Protecting Your CANProtecting Your CAN - microchip.com

1 microchip Technology : October 2006 CAN ProtectionPage 1 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)1 Protecting Your CANP rotecting Your CANP rotecting Your CANC ontroller Area Network Physical Layer ProtectionController Area Network Controller Area Network Physical Layer ProtectionPhysical Layer ProtectionHello, and welcome to the WebSeminar on the EMI and ESD protection for the CAN Bus. My name is Pat Richards. I am an Applications Engineer in the Analog and Interface Products Division. So let s get Technology : October 2006 CAN ProtectionPage 2 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)2 TopicsTopicsTopicsOWhy Does the CAN Bus Need Protecting ?OCommon Protection MethodsOSummaryOOWhy Does the CAN Bus Need Protecting ?Why Does the CAN Bus Need Protecting ?

2 OOCommon Protection MethodsCommon Protection MethodsOOSummarySummaryToday we will be discussing protection methods for the Controller Area Network, or the CAN Bus. We will answer the question, Why does the CAN Bus need extra protection? We will then identify some common protection methods, which include cabling, filters, common mode chokes, split termination, MOVs, TVSs and then we will finish with a brief Technology : October 2006 CAN ProtectionPage 3 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)3 TopicsTopicsTopicsOWhy Does the CAN Bus Need Protecting ?OCommon Protection MethodsOSummaryOOWhy Does the CAN Bus Need Protecting ?Why Does the CAN Bus Need Protecting ?OOCommon Protection MethodsCommon Protection MethodsOOSummarySummarySo, why does the CAN Bus need extra protection anyway? Doesn t the transceiver provide adequate protection?

3 microchip Technology : October 2006 CAN ProtectionPage 4 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)4 Noisy, Harsh EnvironmentsNoisy, Harsh EnvironmentsNoisy, Harsh EnvironmentsOCAN is used in automotive and industrial applicationsONodes must survive high energy transientsOLoad dumpOInductive load switchingORelay noiseOIgnition system noiseOAnd short circuit conditionsOOCAN is used in automotive and industrial CAN is used in automotive and industrial applicationsapplicationsOONodes must survive high energy transientsNodes must survive high energy transientsOOLoad dumpLoad dumpOOInductive load switchingInductive load switchingOORelay noiseRelay noiseOOIgnition system noiseIgnition system noiseOOAnd short circuit conditionsAnd short circuit conditionsCAN was originally developed for data communications in the automotive industry.

4 The industry is a noisy, harsh nodes are subject to high energy transients such as load dump, inductive load switching, relay noise, ignition switching, most popular physical layer in CAN is specified by ISO ISO 11898 compatible transceivers do a good job of filtering out noise, which are coupled on the bus, and Protecting the node from energy spikes and short circuit conditions. For example, the MCP2515 can withstand shorts of + or 42 volts on the CAN bus and can withstand transient energy spikes of + or 250 volts without damage to the , the differential signaling inherent in CAN helps to filter out noise. However, in some systems the environment is harsh enough that extra protection and/or extra filtering is needed to protect the node and ensure clean Technology : October 2006 CAN ProtectionPage 5 2006 microchip Technology Incorporated. All Rights Reserved.

5 Introduction to Controller Area Network (CAN)5 TopicsTopicsTopicsOWhy Does the CAN Bus Need Protecting ?OCommon Protection MethodsOSummaryOOWhy Does the CAN Bus Need Protecting ?Why Does the CAN Bus Need Protecting ?OOCommon Protection MethodsCommon Protection MethodsOOSummarySummaryThe rest of the presentation we will look at some of the common protection methods for the CAN Technology : October 2006 CAN ProtectionPage 6 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)6 Transceivers Provide Some ProtectionTransceivers Provide Some Transceivers Provide Some ProtectionProtectionOISO 11898 requires:OShort circuit protection = to +32 VOTransient protection = -150V to +100 VOMCP2551 protection (bus pins)OShort circuit protection = 42 VOTransient protection = 250 VOOISO 11898 requires:ISO 11898 requires:OOShort circuit protection = Short circuit protection = to + to +32 VOOT ransient protection = Transient protection = --150V to +100V150V to +100 VOOMCP2551 protection (bus pins)MCP2551 protection (bus pins)OOShort circuit protection = Short circuit protection = 42V42 VOOT ransient protection = Transient protection = 250V250 VAs mentioned earlier, ISO 11898 is by far the most popular physical layer for CAN.

