ISL3282EMRTEP Datasheet, PDF (5/8 Pages) Intersil Corporation – RS-485/RS-422 Receiver

ISL3282EMRTEP
ESD protection eliminates the need for board level protection
structures (e.g., transient suppression diodes), and the
associated, undesirable capacitive load they present.
IEC61000-4-2 Testing
The IEC61000 test method applies to finished equipment, rather
than to an individual IC. Therefore, the pins most likely to suffer
an ESD event are those that are exposed to the outside world (the
RS-485 pins in this case), and the IC is tested in its typical
application configuration (power applied) rather than testing
each pin-to-pin combination. The lower current limiting resistor
coupled with the larger charge storage capacitor yields a test
that is much more severe than the HBM test. The extra ESD
protection built into this device’s RS-485 pins allows the design
of equipment meeting level 4 criteria without the need for
additional board level protection on the RS-485 port.
AIR-GAP DISCHARGE TEST METHOD
For this test method, a charged probe tip moves toward the IC pin
until the voltage arcs to it. The current waveform delivered to the
IC pin depends on approach speed, humidity, temperature, etc.,
so it is difficult to obtain repeatable results. The A and B RS-485
pins withstand ±16.5kV air-gap discharges.
CONTACT DISCHARGE TEST METHOD
During the contact discharge test, the probe contacts the tested
pin before the probe tip is energized, thereby eliminating the
variables associated with the air-gap discharge. The result is a
more repeatable and predictable test, but equipment limits
prevent testing devices at voltages higher than ±9kV. The
ISL3282EMRTEP can survive ±9kV contact discharges on the RS-
485 pins.
Data Rate, Cables, and Terminations
RS-485, RS-422 are intended for network lengths up to 4000’,
but the maximum system data rate decreases as the
transmission length increases. Networks operating at 16Mbps
are limited to lengths less than 100’, while a 250kbps network
that uses slew rate limited transmitters can operate at that data
rate over lengths of several thousand feet.
Twisted pair is the cable of choice for RS-485, RS-422 networks.
Twisted pair cables tend to pick up noise and other
electromagnetically induced voltages as common mode signals,
which are effectively rejected by the differential receiver in these ICs.
To minimize reflections, proper termination is imperative for high
data rate networks. Short networks using slew rate limited
transmitters need not be terminated, but terminations are
recommended unless power dissipation is an overriding concern.
In point-to-point, or point-to-multipoint (single driver on bus)
networks, the main cable should be terminated in its
characteristic impedance (typically 120Ω) at the end farthest
from the driver. In multi-receiver applications, stubs connecting
receivers to the main cable should be kept as short as possible.
Multipoint (multi-driver) systems require that the main cable be
terminated in its characteristic impedance at both ends. Stubs
connecting a transmitter or receiver to the main cable should be
kept as short as possible.
Low Power Shutdown Mode
This BiCMOS receiver uses a fraction of the power required by its
bipolar counterparts, and include a shutdown feature that
reduces the already low quiescent I_CC to a 20µA trickle. They
enter shutdown whenever the receiver is disabled (RE = V_CC ).
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FN7595.1
February 15, 2011