AND8169/D

EMI/ESD Protection
Solutions for the CAN Bus

http://onsemi.com

APPLICATION NOTE

INTRODUCTION protection circuits. The CAN bus protection circuits

improve the reliability of the CAN module, without
The Controller Area Network (CAN) is a serial
significantly adding to the cost and complexity of the
communication protocol designed for providing reliable
transceiver circuit.
high−speed data transmission in harsh environments. CAN
system designers are being challenged to meet stringent CAN OVERVIEW
Electromagnetic Interference (EMI) and Electrostatic
Centralized vs. Distributed Control
Discharge (ESD) standards and increase reliability, while Control systems can be implemented using either a
reducing the size and cost of their products. This document centralized or a distributed architecture, as shown in
provides guidelines to select a CAN bus protection circuit Figure 1. A centralized control system typically consists of
that can prevent conducted and radiated EMI and ESD noise a single, relatively complex control unit that is used to
problems. The attributes of several practical CAN bus perform multiple tasks and monitor several sensors. In
protection circuits will be analyzed using discrete filters, contrast, a distributed control system consists of many
common mode chokes and Transient Voltage Suppression controllers that perform a specialized task. The sensors,
(TVS) devices. actuators and motors in a centralized system require point to
Bus protection circuits are used to supplement the noise point wiring in order to exchange information with the
immunity level of CAN transceivers. Many of the second control unit, while a distributed system requires only a few
generation CAN transceivers meet the minimum transient wires to connect all of the control units. Also, each control
overvoltage test levels; however, higher immunity levels unit in a distributed system, such as the CAN bus can be
can be easily achieved by adding external EMI/ESD implemented with a low cost microprocessor.

Solenoid Solenoid
Control Sensor Control Sensor

Sensor Sensor Control Control Control

Unit Unit Unit

Main CAN Bus

Control Unit

Control Control Control

Sensor Sensor Unit Unit Unit

Motor Sensor Motor Sensor

Control Control

Centralized Control System Distributed Control System

Figure 1. Centralized vs. Distributed Control

© Semiconductor Components Industries, LLC, 2014 1 Publication Order Number:

June, 2014 − Rev. 2 AND8169/D
 AND8169/D

Applications
The CAN network is a serial communication protocol The CAN network is also becoming popular in other
initially developed to connect sensors and electronic applications that need a communication bus with a high level
modules in automobiles and trucks. Since its inception in the of data integrity, such as train, marine and medical systems.
mid−1980’s, the CAN bus has also gained wide popularity Figures 2 and 3 provide examples of a typical CAN
in industrial control and building automation applications. automotive and industrial control system, respectively.

Rear Lighting Instrument Engine Control

Control ABS Panel Module

Diagnostic High−Speed CAN Bus

Port

Transmission Environment Active Suspension

Control Module Control System Control Module

Mirror Motor
Passenger Seat Door Module
Module Window Motor
Low Speed, Single Wire CAN Bus
or Local Interconnect Network (LIN)

Figure 2. Example of a Typical Automotive CAN Network

Smart Sensor Smart Valve Smart Motor

Stub Cable

Control
Interface
Termination
Resistor

Termination T Connector
Resistor

Figure 3. Example of an Industrial CAN Network

http://onsemi.com
2
 AND8169/D

Network Model electrical characteristics of the transceiver are given in the

A seven−layer Open Systems Interconnection (OSI) ISO and Society of Automotive Engineers (SAE) physical
network layering model is used to define the CAN network. layer specifications summarized in Table 1. The top layers
The model, shown in Figure 4, was developed by the of the OSI model are not specified by CAN so that users can
International Standards Organization (ISO) to define a create unique interfaces that met their specific requirements.
standard network that can be implemented with components The Rockwell (Allen−Bradley) DeviceNet™, Honeywell
from different manufacturers that are interchangeable. The Smart Distributed System™ (SDS), Kvaser CAN Kingdom,
CAN specification defines the bit encoding, timing and Time Triggered CAN (TTCAN) and SAE J1939 are popular
synchronization information of the transmitted signal. The networks that incorporate the CAN protocol.