6 The spec requires that the transceiver be able to withstand short circuit conditions on the bus of -3 volts to +32 volts and transient energy surges of -150 volts to 100 volts. Compatible transceivers must meet these s MCP2551 exceeds the spec. On that device the short circuit protection is + or -42 volts and transient protection of + or -250 , sometimes extra protection is still needed to further improve the robustness of the CAN Technology : October 2006 CAN ProtectionPage 7 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)7 Shielded Twisted Pair CableShielded Twisted Pair CableShielded Twisted Pair CableOShielded cable reduces radiated interference from introducing noise on the common mode voltageOTwisted pair essentially equalizes the noise across both wiresONoise distributed equally is invisible to the differential busOOShielded cable reduces radiated interference from Shielded cable reduces radiated interference from introducing noise on the common mode voltageintroducing noise on the common mode voltageOOTwisted pair essentially equalizes the noise across Twisted pair essentially equalizes the noise across both wiresboth wiresOONoise distributed equally is invisible to the Noise distributed equally is invisible to the differential busdifferential busTwisted pair wiring is a common method to reduce the effects of radiated emissions from external.

7 Twisted pair reduces the chance of noise coupling on the bus lines, and noise that does couple on the line is equally distributed to both wires and is effectively invisible to the differential twisted pair solution is basically a filter and does not provide any protection from high energy electrical Technology : October 2006 CAN ProtectionPage 8 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)8 FiltersFiltersFiltersORC, LC, or Common Mode ChokeOWill attenuate the magnitude of the noise on the CAN busOMay distort the signalODoes not provide voltage clampingOORC, LC, or Common Mode ChokeRC, LC, or Common Mode ChokeOOWill attenuate the magnitude of the noise on Will attenuate the magnitude of the noise on the CAN busthe CAN busOOMay distort the signalMay distort the signalOODoes not provide voltage clampingDoes not provide voltage clampingThe filters discussed here are used to reduce noise at the transceiver bus and LC are Low Pass filters.

8 Common mode chokes attenuate the noise. It is very important for the components to match. Any mismatch will cause unbalanced filtering or attenuation between CANH and CANL pins which will result in distortion of the , these filter circuits do not provide any voltage clamping, they only , the filter components are susceptible to high energy transients that can be damaged. We ll discuss how to protect these circuits Technology : October 2006 CAN ProtectionPage 9 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)9 Common Mode ChokeCommon Mode ChokeCommon Mode ChokeOIncreases Common Mode Rejection Ratio (CMRR) by:OProviding high impedance for common mode signals (bus in recessive state)OProviding low impedance for differential signals (bus in dominant state)OOIncreases Common Mode Rejection Ratio Increases Common Mode Rejection Ratio (CMRR) by:(CMRR) by.

9 OOProviding high impedance for common mode Providing high impedance for common mode signals (bus in recessive state)signals (bus in recessive state)OOProviding low impedance for differential signals Providing low impedance for differential signals (bus in dominant state)(bus in dominant state)Common mode chokes increase the Common Mode Rejection Ratio, or CMRR, by providing high impedance for the common mode signal, that is, when the bus is in a recessive state with no differential voltage between the CANH and CANL pins and providing low impedance for differential signals, that is, when the bus is are common on CAN buses and can implement filtering on the common mode signal without adding a lot of distortion. That s because the coils are usually matched fairly well. Like RC and LC filters, any mismatch in the inductance of the coils can cause distortion of the differential signals.

10 Another thing to be aware of is that the inductance of the coils and the capacitance of the PCB could theoretically create a tank circuit and cause oscillations, although that is Technology : October 2006 CAN ProtectionPage 10 2006 microchip Technology Incorporated. All Rights Reserved. Introduction to Controller Area Network (CAN)10 Split Termination SchemesSplit Termination SchemesSplit Termination SchemesOSplit termination effectively reduces emissions and improves immunityOThe 120 resistor at each end of the bus is split into two 60 resistors with a capacitor to groundOActs as a low pass filterOOSplit termination effectively reduces emissions and Split termination effectively reduces emissions and improves immunityimproves immunityOOThe 120 The 120 resistor at each end of the bus is split resistor at each end of the bus is split into two 60 into two 60 resistors with a capacitor to groundresistors with a capacitor to groundOOActs as a low pass filterActs as a low pass filterCAN Transceiver60 60 CSCAN BusTermination of an ISO 11898-2 bus consists of two 120 ohm resistors-- one placed at each end of the bus to eliminate those single 120 ohm resistors into two 60 ohm