CAN Newsletter magazine
In the harsh automotive environment, careful design and proper TVS (transient voltage suppression) diodes are crucial for safeguarding the communication networks and interfaces. Semtech's portfolio of TVS products protects CAN.
Motor vehicles have gone through much progress since their inception. Modern cars include autonomous and semi-autonomous driving, anti-lock braking systems, electric power steering, forward and rear collision warning, lane assistant, autonomous parking assistant, and automatic emergency braking. Advanced features such as GPS navigation, interior mood lighting, surround-view camera, advanced infotainment system, active antenna are standard in most modern vehicles.
Each vehicle feature is usually monitored and con- trolled by an individual electronic control unit (ECU). In a modern vehicle, there are about 50 ECUs to 100 ECUs, with each ECU responsible for one or more functions. The ECU works with multiple sensors such as the engine temperature sensor, air pressure sensor, door sensor, etc. It receives information from the sensors and adjusts the vehicle parameters under control accordingly. Sometimes one ECU may need to communicate with other ECUs to perform a designated function. For example, if you forget your car key in the ignition, the corresponding engine control ECU communicates with the door ECU to keep the door open and the speaker ECU to sound the alarm.
There are several in-vehicle network (IVN) protocols for data transmission between ECUs inside a vehicle. The most popular communication mechanism in modern vehicles is CAN. The bi-directional serial communication network allows ECUs to communicate without using any complex wiring. A twisted pair cable with a characteristic impedance of 120 Ω is used to transmit the data. One of the wires is called the CAN_Low (CAN_L) and the other, CAN_High (CAN_H), with a data transfer rate of 1 Mbit/s. The ECU is connected to the CAN network via a CAN controller, similar to a micro-controller that handles all the necessary data processing activities. There is a CAN transceiver that interfaces between the CAN controller and the CAN network. It converts the transistor-transistor logic (TTL) signal into the actual differential voltage signal for the CAN network to read and interpret. The CAN architecture is shown in Figure 1.
When designing a CAN interface system in a harsh auto- motive environment with 50 ECUs to 100 ECUs, ensuring sufficient protection from electrical overstress events (EOS) is imperative. One of the primary causes of EOS is electrostatic discharge (ESD). Moreover, with the miniaturization of the electronic components, it is even more essential to protect the components from ESD threats and meet modern-day vehicles' safety and reliability requirements.
Transient protection can be achieved by placing a transient voltage suppression diode on a CAN network data line to protect against transient events during the fast rise time in less than a nanosecond. Under normal operating conditions, the TVS diode presents a high impedance path to the protected circuit, so the device appears as an open circuit. It does not interfere with the rest of the circuit. During a transient event, the voltage on the terminals of the protected CAN transceiver can exceed safe operational limits. The TVS diode offers protection by providing a low impedance path so that the transient current is diverted away from the transceiver circuit. At the supposition of the ESD event, the TVS diode reverts to a high impedance state.
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