Hardware
OSI Reference Layers Implementation

Application Microcontroller
or
Industry Standard SAE J1939, TTCAN, CAN Kingdom,
Presentation DSP
CAN Networks DeviceNet, SDS
Session

Transport CAN
Controller
Network
CAN Bit encoding protocol,
Specification message identification, etc. Data Link Layer
CAN
Physical Layer
Transceiver
ISO/SAE Electrical specifications:
Physical Layer transceiver characteristics,
Specifications connectors, cable, etc.
CAN_H CAN_L

Figure 4. CAN Uses the Seven−Layer OSI Model to Implement a High−Speed Communication Network

CAN Messages Cycle Redundancy Check (CRC) and Acknowledge (ACK)

The CAN protocol uses a multi−master broadcast fields that enable the system to detect and correct
technique where each node can initiate the transmission of transmission errors. The growing popularity of the CAN
a message that is sent to all the other nodes. Each node can bus results from its ability to provide error−free
also request information from another node. Messages are communications in a high noise environment. Figure 5
marked by an identifier field and are sent with provides the bit definitions of a CAN standard data frame.

2 bits
11 or 29 bits 6 bits 0 to 64 bits 16 bits 7 bits
Start of Frame

ACK Field

Arbitration Field Control Data CRC End of

(Identifier) Field Field Field Frame

4 bits

Data
Length
Code

Figure 5. Example of a Standard CAN Data Frame

http://onsemi.com
3
 AND8169/D

Hardware Implementation The system designer can create a CAN network by using
A CAN node is the portion of the network that consists of either a high−speed, fault tolerant or single wire physical
a controller that implements a function such as measuring layer protocol. Many CAN applications are constructed
the speed and temperature of an automobile’s transmission. using a combination of the three major physical layer
Figure 6 shows that a node can be formed by using a standards. For example, in many automobiles, the power
microcontroller, an external CAN controller, a CAN train will use the high−speed 1.0 Mbits/s differential bus,
Input/Output (I/O) Expander and a CAN transceiver. while less critical functions such as the rear view mirror
Typically the connection to the CAN bus is implemented controls use either a secondary 125 kbits/s differential bus
with a CAN transceiver IC that provides the ability to or a single wire bus.
receive and transmit the messages over the bus. Table 1
provides a summary of the physical layer standards defined
by ISO and SAE that define the electrical characteristics of
the CAN transceivers.

Node 1 Node 2 Node N

Microcontroller

A/D PWM I/O

Microcontroller
with Built−in CAN I/O
CAN Controller CAN Controller Expander

CAN Transceiver CAN Transceiver CAN Transceiver

CAN_H

High−Speed
120 W 120 W
CAN Bus Lines
CAN_L

Figure 6. CAN Node Configuration Options

http://onsemi.com
4
 AND8169/D

Table 1. Popular CAN Physical Layer Standards

Parameter High−Speed CAN Fault Tolerant CAN Single Wire CAN

Physical Layer ISO 11898−2 ISO 11519−2, ISO 11898−3, SAE J2411
Specification ISO 11992

Features High−speed, differential bus, Ability to detect a wiring error and Low cost
good noise immunity switch to a single wire mode

Popular Applications Automotive and industrial Large trucks and trailers Automotive, GM−LAN network
controls

Transmission Speed 1.0 Mbits/s @ 40 meters 125 kbits/s 33.3 kbits/s (normal mode)
125 kbits/s @ 500 meters 83.3 kbits/s (diagnostic mode)
Cable Twisted or parallel pair wires, Twisted or parallel pair wires, Single unshielded wire
shielded or unshielded cable shielded or unshielded cable

Termination Resistance 120 W resistors located at each Separate CAN_H and CAN_L Termination resistor located
end of the bus termination resistors located at each node, resistance
at each node, resistance determined by number of CAN
determined by number of CAN nodes
nodes

Min/Max Bus Voltage 12 V System: −3.0/16 V 12 V System: −3.0/16 V 12 V System: −3.0/16 V

24 V System: −3.0/32 V 24 V System: −3.0/32 V

Min/Max Common CAN_L: −2.0 (min)/2.5 V (nom) CAN_L: −2.0 (min)/2.5 V (nom) CAN_Bus offset voltage = 1.0 V
Mode Bus Voltage CAN_H: 2.5 (nom)/7.0 V (max) CAN_H: 2.5 (nom)/7.0 V (max) (max)

Transceiver Schematic Figure 7 Figure 8 Figure 9

and Waveform

http://onsemi.com
5
 AND8169/D

High−Speed CAN The ISO 11898−2 bus consists of the CAN_H (high) and
The ISO 11898−2 high−speed differential bus is the most CAN_L (low) data lines and a common ground signal. A
popular CAN transmission protocol. A differential protocol 120 W termination resistor is located at each end of the bus
is good for high−speed (1.0 Mbits/s) and medium−speed to minimize reflections and ringing on the waveforms. The
(125 kbits/s) applications that require the transfer of large logic states of the bits are determined by the differential
amounts of data. A differential bus also provides excellent voltage between the CAN_H and CAN_L signals. In most
noise immunity due to the inherent noise cancellation systems, the recessive state represents logic ‘1’, while logic
characteristics achieved by using a shielded cable with a ‘0’ is provided by the dominant state. Figure 7 shows
twisted wire pair and a receiver with a differential amplifier. simplified transceiver and system schematics, along with
the voltage waveforms of the data line signals.

