LTC2864 datasheet(13/24 Pages) AD

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LTC2862A

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

±60V Fault Protection

The LTC2862A is an improved overvoltage fault-tolerant

RS485/RS422 transceiver that operates from 3V to 5.5V

power supplies. Industrial installations may encounter

common mode voltages between nodes far greater than

the –7V to 12V range specified by the RS485 standards.

Standard RS485 transceivers can be damaged by voltages

above their typical absolute maximum ratings of –8V to

12.5V.ThelimitedovervoltagetoleranceofstandardRS485

transceivers makes implementation of effective external

protectionnetworksdifficultwithoutinterferingwithproper

data network performance within the –7V to 12V region of

RS485 operation. Replacing standard RS485 transceivers

with the rugged LTC2862A devices may eliminate field

failures due to overvoltage faults without using costly

external protection devices.

The ±60V fault protection of the LTC2862A is achieved by

usingahigh-voltageBiCMOSintegratedcircuittechnology.

The naturally high breakdown voltage of this technology

provides protection in powered-off and high-impedance

conditions. The driver outputs use a progressive foldback

current limit design to protect against overvoltage faults

while still allowing high current output drive.

The LTC2862A is protected from ±60V faults even with

the loss of GND or VCC (GND open faults not tested in

production). Additional precautions must be taken in the

case of VCC present and GND open. The LTC2862A chip

willprotectitselffromdamage,butthechipgroundcurrent

may flow out through the ESD diodes on the logic I/O pins

and into associated circuitry. The system designer should

examine the susceptibility of the associated circuitry to

damage if the condition of a GND open fault with VCC

present is anticipated.

The high voltage rating of the LTC2862A makes it simple

to extend the overvoltage protection to higher levels using

external protection components. Compared to lower

voltage RS485 transceivers, external protection devices

with higher breakdown voltages can be used, so as not to

interfere with data transmission in the presence of large

common mode voltages. The Typical Applications section

shows a protection network against faults up to ±360V

peak, while still maintaining the extended ±25V common

mode range on the signal lines.

±25V Extended Common Mode Range

To further increase the reliability of operation and extend

functionality in environments with high common mode

voltages due to electrical noise or local ground potential

differences due to ground loops, the LTC2862A features

an extended common mode operating range of –25V

to 25V. This extended common mode range allows the

LTC2862A to transmit and receive under conditions that

would cause data errors and possible device damage in

competing products.

±40kV ESD Protection

The LTC2862A features exceptionally robust ESD

protection. The transceiver interface pins (A,B) feature

protection to ±40kV HBM with respect to GND, VCC (with a

1µF capacitor to GND), A or B without latchup or damage,

during all modes of operation or while unpowered. All

the other pins are protected to ±15kV HBM to make this

a component capable of reliable operation under severe

environmental conditions.

Level 4 IEC ESD and EFT Protection

The improved ESD protection of the LTC2862A provides a

high level of protection in the IEC ESD and EFT (Electrical

Fast Transient) tests. The IEC ESD stress exceeds that of

the HBM test in peak current, amplitude, and rise time,

while the EFT test provides a prolonged repetitive stress.

Combined with the HBM test, the IEC tests help ensure

that the LTC2862A is robust under a wide range of real

world hazards. The LTC2862A passes the following tests

on the A, B pins:

• IEC 61000-4-2 Edition 2.0 2008-12 ESD Level 4: ±8kV

contact (A or B to GND, direct discharge to bus pins

with transceiver and protection circuit mounted on a

test card with a low impedance ground discharge path

from board GND to ESD gun return lead, per Figure 4

of the standard)

• IEC 61000-4-4 Second Edition 2004-07 EFT Level 4:

±5kV (line to GND, 5kHz repetition rate, 15ms burst

duration, 60 second test duration, discharge coupled

to bus pins through 100pF capacitor per paragraph

7.3.2 of the standard)

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