout.The combination of smaller
boards and less complex layout reduces board costs. In
addition, circuit board assembly costs typically decrease
proportionally as the number of devices required for the
board assembly process decreases. Therefore, designing
with fewer devices results in lower manufacturing costs.
Cost Requirements
Cost constraints and concerns are a reality in virtually all
system design work, and therefore must be considered.
Cost considerations can have an effect on the design choices
for a system. As noted previously, isolator device cost
rises in proportion with data rate performance. Specifying
a device with only the system performance required can
reduce costs.
ADI i COUPLER PRODUCTS
ADI’s iCoupler device technology has created products
that possess distinct advantages for the system designer
in comparison to other available isolation options. The
iCoupler products provide superior performance, lower
power consumption, higher reliability, and lower component count, with cost characteristics that are comparable
with optocouplers.
Other cost issues include a consideration of the number
of devices used. The iCoupler device cost increases with
channel count. However, the cost per channel decreases
as the device channel count increases.
ADI iCoupler Technology Overview
ADI’s unique iCoupler technology provides isolation based
on chip scale transformers rather than on the LEDs and
photodiodes used in optocouplers. By fabricating the
transformers directly on-chip using wafer-level processing,
iCoupler channels can be integrated with each other and
with other semiconductor functions at low cost (Figure 7).
Additional cost benefits of integrating as many channels
into one device as possible include reduction in board
space and assembly costs. A lower device count results in
smaller boards. Also, lower device count typically results
ISOLATED DC/DC
SUPPLY
LOCAL
VCC
LOCAL VCC
iCoupler
ADuM1301
VDD1
VDD2
GND1
GND2
CAN CONTROLLER
VIA
LOCAL
GROUND
ISOLATED
SUPPLY VCC
ISOLATED
SUPPLY
GROUND
CANL
VOA
Tx
CANH
CAN BUS
TRANSCEIVER
VIB
VOB
VOC
VIC
NC
VE1
NC
VE2
GND1
STB
Rx
GND2
LOCAL GROUND
Figure 6. CAN Bus Node Network with Isolation Using ADuM1301
TOP COIL
BOTTOM COIL
POLYIMIDE LAYERS
Figure 7. Cross-Sectional View of iCoupler Configuration
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The technology used in iCoupler design eliminates the
inefficient electro-optical conversions that take place in
optocouplers. This is because iCouplers eliminate the
LEDs used in optocouplers. Also, because channels are
fabricated entirely with wafer-level processing, multiple
iCoupler channels can be easily integrated within a
single package. iCoupler technology provides increased
performance, reduced power consumption, smaller size,
increased reliability, and lower cost.
In the absence of logic transitions at the input for more
than 2 s, a periodic set of refresh pulses indicative of the
correct input state is generated to ensure dc correctness
at the output. If the iCoupler output side circuit receives
no pulses for more than about 5 s, the input side circuit
is assumed to be unpowered or nonfunctional, in which
case the isolator output is forced to a default state by the
watchdog timer.
ADI iCoupler Selection
The iCoupler family contains a broad portfolio of products,
allowing the system designer to select a product ideally
suited for the design. In addition, features and options
allow the design of a system using fewer devices.
Another distinct advantage of iCouplers over optocouplers
is the elimination of external components. In addition
to bypass capacitors, optocouplers require external
discrete devices to bias the output transistors and drive
the LEDs. Conversely, iCoupler devices require no
external components other than decoupling capacitors.
The iCoupler solution results in less circuit complexity
and lower cost.
The iCoupler portfolio includes 1-channel through 4-channel
options. Among the choices are devices designed for
bidirectional communication that enhance flowthrough
board design. iCoupler devices are also available for a
range of data rate performances.
The iCoupler products also incorporate unique refresh
and watchdog circuits.
Table I shows a comparison of product options, including
the number of channels, and data rate performance.
Table I. iCoupler and Isolated RS-485 Transceiver Products
Product
Model
Max
Max
Max
Number of Channel
UL Insulation Data Rate, Prop. Delay Operating
Channels Configuration* Rating (kV) 5 V (Mbps) 5 V (ns)
Temp. (°C) Package
ADuM1100 ADuM1100AR
ADuM1100BR
ADuM1100UR
1
1
1
1/0
1/0
1/0
2.5
2.5
2.5
25
100
100
18
18
18
105
105
125
8-Lead Narrow Body SOIC
8-Lead Narrow Body SOIC
8-Lead Narrow Body SOIC
ADuM120x ADuM1200AR
ADuM1200BR
ADuM1200CR
ADuM1201AR
ADuM1201BR
ADuM1201CR
2
2
2
2
2
2
2/0
2/0
2/0
1/1
1/1
1/1
2.5
2.5
2.5
2.5
2.5
2.5
1
10
25
1
10
25
150
50
45
150
50
45
105
105
105
105
105
105
8-Lead Narrow Body SOIC
8-Lead Narrow Body SOIC
8-Lead Narrow Body SOIC
8-Lead Narrow Body SOIC
8-Lead Narrow Body SOIC
8-Lead Narrow Body SOIC
ADuM130x ADuM1300ARW
ADuM1300BRW
ADuM1300CRW
ADuM1301ARW
ADuM1301BRW
ADuM1301CRW
3
3
3
3
3
3
3/0
3/0
3/0
2/1
2/1
2/1
2.5
2.5
2.5
2.5
2.5
2.5
1
10
90
1
10
90
100
50
32
100
50
32
105
105
105
105
105
105
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
ADuM140x ADuM1400ARW
ADuM1400BRW
ADuM1400CRW
ADuM1401ARW
ADuM1401BRW
ADuM1401CRW
ADuM1402ARW
ADuM1402BRW
ADuM1402CRW
4
4
4
4
4
4
4
4
4
4/0
4/0
4/0
3/1
3/1
3/1
2/2
2/2
2/2
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1
10
90
1
10
90
1
10
90
100
50
32
100
50
32
100
50
32
105
105
105
105
105
105
105
105
105
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
16-Lead Wide Body SOIC
*Channel configuration refers to the directionality of the isolation channels. For example, 2/1 means two channels communicate in one direction while the
third channel communicates in the reverse direction.
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Bypass Capacitors
The iCoupler products need no external components
other than bypass capacitors. A bypass capacitor is
strongly recommended for the input and output supply
pins. The bypass capacitor value should be between
0.01 F and 0.1 F. The total lead length between both
ends of the capacitor and the power supply pins should
not exceed 20 mm.
SUMMARY
The flexibility and high noise immunity of the CAN
bus specification make this protocol very popular for
intersystem communication. However, intersystem
communication cable systems are highly susceptible to
interference or damage from overvoltage transients and
ground loops.
Digitally isolating the CAN bus from the systems connected
to the bus reduces signal distortion and errors. This also
provides system and component protection from system
and bus voltage and ground mismatches.
Output Enable Control
Many of the iCoupler products have output enable control
pins (VEX) to allow outputs to be placed into a high impedance state. The outputs are in an active logic state when
the output enable pins are high or floating.The outputs are
disabled when the output enable pin is low. It is recommended that the output enable pins be pulled to a known
logic level, either high or low, in noisy applications.
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Analog Devices’ iCoupler products cover a broad range
of performance, channel counts, and configurations. The
combination of performance and channel configuration
allow the system designer multiple options, allowing
system design optimization.The iCoupler products provide
a cost-effective method for including critical isolation into
a system design.
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© 2005 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners.
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