VDD 3.5 V

Transmitter CAN_H 2.5 V 2.5 V

DV
CAN_L 2.5 V 2.5 V
CAN_H

Receiver VDD/2 +
−

CAN_L 1.5 V

Recessive Dominant Recessive

High−Speed
CAN Transceiver
Bus Logic States
Recessive: DV v 0.5 V
Dominant: DV w 0.9 V

Node 1 Node 2 Node N

High−Speed High−Speed High−Speed

CAN Transceiver CAN Transceiver CAN Transceiver

CAN_H

120 W 120 W
CAN_L

Figure 7. ISO 11898−2 Differential High−Speed CAN Bus

http://onsemi.com
6
 AND8169/D

Fault Tolerant CAN There are three different ISO specifications that are used
Fault tolerant CAN transceivers normally use a two wire to define the fault tolerant bus. The ISO 11519−2 and
differential bus that is identical to the high−speed bus; ISO 11898−3 specifications are popular in automobiles,
however, the transceivers automatically switch to a single while ISO 11992 is widely used in truck and trailer
wire mode if either the CAN_H and CAN_L signal lines are applications. The voltage levels of the ISO 11519−2 and
shorted to ground or power. The fault tolerant bus has a ISO 11898−3 correspond to a 5.0 V data bus. In contrast, the
maximum specified data rate of 125 kbits/s. The composite voltage levels of the ISO 11992 signals are specified at a
bus termination resistance is equal to 100 W and the resistors voltage level of 1/3 and 2/3 of the supply voltage which is
are located next to each transceiver. typically 24 V. Figure 8 shows a simplified system
schematic of the ISO 11898−3 protocol, along with the
voltage waveforms of the data line signals.

5.0 V
3.6 V
CAN_H CAN_L
Bus Logic States
Dominant: CAN_H > CAN_L
Recessive: CAN_H < CAN_L
CAN_L
CAN_H
1.4 V
0V

Recessive Dominant

Node 1 Node 2 Node N

Fault Tolerant Fault Tolerant Fault Tolerant

CAN Transceiver CAN Transceiver CAN Transceiver
CAN_H

CAN_H

CAN_H
CAN_L

CAN_L

CAN_L
RTH

RTH

RTH
RTL

RTL

RTL
CAN_H

CAN_L

Figure 8. ISO 11898−3 Fault Tolerant CAN Bus

http://onsemi.com
7
 AND8169/D

Single Wire CAN has the provision for an 83.3 kbits rate diagnostic mode. The
The single wire SAE J2411 bus is used in CAN network bus is typically implemented with an unshielded cable with
applications with low bit rate transmissions requirements a signal and ground wire. The bus termination resistors are
and relatively short bus lengths. Typical applications of the located next to each transceiver and have a value determined
single wire bus include non−critical comfort accessories on by the parallel resistance of all the individual node resistors.
the automobile with bus lengths less than 1 meter, such as Figure 9 shows a simplified system schematic, along with
adjustable driver and passenger seats. The normal the voltage waveform of the data line signal.
transmission rate is 33.3 kbits/s; however, the system also

4.0 V
Single Wire
Transceiver

CAN_Bus 0.1 V 0.1 V

Transmitter
CAN_Bus

Recessive Dominant Recessive

Receiver

Bus Logic States

Recessive: CAN_Bus < 1.6 V
Loss of Dominant: CAN_Bus > 3.4 V
Ground
Protection Load

