The EVAL-CN0313SDPZ is a certified EMC Compliant RS-485 Interface Design Tool.
The main reason for the different protection circuitry is related to the difference in energy levels associated with each of the transient waveforms. According to the IEC61000-4-2 ESD specification, the ESD transient waveform has fast rise times of less than 1 ns and short pulse widths of approximately 60 ns. The individual EFT pulses are described in the IEC61000-4-4 spec as having rise times of 5 ns and a pulse duration of 50 ns, measured between the 50% point on the rising and falling edges of the waveform. The ESD and EFT pulses have similar characteristics of fast rise times and short pulse widths, hence the energy levels in an EFT pulse are similar to those in an ESD pulse.
In IEC61000-4-5 the surge transient has a rise time of the 1.25 µs and the pulse width is 50 µs. The surge pulse energy can have energy levels that are three to four orders of magnitude larger than the energy in an ESD or EFT pulse. Therefore, the surge transient is considered the most severe of the EMC transient specs. Due to the similarities between ESD and EFT, the design of the circuit protection can be similar, however, due to its high energy, surge must be dealt with differently. This is one of the main issues in developing protection circuitry that improves the immunity of RS485 transceiver data ports to all three transients while remaining cost effective.
The first circuit containing the CDSOT23-SM712 and the ADM3485E will protect up to level 4 ESD and EFT, but only level 2 surge. To improve the surge protection level, the protection circuitry gets more complex. The additional transient blocking unit (TBU-CA065-200-WH) and thyristor surge protectors (TISP4240M3BJR-S) or gas discharge tube (2038-15-SM-RPLF) in the remaining two circuits are required to deal with the large energy levels associated with the surge pulse. The two different circuits offer different levels of surge protection while giving level 4 ESD and EFT protection. Each of the circuits were verified by an external independent EMC compliance test house.
In summary due to the large energy contained in the surge pulse compared to the ESD and EFT pulse there is a need for a more complex level of circuit protection when dealing with high levels of surge.