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±60V Fault Protection
The LTC2862-LTC2865 devices answer application needs
for overvoltage fault-tolerant RS485/RS422 transceivers
operating 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 LTC2862-LTC2865 devices may eliminate
field failures due to overvoltage faults without using costly
external protection devices.
The ±60V fault protection of the LTC2862 series 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 LTC2862 series is protected from ±60V faults even with
GND open, or V
open or grounded. Additional precautions
must be taken in the case of V
present and GND open.
The LTC2862 series 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
present is anticipated.
The high voltage rating of the LTC2862 series 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 LTC2862-LTC2865
devices feature an extended
common mode operating
range of –25V to 25V. This extended common mode
range allows the LTC2862-LTC2865 devices to transmit
and receive under conditions that would cause data errors
and possible device damage in competing products.
±15kV ESD Protection
The LTC2862 series devices feature exceptionally robust
ESD protection.
The transceiver interface pins (A,B,Y,Z)
feature protection to ±15kV HBM with respect to GND
without latchup or damage, during all modes of operation
or while unpowered. All the other pins are protected to ±8kV
HBM to make this a component capable of reliable operation
under severe environmental conditions.
The driver provides full RS485/RS422 compatibility. When
enabled, if DI is high, Y–Z is positive for the full-duplex
devices (LTC2863-LTC2865) and A–B is positive for the
half-duplex device (LTC2862).
When the driver is disabled, both outputs are high-
impedance. For the full-duplex devices, the leakage on
the driver output pins is guaranteed to be less than 30µA
over the entire common mode range of –25V to 25V. On
the half-duplex LTC2862, the impedance is dominated by
the receiver input resistance, R
Driver Overvoltage and Overcurrent Protection
The driver
outputs are protected from short circuits to any
within the Absolute Maximum range of –60V to
60V. The maximum current in a fault condition is ±250mA.
The driver includes a progressive foldback current limiting
circuit that continuously reduces the driver current limit
with increasing output fault voltage. The fault current is
less than ±15mA for fault voltages over ±40V.
All devices also feature thermal shutdown protection that
disables the driver and receiver in case of excessive power
dissipation (see Note 4).