Node 1 Node 2 Node N

Single Wire Single Wire Single Wire

CAN Transceiver CAN Transceiver CAN Transceiver

CAN_Bus Load CAN_Bus Load CAN_Bus Load

CAN_Bus

Figure 9. SAE J2411 Single Wire CAN Bus

http://onsemi.com
8
 AND8169/D

CAN TRANSCEIVER SPECIFICATIONS Maximum Transmission Speed

The transmission rate of the network is a critical
There are several CAN transceiver specifications that must parmenter in the selection process of the CAN bus protection
be evaluated in order to pick an appropriate EMI / ESD bus circuit. The inductance and capacitance of the protective
protection circuit. The critical transceiver characteristics devices will cause distortion in the signal waveforms, which
include: often becomes a major design concern at high transmission
1. Maximum supply voltage frequencies. The high−to−low and low−to−high transitions
2. Common mode voltage on the CAN signal lines will have rounded transitions that
3. Maximum transmission speed can result in the system being unable to clearly identify a
4. ESD rating high and low logic state.
5. Coupled electrical disturbance It is important to match the filter attenuation on each
Maximum Supply Voltage signal line. Minor distortion on the signal lines in a
The ISO and SAE physical layer specifications require differential system is acceptable if the amount of filtering on
that the transceiver must survive an indefinite short between the CAN_H and CAN_L lines are identical. It is typically
the battery power lines and CAN signal lines. Most CAN not possible to tightly match discrete capacitors or the
transceivers require a 3.3 V or a 5.0 V supply voltage that is capacitance of a MOV or TVS device; thus, the practical
created from either a 12 V or 24 V battery. The external approach is to select a protection device with the minimum
protection circuit devices may not be needed to meet the possible capacitance. However, a design trade−off exists
maximum supply voltage requirement because the majority because the energy absorption rating of Zener TVS diodes
of CAN transceivers are designed to withstand a DC voltage and MOVs typically increases with capacitance. It is
of ±40 V or greater on the signal lines. recommended that the maximum capacitance of the
The maximum battery voltage is an important factor in the protective network measured from each signal line to
selection of the TVS devices. The TVS devices should be ground should be less than 35 pF for 1.0 Mbits/s and 250 pF
chosen so that the minimum breakdown voltage of the Zener for 125 kbits/s.
diode or MOV over the operating temperature range is
ESD Rating
greater than the maximum system supply voltage. TVS There are many ways that ESD can enter a CAN node. An
devices are designed to dissipate the large peak power of a ESD event can occur when a charged object such as a person
transient event; however, they should not be used to regulate touches the CAN module’s connector pins or cable. CAN
a steady−state voltage. transceivers are designed to have a higher ESD rating than
Common Mode Voltage a standard IC because they drive lines that are connected to
The common mode voltage specification is an important an external input/output (I/O) connector; however, their
parameter in selecting a TVS device. Often in the case of ESD levels are typically below the rating that can be
networks such as CAN, there can be a significant difference achieved by using an external TVS device.
in the voltage potential between the ground reference of the The ISO 11898−2, SDS and DeviceNet physical layer
transmitting and receiving nodes. The ISO 11898−2 specifications do not list an ESD requirement; however, it is
specification requires that a transceiver function with a generally recommended that a network system should have
signal line voltage that can be offset by as much as 2.0 V a contact rating of at least ±8.0 kV and a non−contact or air
above or below the nominal voltage level of the CAN_H and rating of ±15 kV. There are several different specifications
CAN_L signal lines. used to measure ESD immunity, including the human body
A solution to the common mode problem is to use model (HBM) and the IEC 61000−4−2 tests. The HBM test
bidirectional TVS devices that will not clamp if the voltage is typically the specification listed on CAN transceiver data
at the signal lines is offset. MOVs are inherently sheets, while the IEC specification is gaining popularity as
bidirectional and have a breakdown voltage that is equal for a system level test. Both ESD specifications are designed to
both positive and negative voltages. In contrast, a standard simulate the direct contact of a person to an object such as
TVS diode will function as a Zener diode for a positive the I/O pin of a connector; however, the IEC test is more
voltage (cathode voltage > anode voltage) and as a standard severe than the HBM. The IEC test is defined by the
diode for a negative voltage (cathode voltage < anode discharge of a 150 pF capacitor through a 330 W resistor,
voltage). A differential transceiver functions by monitoring while the HBM uses a 100 pF capacitor and 1500 W resistor.
the voltage difference in two signal lines, rather than the
absolute voltage levels. It is essential that the TVS devices
do not clamp the transmission signals during normal
operating conditions.

