ISL31490E_10 Datasheet, PDF (14/27 Pages) Intersil Corporation – ±60V Fault Protected, 5V, RS-485/RS-422 Transceivers with ±25V Common Mode Range

ISL31490E, ISL31491E, ISL31492E, ISL31493E, ISL31495E, ISL31496E,
Application Information
RS-485 and RS-422 are differential (balanced) data
transmission standards used for long haul or noisy
environments. RS-422 is a subset of RS-485, so RS-485
transceivers are also RS-422 compliant. RS-422 is a
point-to-multipoint (multidrop) standard, which allows
only one driver and up to 10 (assuming one unit load
devices) receivers on each bus. RS-485 is a true
multipoint standard, which allows up to 32 one unit load
devices (any combination of drivers and receivers) on
each bus. To allow for multipoint operation, the RS-485
specification requires that drivers must handle bus
contention without sustaining any damage.
Another important advantage of RS-485 is the extended
common mode range (CMR), which specifies that the
driver outputs and receiver inputs withstand signals that
range from +12V to -7V. RS-422 and RS-485 are
intended for runs as long as 4000’, thus the wide CMR is
necessary to handle ground potential differences, as well
as voltages induced in the cable by external fields.
The ISL3149xE is a family of ruggedized RS-485
transceivers that improves on the RS-485 basic
requirements, and therefore increases system reliability.
The CMR increases to ±25V, while the RS-485 bus pins
(receiver inputs and driver outputs) include fault
protection against voltages and transients up to ±60V.
Additionally, larger than required differential output
voltages (V_OD) increase noise immunity.
Receiver (Rx) Features
These devices utilize a differential input receiver for
maximum noise immunity and common mode rejection.
Input sensitivity is better than ±200mV, as required by
the RS-422 and RS-485 specifications.
Receiver input (load) current surpasses the RS-422
specification of 3mA, and is four times lower than the
RS-485 “Unit Load (UL)” requirement of 1mA maximum.
Thus, these products are known as “one-quarter UL”
transceivers, and there can be up to 128 of these devices
on a network while still complying with the RS-485
loading specification.
The Rx functions with common mode voltages as great
as ±25V, making them ideal for industrial, or long
networks where induced voltages are a realistic concern.
All the receivers include a “full fail-safe” function that
guarantees a high level receiver output if the receiver
inputs are unconnected (floating), shorted together, or
connected to a terminated bus with all the transmitters
disabled (i.e., an idle bus).
Rx outputs feature high drive levels (typically 22mA @
V_OL = 1V) to ease the design of optically coupled
isolated interfaces.
Receivers easily meet the data rates supported by the
corresponding driver, and all receiver outputs are
three-statable via the active low RE input (except on the
ISL31491E).
The Rx in the 250kbps and 1Mbps versions include noise
filtering circuitry to reject high frequency signals. The
1Mbps version typically rejects pulses narrower than
50ns (equivalent to 20Mbps), while the 250kbps Rx
rejects pulses below 150ns (6.7Mbps).
Driver (Tx) Features
The RS-485/RS-422 driver is a differential output device
that delivers at least 1.5V across a 54Ω load (RS-485),
and at least 2.4V across a 100Ω load (RS-422). The
drivers feature low propagation delay skew to maximize
bit width, and to minimize EMI, and all drivers are
three-statable via the active high DE input.
The 250kbps and 1Mbps driver outputs are slew rate
limited to minimize EMI, and to minimize reflections in
unterminated or improperly terminated networks.
Outputs of the ISL31496E and ISL31498E drivers are not
limited, thus faster output transition times allow data
rates of at least 15Mbps.
High Overvoltage (Fault) Protection
Increases ruggedness
Note: The available smaller pitch packages (e.g., MSOP
and TDFN) may not meet the creepage and clearance
(C&C) requirements for ±60V levels. The user is advised
to determine his C&C requirements before selecting a
package type.
The ±60V (referenced to the IC GND) fault protection on
the RS-485 pins, makes these transceivers some of the
most rugged on the market. This level of protection
makes the ISL3149xE perfect for applications where
power (e.g., 24V and 48V supplies) must be routed in the
conduit with the data lines, or for outdoor applications
where large transients are likely to occur. When power is
routed with the data lines, even a momentary short
between the supply and data lines will destroy an
unprotected device. The ±60V fault levels of this family
are at least five times higher than the levels specified
for standard RS-485 ICs. The ISL3149xE protection is
active whether the Tx is enabled or disabled, and even if
the IC is powered down.
If transients or voltages (including overshoots and
ringing) greater than ±60V are possible, then additional
external protection is required.
Widest Common Mode Voltage (CMV)
Tolerance Improves Operating Range
RS-485 networks operating in industrial complexes, or
over long distances, are susceptible to large CMV
variations. Either of these operating environments may
suffer from large node-to-node ground potential
differences, or CMV pickup from external electromagnetic
sources, and devices with only the minimum required
+12V to -7V CMR may malfunction. The ISL3149xE’s
extended ±25V CMR is the widest available, allowing
operation in environments that would overwhelm lesser
transceivers. Additionally, the Rx will not phase invert
(erroneously change state) even with CMVs of ±40V, or
differential voltages as large as 40V.
14
FN7637.0
June 17, 2010