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ISL8485ECB-T(2005) View Datasheet(PDF) - Intersil

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ISL8485ECB-T Datasheet PDF : 14 Pages
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ISL8483E, ISL8485E
Receivers easily meet the data rates supported by the
corresponding driver.
ISL8483E/85E receiver outputs are three-statable via the
active low RE input.
Driver Features
The RS-485/422 driver is a differential output device that
delivers at least 1.5V across a 54load (RS-485), and at
least 2V across a 100load (RS-422). The drivers feature
low propagation delay skew to maximize bit width, and to
minimize EMI.
Drivers of the ISL8483E/85E are three-statable via the active
high DE input.
The ISL8483E driver outputs are slew rate limited to
minimize EMI, and to minimize reflections in unterminated or
improperly terminated networks. Data rate on these slew
rate limited versions is a maximum of 250kbps. Outputs of
the ISL8485E driver are not limited, so faster output
transition times allow data rates of at least 5Mbps.
Data Rate, Cables, and Terminations
RS-485/422 are intended for network lengths up to 4000’,
but the maximum system data rate decreases as the
transmission length increases. Devices operating at 5Mbps
are limited to lengths less than 100’, while the 250kbps
versions can operate at full data rates with lengths in excess
of 1000’.
Twisted pair is the cable of choice for RS-485/422 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
receivers in these ICs.
Proper termination is imperative, when using the 5Mbps
devices, to minimize reflections. Short networks using the
250kbps versions 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 120) 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 transceiver to
the main cable should be kept as short as possible.
Built-In Driver Overload Protection
As stated previously, the RS-485 spec requires that drivers
survive worst case bus contentions undamaged. The
ISL848XE devices meet this requirement via driver output
short circuit current limits, and on-chip thermal shutdown
circuitry.
The driver output stages incorporate short circuit current
limiting circuitry which ensures that the output current never
exceeds the RS-485 spec, even at the common mode
voltage range extremes. Additionally, these devices utilize a
foldback circuit which reduces the short circuit current, and
thus the power dissipation, whenever the contending voltage
exceeds either supply.
In the event of a major short circuit condition, ISL848XE
devices also include a thermal shutdown feature that
disables the drivers whenever the die temperature becomes
excessive. This eliminates the power dissipation, allowing
the die to cool. The drivers automatically reenable after the
die temperature drops about 15 degrees. If the contention
persists, the thermal shutdown/reenable cycle repeats until
the fault is cleared. Receivers stay operational during
thermal shutdown.
Low Power Shutdown Mode (ISL8483E Only)
These CMOS transceivers all use a fraction of the power
required by their bipolar counterparts, but the ISL8483E
includes a shutdown feature that reduces the already low
quiescent ICC to a 1nA trickle. The ISL8483E enters
shutdown whenever the receiver and driver are
simultaneously disabled (RE = VCC and DE = GND) for a
period of at least 600ns. Disabling both the driver and the
receiver for less than 50ns guarantees that the ISL8483E
will not enter shutdown.
Note that receiver and driver enable times increase when
the ISL8483E enables from shutdown. Refer to Notes 5-8, at
the end of the Electrical Specification table, for more
information.
ESD Protection
All pins on these interface devices include class 3 Human
Body Model (HBM) ESD protection structures, but the
RS-485 pins (driver outputs and receiver inputs) incorporate
advanced structures allowing them to survive ESD events in
excess of ±15kV HBM. The RS-485 pins are particularly
vulnerable to ESD damage because they typically connect to
an exposed port on the exterior of the finished product.
Simply touching the port pins, or connecting a cable, can
cause an ESD event that might destroy unprotected ICs.
These new ESD structures protect the device whether or not
it is powered up, protect without allowing any latchup
mechanism to activate, and without degrading the RS-485
common mode range of -7V to +12V. This built-in ESD
protection eliminates the need for board level protection
structures (e.g., transient suppression diodes), and the
associated, undesirable capacitive load they present.
Human Body Model Testing
As the name implies, this test method emulates the ESD
event delivered to an IC during human handling. The tester
delivers the charge stored on a 100pF capacitor through a
1.5kcurrent limiting resistor into the pin under test. The
HBM method determines an IC’s ability to withstand the ESD
events typically present during handling and manufacturing.
The RS-485 pin survivability on this high ESD family has
been characterized to be in excess of ±15kV, for discharges
to GND.
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