http://onsemi.com
9
 AND8169/D

Coupled Electrical Disturbances Filters

A CAN transceiver must be able to survive the high Filters can provide bus protection with either discrete
energy transients that are produced by a number of resistor−capacitor (RC) and inductor−capacitor (LC) filters
disturbances including load dump, inductive load switching, or a common mode choke. Filters will attenuate the
relay contact chatter and ignition system noise. These magnitude of the noise on the CAN signal lines; however,
transient signals are usually generated on the CAN nodes they may distort the signal waveform and do not provide
supply voltage; however, the noise can be coupled into the voltage clamping. A filter approach is also limited by the
adjacent data line signals because the power and CAN data physical layer specifications that specify a maximum
lines are typically located inside the same wire bundle. capacitance load for each CAN node. A TVS device should
The severest transient in an automotive application is the always be used in combination with a filter to protect not
load dump which occurs when the battery is inadvertently only the CAN transceiver, but also the discrete filter
disconnected from the generator. Studies by the SAE and components such as the capacitors.
ISO groups have shown that the load dump produces an
exponentially decaying positive voltage with a magnitude of TVS Devices
25 to 125 V and a pulse between 40 to 500 ms. A common TVS devices can be used to absorb the transient energy of
method to protect against the high energy load dump surge an overvoltage event to prevent damage to the CAN
is to use a TVS device on the power supply line entering the transceiver. The preferred TVS devices to use for a digital
CAN module. The CAN physical layer specifications do not bus circuit are TVS Zener diodes and Metal Oxide Varistors
require the transceiver to pass a load dump test. Protection (MOVs). These clamping devices have a very fast turn−on
against the load dump surge is provided by TVS devices that time (< 1.0 ns) and limit the overvoltage to a safe value
are part of the CAN module’s power supply circuit, which within the transceiver’s operating range. Zener diodes and
typically is a low dropout (LDO) linear voltage regulator. MOVs also have the feature that they function as a capacitor
The specifications used to verify the CAN systems for normal signal transmissions below their breakdown
immunity to coupled transient noise on the power supply voltage. Low capacitance TVS Zener diodes and MOVs are
lines are given in ISO 7637−1 (12 V systems) and 7637−2 readily available that make these devices essentially
(24 V systems). The CAN data line immunity requirement transparent to the operation of a high−speed data bus.
for a repetitive high frequency disturbance such as the noise PCB Layout Recommendations
produced by the arcing contacts in a relay is typically tested The location and circuit board layout is critical to
with ISO 7637−3. The IEC 61000−4−4 Electrical Fast maximize the effectiveness of the CAN protection circuit.
Transient (EFT) test is similar to ISO 7637−3 and is a The following guidelines are recommended:
required test for the SDS CAN network. 1. Locate the protection devices as close as possible
to the I/O connector. This allows the protection
CAN BUS PROTECTION OPTIONS devices to absorb the energy of the transient
The options available to protect the CAN signal lines from voltage before it can be coupled into the adjacent
EMI and ESD interference include shielded twisted wire traces on the PCB.
pair cables, differential and common mode filters and TVS 2. Minimize the loop area for the high−speed data
devices. In most applications a combination of multiple bus lines, power and ground lines to reduce the
protection devices is required to provide a robust radiated emissions and the susceptibility to RF
communication system. Also, the location and circuit board noise.
layout is critical to maximize the effectiveness of the CAN 3. Minimize the path length between the CAN signal
protection circuit. lines and protective devices.
4. Use ground planes wherever possible to reduce the
Shielded Twisted Wire Pair Cable parasitic capacitance and inductance of the PCB
A shielded cable is an effective tool to prevent radiated that degrades the effectiveness of a filter device.
interference from introducing a common mode noise
voltage on signal wires. A shielded twisted wire pair cable CAN PROTECTION CIRCUITS
minimizes the voltage induced on the bus signal lines. There
will still be some noise signal coupled into the two signal There are several different options available to provide
lines, but the noise level on lines will be essentially equal. EMI and ESD protection to a CAN transceiver. The
The transceiver’s differential amplifier ability to cancel the protective device options available include TVS diodes,
majority of the common mode noise is specified by its MOVs, a common mode choke, a split termination circuit
common mode rejection ratio (CMRR) specification. The and RC/LC filters. A combination of multiple protection
CMRR of the transceiver can be increased by discrete filters devices is required in many applications to ensure a reliable
or by using a common mode choke. communication system in a noisy electrical environment.

