SLYT441A November 2011 – November 2025 ISO7242C , SN65C1167
Electrical overstress transients caused by electrostatic discharge (ESD), switching of inductive loads, or lightning strikes will corrupt data transmission and damage bus transceivers unless effective measures are taken to diminish their impact. Modern transient-voltage suppressors, such as the ones in Figure 3, are the preferred protection components for high-speed data transmission due to their low capacitance, which allows them to be designed into every node of a multinode network without requiring a reduction in data rate.
Depending on the power rating of the transient-voltage suppressor chosen, the maximum clamp voltages can range from 25 to 35 V, which is higher than a standard transceiver’s maximum bus voltage of 14 V. In this case, the internal protection circuit of the transceiver must absorb the remaining clamp energy to protect the device from damage.
For ESD and burst transients, the clamp energy is rather low due to the short pulse duration and does not pose a problem to the internal ESD cells. Clamp energy from surge transients, however, can present a serious challenge due to the much longer pulse duration. For transceivers specified with low ESD immunity, series resistors might be necessary to reduce the remaining current flowing into the transceiver. Common resistor values range from 5 to 10 Ω. Note that these resistors must be surge-rated to provide high pulse robustness.
Although the transient-voltage suppressor’s diodes divert large transient currents to ground, it must be ensured that these currents are further diverted to true earth potential without disturbing the ground reference of the remaining circuitry. Often this is accomplished by implementing a high-voltage capacitor that has one plate connected to ground and the other plate connected to a protective-earth (PE) island. This island is then connected via a short, low-inductance earthing wire to the PE terminal of the local mains supply.
In addition to the suppressor’s action on the bus side, further protection against signal degradation is required on the transceiver’s single-ended sides. This is accomplished with R-C low-pass filters, which filter transient remnants in the reception path and stop high-frequency noise from entering the transmission path.