Application Note No. 104

Transcription

1 Application Note, Rev. 1.0, August 2007 Application Note No channel bi/uni-directional TVS diodes for ESD protection in /LIN bus applications ESD24VS2B, ESD24VS2U Small Signal Discretes

2 Edition Published by Infineon Technologies AG München, Germany Infineon Technologies AG All Rights Reserved. LEGAL DISCLAIMER THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE IMPLEMENTATION OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE REGARDED AS ANY DESCRIPTION OR WARRANTY OF A CERTAIN FUNCTIONALITY, CONDITION OR QUALITY OF THE INFINEON TECHNOLOGIES COMPONENT. THE RECIPIENT OF THIS APPLICATION NOTE MUST VERIFY ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. INFINEON TECHNOLOGIES HEREBY DISCLAIMS ANY AND ALL WARRANTIES AND LIABILITIES OF ANY KIND (INCLUDING WITHOUT LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE. Information For further information on technology, delivery terms and conditions and prices please contact your nearest Infineon Technologies Office ( Warnings Due to technical requirements components may contain dangerous substances. For information on the types in question please contact your nearest Infineon Technologies Office. Infineon Technologies Components may only be used in life-support devices or systems with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system, or to affect the safety or effectiveness of that device or system. Life support devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustain and/or protect human life. If they fail, it is reasonable to assume that the health of the user or other persons may be endangered.

3 Application Note No. 104 Revision History: , Rev. 1.0 Previous Version: Page Subjects (major changes since last revision) Application Note 3 Rev. 1.0,

4 TVS Diodes for ESD Protection in /LIN Bus 1 TVS Diodes for ESD Protection in /LIN Bus Applications High-Speed Bus Low-Speed Fault Tolerant Bus Single wire Bus LIN Bus Introduction SOT23 Electrostatic discharge (ESD) protection is one of the main issues in electronics applications. It gains more importance in high reliability applications such as automotive electronics. Controller Area Network () is a communication robust differential serial bus standard [1] for connecting electronic control units in electromagnetically noisy environments. It creates a means for various control units to communicate with each other. In general transceivers have integrated ESD protection circuitry up to several kvs. This may be sufficient for handling but not for a reliable system operation. Currently most of automobile manufacturers set a requirement of at least 6 kv ESD protection. This can be achieved by using an external circuitry containing varistors or transient voltage suppression (TVS) diodes. Varistors have high and imprecise clamping voltages and they face also to performance degradation in long term. This is a big risk because the transceiver ICs in the network may be easily damaged. On the other hand, TVS diodes have low, precise and robust clamping voltages fitting perfectly to high reliability systems. They are also faster than varistors which is an important aspect because long turn on times of varistors will result in high peak current flowing into transceivers and thus increase damage probability. Another advantage of TVS diodes compared to varistors is their much lesser leakage current meaning high system efficiency. However, both varistors and TVS diodes suffer from their high parasitic capacitance, which may be a limiting factor for achievable data rate and number of nodes in bus. TVS diodes are suitable not only for automotive networks but also for other applications such as industrial automation networks (Smart Distribution Systems, CanOpen, DeviceNet etc.), marine applications, other transportation vehicles (trains, trucks, etc.), building automation, server/it equipment monitoring, elevators, electric wheelchairs etc. Local Interconnect Network (LIN) bus is another widely used communication bus in automobiles. It is a single-ended system used for low data rate communications. LIN transceivers have similar ESD protection requirements as transceivers. Regarding ESD protection TVS diodes are the best solution for both and LIN transceivers. In the following general information is provided about Automotive bus and Infineon Technologies TVS diode ESD24VS2B for bus protection. ESD24VS2B is a dual line bi-directional device designed for protection against ESD, Surge and Electrical Fast Transient (EFT) pulses. It provides ESD contact discharge immunity up to 30kV, Surge pulse peak current and power capability of more than 5A and 230W per line respectively, and EFT pulse immunity of more than 4kV. It has low clamping voltage of less than 41 V and extremely low reverse current of less than 1nA. The package is the widely used industry standard Surface-Mounted Device SOT23. ESD24VS2B is a reliable and highly efficient solution for transceiver protection. Another member of the ESD24V family is the two line, uni-directional ESD24VS2U in SOT23 package. This device can be used in systems requiring single channel bi-directional protection e.g. single wire or LIN bus. Application Note 4 Rev. 1.0,