http://onsemi.com
10
 AND8169/D

TVS Diode Protection Circuits breakdown voltage of either diode D1 or D2. The main
TVS diodes provide protection to a transceiver by advantage of the three−diode configuration is that a smaller
clamping a surge voltage to a safe level. TVS diodes have IC package can be used to house the array.
high impedance below and low impedance above their
breakdown voltage. A TVS Zener diode has its junction CAN_H
optimized to absorb the high peak energy of a transient CAN
CAN_L CAN Bus
event, while a standard Zener diode is designed and Transceiver
specified to clamp a steady state voltage. D1 D2
A bidirectional TVS diode can be created by combining
two unidirectional diodes, as shown in Figure 10. A
bidirectional TVS diode is typically required for data line
D3
signals that may have an offset voltage. The bidirectional
diode can be created from either dual common cathode or
common anode arrays and both configurations are
equivalent in their clamping characteristics. Figure 12. Alternative High−Speed and Fault
Tolerant CAN TVS Protection Circuit
Bidirectional
TVS Diode Anode 1 Cathode 1 A third circuit configuration using TVS diodes is shown
in Figure 13. This array consists of four standard diodes and
Common Common a unidirectional TVS Zener diode. Protection to the CAN
Cathode Anode bus lines is provided by clamping the signal lines to either
a forward diode voltage drop above the supply voltage
Anode 2 Cathode 2 (VDD) or a forward diode drop below ground. One
advantage of this circuit is that the diode array clamps at a
Figure 10. Bidirectional TVS Diodes can be voltage closer to the normal amplitude of the waveform and
Implemented with Two Zeners and a Common the diodes can be used to remove overshoot or ringing on the
Cathode or a Common Anode Configuration
signal lines.
Figure 11 provides an example of a dual bidirectional VDD
TVS diode array that can be used for protection with the
high−speed CAN network. The bidirectional array is created CAN_H
from four identical Zener TVS diodes. The clamping CAN CAN Bus
Transceiver CAN_L
voltage of the composite device is equal to the breakdown
voltage of the diode that is reversed biased, plus the diode VDD
drop of the second diode that is forwarded biased.
D1 D3
CAN_H NUP2202W1
CAN Z1
CAN_L CAN Bus D2 D4
Transceiver

NUP2105L
Figure 13. Alternative High−Speed CAN TVS
Diode Array Protection Circuit

Another advantage of this configuration is that the

Figure 11. High−Speed and Fault Tolerant CAN
capacitive load on the signal lines is typically less than the
TVS Protection Circuit
bidirectional Zener diode circuits shown in Figures 11 and
12. Diodes D1, D2, D3 and D4 have a low capacitance and are
The circuit shown in Figure 12 is functionally equivalent
designed to have a fast turn−on time. The TVS diode Z1 is
to the Zener array of Figure 11; however, only three diodes
used to dissipate the majority of the energy when an
are required to provide the bidirectional feature. The
overvoltage condition occurs. A large capacitance, high
clamping voltage on the data lines for a positive voltage
energy rated Zener can be used for Z1 because the capacitive
surge will be equal to the forward voltage drop of diode D1
load of the device is on the power supply lines rather than a
or D2 plus the breakdown voltage of the reversed biased
data line.
diode D3. The clamping voltage for a negative voltage will
be equal to the forward voltage drop of diode D3 plus the

http://onsemi.com
11
 AND8169/D

The main disadvantage of the diode array circuit is that it were prone to early high voltage repetitive strike wear out.
may clamp the CAN_L and CAN_H waveforms if a In contrast, multi−layer varistors are now available that are
common mode or offset voltage exists. The ISO 11898−2 not prone to the inconsistent grain size problem of single
specification lists a common mode voltage specification of layer devices which resulted in a limited lifetime.
−2.0 V to +7.0 V for the data lines and the array circuit will
provide undesired clamping for signals at the minimum and Common Mode Choke Circuit
maximum limits of the specification when the transceiver’s Common mode chokes are an effective tool for
supply voltage (VDD) equals +5.0 V. The common mode attenuating the noise that is common to both of the
voltage corresponds to the possible offset voltage that may transceiver bus lines, as shown in Figure 15. Chokes
exist from the difference between the two ground references function by providing high impedance for common mode
of the transmitting and receiving CAN modules. The diode signals and a low impedance for differential signals, which
array circuit of Figure 13 should only be used in a system increases the common mode rejection ratio (CMRR) of the
that can ensure that the offset voltage between the CAN transceiver. Chokes are an effective device to implement
module’s ground references will be relatively small and less filtering without adding a large amount of distortion on
than the turn−on voltage of the diodes. high−speed data lines. The common mode choke functions
limits the magnitude of an overvoltage surge on the data
MOVs Protection Circuit lines by functioning as a filter; thus, it is recommended that
MOVs can be used to provide clamping protection for TVS devices be added to the circuit to provide clamping
CAN transceivers as shown in Figure 14. A varistor is a protection.
non−linear resistor which has electrical characteristics
similar to a bidirectional Zener diode. At low voltages below Common
its breakdown voltage, a MOV can be modeled as a very Mode
CAN_H Choke
large resistance in parallel with a capacitance. When the
voltage of the surge exceeds the breakdown voltage, the CAN
CAN Bus
Transceiver CAN_L
resistance of the device decreases to a low value that will
clamp the transient event via the resistive divider effect of a
low impedance in series with the resistance of the voltage NUP2105L
source.

