LTC2855 datasheet(19/24 Pages) AD

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LTC2862A

Rev. A

For more information www.analog.com

A protection network shown in the Typical Applications

section (Network for IEC Level 4 Protection Against 5kV

Surge, 5kV EFT and 30kV IEC ESD Plus ±360V Overvoltage

Protection) meets this challenge. The network provides

the following protection:

• IEC 61000-4-2 ESD Level 4: ±30kV contact, ±30kV

air (line to GND, direct discharge to bus pins with

transceiverandprotectioncircuitmountedonaground

referenced test card per Figure 4 of the standard)

• IEC 61000-4-4 EFT Level 4: ±5kV (line to GND, 5kHz

repetition rate, 15ms burst duration, 60 second test

duration,dischargecoupledtobuspinsthrough100pF

capacitor per paragraph 7.3.2 of the standard)

• IEC61000-4-5SurgeLevel4:±5kV(linetoGND,lineto

line, 8/20µs waveform, each line coupled to generator

through 80Ω resistor per Figure 14 of the standard)

This protection circuit adds only ~8pF of capacitance per

line(linetoGND),therebyprovidinganextremelyhighlevel

ofprotectionwithoutsignificantimpacttotheperformance

of the LTC2862A transceivers at high data rates.

APPLICATIONS INFORMATION

The gas discharge tubes (GDTs) provide the primary

protection against electrical surges. These devices

provide a very low impedance and high current carrying

capability when they fire, safely discharging the surge

current to GND. The transient blocking units (TBUs) are

solid state devices that switch from a low impedance pass

through state to a high impedance current limiting state

when a specified current level is reached. These devices

limit the current and power that can pass through to the

secondary protection. The secondary protection consists

of a bidirectional thyristor, which triggers above 35V to

protectthebuspinsoftheLTC2862Atransceiver.Thehigh

trigger voltage of the secondary protection maintains the

full ±25V common mode range of the receivers. The final

component of the network is the metal oxide varistors

(MOVs) which are used to clamp the voltage across the

TBUs to protect them against fast ESD and EFT transients

which exceed the turn-on time of the GDT.

The high performance of this network is attributable to

the low capacitance of the GDT and thyristor primary

and secondary protection devices. The high capacitance

MOV floats on the line and is shunted by the TBU, so it

contributes no appreciable capacitive load on the signal.

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