5 bus in automotive applications 2 bus in automotive applications The number of electronic systems in todays automobiles increase gradually. They must have information exchange for a proper operation. These systems were connected with individual cables previously. Nowadays some bus systems with digital communication protocols are used to avoid massive usage of wires between modules. The most widely used protocol is which is offering a communication data rate of up to 1Mbits/s. There are different networks such as low-speed fault tolerant, high-speed and single wire bus, which are suitable for different applications. There are also LIN systems for low data rate transmission up to 20 kbits/s. In todays automobiles 40 to 70 microcontrollers are used which communicate via these networks. If 15 to 20 nodes per bus are considered, then the cars may have 3 to 4 networks for data communications. Figure 1 shows an example bus in automotive applications. The control units are microprocessors for dedicated applications. Each control unit needs a transceiver in order to transmit and receive data via bus. For example high-speed achieving 1 Mbits/s data rate is used for real time systems such as engine control, anti-lock brakes whereas low-speed with its data rate up to 125 kbits/s is used for lighting, comfort units, infotainment etc. Single wire is a possible low cost solution with data rate of 33.3 kbits/s. Low-speed fault tolerant bus is similar to high-speed but it is capable of switching single wire transmission in case of a malfunction such as short circuits to ground. Engine Control Anti-Lock Brakes Lighting Air Condition Power Locks High-Speed Dash Board Low-Speed Active Suspension Transmission Control Power Seats Power Windows AN104 bus_ Figure 1 Automotive bus system with various functions Figure 2 is a simplified block diagram of a transceiver. It shows the connection of transceiver to the bus and its signals. Differential signals with twisted pair lines are used in bus (except single wire ) in order to increase the immunity against noisy communication environment due to strong electromagnetic emission. The information is transferred via high and low signals named H and L respectively. If H is close to Vcc and L to ground, then this mode is called dominant, if both signals are close to the bias voltage Vcc/2, then the mode is recessive. The recessive state represents logic 1 and the dominant state logic 0 in most systems. receiver obtains data by subtracting H signal from L. This subtraction results in high common mode parasitic rejection and improves system immunity against disturbances. For details please refer to [2]. Application Note 5 Rev. 1.0,

6 bus in automotive applications 120Ohm Vcc Transmitter V CC H BUS H L Output Stage Driver GND L bus dominant bus recessive t 120Ohm Receiver Figure 2 transceiver connected to the bus via twisted pair lines and the signals on differential lines AN104 TRX_ Maximum data rate in a bus depends mainly on the bus length and the total capacitive load of the bus. The cable length is important because a transmitting node must be able to see a response in a given bit period meaning that the total delay in both directions must be accounted for. Figure 3 shows the relation between bus data rate and the bus length under ideal conditions. Data rate is reduced very fast with increasing bus length. For maximum data rate of 1Mbits/s the calculated bus length is about 50m. However, in reality the bus length is much shorter than the given theoretical values because the data rate strongly depends on system configuration, number of nodes, terminations etc. The capacitive load of the bus is also varying with the bus length due to twisted pair cable capacitance of about ~100pF/m. The other contributors of total load capacitance are the transceivers and their external ESD protection circuitry such as TVS diodes. High total load capacitance means lower data rates because charging and discharging of the capacitance takes long time. Keeping the data rate high will increase the error rate in a bus with high capacitive load because of H and L signal degradation. Therefore the line capacitance of TVS diodes should be low for better system performance Bit Rate [kbps] Bus Length [m] AN104 datarate Figure 3 Calculated maximum data rate versus bus length in bus under ideal conditions Application Note 6 Rev. 1.0,