CAN_H
CAN
CAN_L CAN Bus
Transceiver Figure 15. High−Speed and Fault Tolerant CAN
Common Mode Choke Circuit

There are several disadvantages of chokes. One issue with

MOVs chokes is that their inductance and the capacitance of the
board and transceiver can form a resonant tank circuit that
will oscillate. Oscillations on the CAN signal lines will
Figure 14. High−Speed and Fault Tolerant CAN result in false bit detections at the transceiver. Another issue
MOV Protection Circuit with choke filters is that any mismatch in the inductance of
the two coils will cause distortion in the signal waveforms.
The main advantage of MOVs is that they provide
clamping protection at a relatively low cost. The main Split Termination Circuit
disadvantage of a MOV is that its clamping voltage is Figure 16 shows a split termination circuit that can be
typically higher than a comparable Zener diode. Also, used to provide noise protection to a CAN transceiver. The
MOVs traditionally have been used only in cost sensitive termination circuit functions as a low−pass filter and is
applications because their clamping voltage decreased over formed by two equal valued resistors and a capacitor. The
the life of the part. However, this problem has been common mode signal is terminated through a capacitor that
minimized in multi−layer MOVs by improvements in their shunts a high frequency noise signal to ground. The
manufacturing process. MOVs are made from ZnO and tolerance of the termination resistors should be as good as
metal oxides that form a polycrystalline structure with a possible (R tol. ≤ 1%) in order to maintain waveform
granular structure. Single layer or “pressed pill” MOVs symmetry between the CAN_H and CAN_L signals.
often had a large variance in their breakdown voltage and

http://onsemi.com
12
 AND8169/D

line voltage does not exceed the maximum voltage rating of

CAN_H RH CAN the transceiver and capacitors.
CAN Bus Typically the termination resistors for the high−speed
Transceiver CAN_L CAN bus are located at the two ends of the network. If the
CS
RL CAN node is located at the end of the bus, two resistors of
60 W are used instead of one 120 W resistor. Otherwise, if
the transceiver is located at a CAN stub node that would not
normally contain a termination resistor, higher value
NUP2105L
resistors are required so that the parallel value of the
termination resistance remains at 60 W.

Multiple Suppression Device Circuit

Figure 16. High−Speed and Fault Tolerant CAN A combination of a common mode choke, capacitors and
Protection Circuits Using Split Termination TVS diodes can be used to solve the most stringent EMI
emission and immunity requirements, as shown in
The split termination circuit can be combined with either Figure 17. Noise entering the CAN node is attenuated by the
a TVS diode or a MOV clamping diode. A resistor−capacitor second order filters formed by the inductance of the choke
(RC) circuit provides protection by functioning as a filter and capacitors CH1 and CL1. In contrast, capacitors
low−pass filter and by limiting the slew rate of an ESD or CH2 and CL2 provide a filter to reduce the emissions or noise
transient overvoltage signal. A TVS clamping device should that exits the transceiver. The bidirectional TVS diodes
be added to the split termination circuit to ensure that the bus function to clamp a transient voltage disturbance on the
CAN bus lines to a safe value.

Common
Mode
CAN_H Choke
CAN CAN
Transceiver CAN_L Bus

CH1 CL1 CH2 CL2

NUP2105L

Figure 17. High−Speed CAN Protection Circuits Combining Choke, Capacitors and TVS Diodes

Single Wire CAN Protection Circuit CAN_Bus 47 mH

Figure 18 shows the protection circuit that is CAN Bus
recommended by the SAE J2411 specification for a single CAN
wire CAN transceiver. The circuit consists of a bidirectional Transceiver RLoad
TVS diode to provide overvoltage protection and a discrete 220 pF
filter. The inductor and capacitor form a low−pass filter to Load MMBZ27VCLT1
attenuate the emissions or noise exiting the CAN node. In
contrast, the inductor and resistor combination form a filter
that reduces the noise entering the node. The transceiver’s
“load” pin is connected to a “loss of ground protection
circuit” that serves to compensate for a broken ground Figure 18. J2411 Single Wire CAN
Recommended Bus Interface Circuit
connection.

http://onsemi.com
13
 AND8169/D

CONCLUSION
The Controller Area Network (CAN) is a popular serial be used to increase the CAN transceivers noise immunity for
communication protocol that provides reliable high−speed EMI and ESD. The bus protection circuits improve the
data transmission in a multitude of applications ranging from reliability of the CAN module, without significantly adding
automotive to industrial control. Bus protection circuits can to the cost and complexity of the transceiver circuit.

