LTC2862A
12
2862af
For more information www.linear.com/LTC2862A
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. The limited overvoltage tolerance of standard RS485
transceivers makes implementation of effective external
protection networks difficult without interfering with proper
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
using a high-voltage BiCMOS integrated circuit technology.
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 V
CC
(GND open faults not tested in
production). Additional precautions must be taken in the
case of V
CC
present and GND open. The LTC2862A chip
will protect itself from damage, but the chip ground current
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 V
CC
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, V
CC
(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)