7 ESD protection for s 3 ESD protection for s ESD is one of the most critical disturbances in a vehicle. Since the systems must always be functional in the whole vehicle, the risk of malfunction due to ESD must be minimized. In this document two types of ESD are considered. The unpowered situation corresponds to handling (JESD22-A114D, Human Body Model (HBM), 100pF/1.5kOhm [3]) whereas the powered situation corresponds to system level specification (IEC Gun Test, 150pF/330Ohm [4]). Normally all transceivers must have integrated ESD protection of +/-2 kv HBM, which is the general requirement for handling. This is adequate to protect against damage during manufacture of the module. Most vehicle manufacturers require at least 4kV IEC of immunity from the transceiver. Neither of these levels is sufficient on it's own as system specifications for operation in a vehicle are set by most of the automobile manufacturers of at least 6kV IEC These requirements drive the need for external ESD protection. There are a few choices for this external protection including TVS diodes and varistors. Depending on protection requirement some more external components for filtering purposes may be required. In general TVS diodes are much faster than varistors. They have low, precise and robust clamping voltages and low leakage currents. Therefore they are preferred in automotive bus applications Figure 4 shows a general ESD protection circuitry for transceivers. TVS diodes are connected in shunt and choke or ferrite is connected in series. TVS diodes lead the ESD power to ground while choke and ferrite behave like a filter and reduce the energy of pulses on the lines connected to transceiver ports. They delay the ESD signal (especially high frequency part) to give some time to TVS diodes in order to absorb the current. If the transceiver has internal ESD protection such as thyristor, zener diode etc., then external ESD protection can be more relaxed. There will be still some protection in case that some ESD current flows into the transceiver. Otherwise the external ESD protection circuitry must fulfil more stringent requirements. Normally TVS diodes are faster than internal ESD protection circuits but combination of TVS diodes and choke or ferrite results in more reliable ESD protection for transmitters due to finite response times of TVS diodes. Choke or Ferrite H L TVS Diodes Figure 4 AN104 TRX2 Simplified block diagram of a transceiver with external ESD protection circuitry Figure 5 shows ESD measurement results obtained with the Infineon Technologies transceiver TLE6250G to see the ESD performance comparison of TVS diodes and varistors. The used TVS diode has breakdown voltage of 20 V and the varistor has capacitance of 30pF. An ESD pulse of 4kV IEC is applied on the _H pin, and the current is measured on the transceiver ground line. Grey curve is the reference and shows the current flowing into transceiver if no external ESD protection is used, which is about 7 A. TVS diode with choke (brown colored curve) results in almost zero current going to the transceiver. However, varistor with choke (green colored curve) let too much current flow through transceiver. The current flowing through transceiver increases gradually up to 3 A at 90 ns and it decreases with decaying ESD pulse. This behavior indicates that the current flowing through will increase further with increasing ESD test pulse. Another varistor test has been done with two chokes in series (yellow colored curve) in order to improve protection but there is still about 1.5 A peak current flowing through transceiver. This is comparable to the test result obtained with just TVS diode (blue colored curve). This means TVS diodes can protect a transceiver as good as the Application Note 7 Rev. 1.0,

8 Application example combination of varistors and two chokes. It is clearly seen from measurement results that TVS diode absorbs much more current and it is much faster than the varistors. The measurements have been repeated with ESD pulse values higher than 4kV IEC and it has been observed that TVS diode with choke provides protection up to 30kV while varistor with choke achieves only up to 22kV. TVS diode and choke provide the best protection for transceiver IC. Another important aspect in ESD protection is PCB design. ESD components must be located close to I/O points. This will eliminate ESD pulse at the point where it appears. Short length from ESD event point to TVS diode for example will reduce connection impedance and thus the power dissipated on the PCB [2] Without ext. ESD protection TVS diode TVS diode + choke Varistor + choke Varistor + 2 chokes in series [A] [ns] AN104 ESD Figure 5 ESD current flowing into transceiver with and without external ESD protection 4 Application example Figure 6 shows Infineon TVS diode ESD24VS2B in a bus application. ESD24VS2B comes in a robust and automotive qualified SOT23 package. Since ESD24VS2B is dual channel, one device is sufficient for a differential transceiver. H and L lines need separate ESD protection paths to ground as shown in Figure 6. It shows the bi-directional configuration with two diodes in series (reverse order) so that it can be used for both positive and negative ESD pulses. The clamping voltage of an ESD protection diode in bi-directional configuration is higher than the clamping voltage of a single diode because of higher resistance. Connecting two diodes in series reduces the line capacitance (equal to half of a single diode if the diodes have equal size). ESD24VS2B has bidirectional line capacitance of 24pF, which is suitable for latest generation bus communication interface. The protection diode should be placed very close to the location where the ESD or other transients can occur to keep loops and inductances as small as possible. Pin 3 should be connected directly to a ground plane on the board. Application Note 8 Rev. 1.0,