Bibliography Industry Websites for Further Information

1. Broyles, Sam, “Application Report SLLA109 – A • CAN In Automation (CIA) Industry Group,
System Evaluation of CAN Transceivers”, Texas www.can−cia.de
Instruments, 2002. • CAN Kingdom, www.kvaserinc.com/
2. Corrigan, Steve, “Application Report SLOA101 –
• CANopen, http://www.canopensolutions.com/
Introduction to the Controller Area Network
(CAN)”, Texas Instruments, 2002. • International Organization for Standardization (ISO),
3. Demcko, R., “Multilayer Varistors in Automobile www.iso.org
MUX Bus Applications”, SAE International • IXXAT Automation GmbH, www.ixxat.com/
Congress and Exposition, Detroit, February, 1998. • Open DeviceNet Vendor Association (ODVA),
4. Durham, M., Durham, K. and Durham, R., “A www.odva.org
Performance Evaluation of Transient Voltage • Robert Bosch GmbH, www.can.bosch.com
Suppression Devices”, IEEE Industry Applications • Smart Distributed System (SDS),
Magazine, Sept./Oct., 2002.
http://content.honeywell.com/sensing/prodinfo/sds/
5. Eisele, H. and Jöhnk, E., “AN96116 –
PCA82C250/251 CAN Transceiver”, Philips • Society of Automotive Engineers (SAE), www.sae.org
Semiconductors, 1996.
6. Ix, A., “AN10211 – TJA1040 CAN High−Speed
Transceiver”, Philips Semiconductors, 2003.
7. Richards, Pat, “AN228 – A CAN Physical Layer
Discussion”, Microchip Technology, 2002.
8. Suermann, T., “AN00020 – TJA1050 CAN
High−Speed Transceiver”, Philips
Semiconductors, 2000.
9. van Beneden, B., “Varistors: Ideal Solution to
Surge Protection”, Power Electronics Technology,
May, 2003.
10. “AN2005 – AU5790 Single Wire CAN
Transceiver”, Philips Semiconductors, 2001.
11. “The CAN Physical Layer”, CAN in Automation,
GmbH, May, 2004.
12. “Application Hints – Fault−tolerant CAN
Transceiver”, Philips Semiconductors, 2001.

http://onsemi.com
14
 AND8169/D

APPENDIX I: ON SEMICONDUCTOR TVS SOLUTIONS FOR CAN

TVS Zener Diode Selection Guidelines
1. Select a device with a working reverse voltage 3. A bidirectional TVS device should be used in most
(VRWM) that is greater than or equal to the applications to meet the common mode voltage
maximum bus voltage. The maximum bus voltage specification. The common voltage specification is
or DC voltage is equal to 16 V for a 12 V system required because there can be a significant
and 32 V for a 24 V system, per the ISO and SAE difference in the voltage potential between the
physical layer specifications. ground reference of the transmitting and receiving
2. Select a device with a clamping voltage (VC) less nodes.
than the maximum specified voltage for the CAN 4. The diode array circuit of Figure 13 should only be
transceiver’s bus lines. used in a system that can ensure that the offset
voltage between the CAN module’s ground
references will be relatively small and less than the
turn−on voltage of the diodes.

Table 2. Recommended ON TVS Devices

System Voltage High−Speed CAN Fault Tolerant CAN Single Wire CAN

12 V NUP2105L NUP2105L MMBZ27VCLT1

• SOT−23 Package • SOT−23 Package • SOT−23 Package
• Dual Bidirectional TVS Zener • Dual Bidirectional TVS Zener • Dual Common Cathode TVS
Zener
LC03−6R2 LC03−6R2
• SO−8 Package • SO−8 Package
• Low Capacitance Diode Array • Low Capacitance Diode Array
Plus TVS Zener Plus TVS Zener
• Recommended only for short ca- • Recommended only for short ca-
ble lengths ble lengths

ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks,
copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC
reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any
particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without
limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications
and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC
does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for
surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where
personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and
its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly,
any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture
of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.

PUBLICATION ORDERING INFORMATION

LITERATURE FULFILLMENT: N. American Technical Support: 800−282−9855 Toll Free ON Semiconductor Website: www.onsemi.com
Literature Distribution Center for ON Semiconductor USA/Canada
P.O. Box 5163, Denver, Colorado 80217 USA Europe, Middle East and Africa Technical Support: Order Literature: http://www.onsemi.com/orderlit
Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada Phone: 421 33 790 2910
Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada Japan Customer Focus Center For additional information, please contact your local
Email: orderlit@onsemi.com Phone: 81−3−5817−1050 Sales Representative

http://onsemi.com AND8169/D
15