9 ESD and EMI pulse tests Bus 2 1 _H line _L line I/O I/O transceiver ESD24VS2B 3 AN104_ESD24VS 2B bus_ appl Figure 6 bus protection application example with ESD24VS2B 5 ESD and EMI pulse tests ESD24VS2B is qualified for the following system level specifications: IEC standard up to +/-30 kv, IEC standard up to +/-4 kv (5/50 ns), IEC standard up to 5A (8/20 us). ISO pulse capability (ISO7637-2: 2004(E)): Pulse 1 (max. -50 V), Pulse 2 (max. 125 V), Pulses 3a and 3b The test conditions and the obtained results for ESD (IEC ), EFT (IEC ) and surge (IEC ) pulses are listed in Table 1. Table 1 ESD, EFT and Surge test measurement results Test Condition Value IEC Air/contact +/-30 kv IEC (5/50 ns) Pulse rate: 5 khz Duration: 1 min +/-4 kv IEC (8/20 us) # of surges: 10 Repetition rate: 1 min ESD24VS2B has also been tested with ISO pulses according to ISO7637-2: 2004(E). Figure 7, Figure 8 and Figure 9 show the measured test pulses and corresponding clamping voltage curves. In Figure 7 on the left hand side the applied ISO Pulse 1 with an open circuit voltage of -50 V is shown whereas on the right hand side the achieved performance by using ESD24VS2B is shown. ESD24VS2B limits the voltage to V. In Figure 8 the similar measurement with ISO Pulse 2 is shown. The open circuit voltage on the left hand side is 125 V and the achieved clamping voltage is 36.0 V. Figure 9 shows the performance with ISO Pulse 3b. The open circuit voltage is 800 V and the clamping voltage is 40 V. The performance with ISO Pulse 3a is similar to 3b. ISO test conditions and obtained results are summarized in Table 2. The results reveal that ESD24VS2B can protect transceiver also against ISO Pulses to some levels. Since the level of possible ISO pulses reaching transceiver input is > 5 A Application Note 9 Rev. 1.0,

10 ESD and EMI pulse tests expected to be attenuated (depending on system architecture), it can be possible to protect bus without additional ISO protection circuitry. Open circuit voltage: -50 V With connected diode: V Figure 7 ISO Pulse 1 test according to ISO7637-2: 2004(E) Open circuit voltage: 125 V With connected diode: 36.0 V Figure 8 ISO Pulse 2 test according to ISO7637-2: 2004(E) Application Note 10 Rev. 1.0,

11 ESD and EMI pulse tests Open circuit voltage: 800 V With connected diode: 40.0 V Figure 9 ISO Pulse 3b test according to ISO7637-2: 2004(E) Table 2 ISO Pulse (ISO7637-2: 2004(E)) test measurement results Test Condition Value Pulse 1 Ri: 10 ohm td: 2 ms # of pulses: V Pulse 2 Pulse 3a Pulse 3b Ri: 10 ohm td: 50 us # of pulses: 4000 Ri: 50 ohm td: 100 ns Test duration: 10 min Ri: 50 ohm td: 100 ns Test duration: 10 min 125 V -800 V 800 V Application Note 11 Rev. 1.0,

12 ConclusionsReferences 6 Conclusions bus is a widely used communication system in automotive applications. High reliability is a must for this application. Precise and durable low clamping voltages of TVS diodes make them first choice for protection in highly reliable systems. Their fast response time and low leakage current values are also very important for systems. The best ESD protection is achieved if they are used together with a common mode choke. It has been shown that dual line bi-directional TVS diode ESD24VS2B from Infineon Technologies can improve transceiver immunity against ESD, surge, EFT and ISO pulses significantly. ESD24VS2U is another member of the ESD24V family with two line, uni-directional configuration. It provides same bi-directional ESD protection as ESD24VS2B if it is connected in series (reverse order). This is applicable to single channel systems such as single wire and LIN bus. References [1] International Standard ISO , Road Vehicles - Controller Area Network () - part 2: High speed medium access unit, [2] High speed s Application Note, Infineon Technologies, Sep ( [3] JESD22-A114D, Electrostatic Discharge (ESD) sensitivity testing Human Body Model (HBM), JEDEC Solid State Technology Association, March [4] IEC , Electromagnetic Compatibility (EMC) Part 4: Testing and measurement techniques Section 2: Electrostatic discharge immunity test, International Electrotechnical Commission, Application Note 12 Rev. 1.0,