isl83086e-isl83088e Intersil Corporation, isl83086e-isl83088e Datasheet - Page 12

• Manufacturer Part Number: isl83086e-isl83088e
• Description: +-15kv Esd, 5v, Full Fail-safe, 1/8 Unit Load, Rs-485/rs-422 Transceivers
• Manufacturer: Intersil Corporation

Hot Plug Function
When a piece of equipment powers up, there is a period of
time where the processor or ASIC driving the RS-485 control
lines (DE, RE) is unable to ensure that the RS-485 Tx and
Rx outputs are kept disabled. If the equipment is connected
to the bus, a driver activating prematurely during power up
may crash the bus. To avoid this scenario, the ISL83080,
ISL83082, ISL83083, ISL83085 versions incorporate a “Hot
Plug” function. Circuitry monitoring V
power up and power down, the Tx and Rx outputs remain
disabled, regardless of the state of DE and RE, if V
than ~3.4V. This gives the processor/ASIC a chance to stabilize
and drive the RS-485 control lines to the proper states.
ESD Protection
All pins on these 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.
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. Devices operating at 10Mbps
are limited to lengths less than 100’, while the 115kbps
2.5
FIGURE 7. HOT PLUG PERFORMANCE (ISL83080E) vs
5
0
V
A/Y
ISL83080E, ISL83082E, ISL83083E, ISL83084E, ISL83085E, ISL83086E, ISL83088E
RO
CC
DEVICE WITHOUT HOT PLUG CIRCUITRY
(ISL83086E)
3.4V
TIME (40µs/DIV)
ISL83080E
ISL83080E
12
CC
3.2V
ensures that, during
R
R
DI = V
L
L
CC
= 1k:
= 1k:
CC
is less
2.5
2.5
5
0
5
0
versions can operate at full data rates with lengths of several
1000’.
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 receivers in these ICs.
Proper termination is imperative (when using the 10Mbps
devices) to minimize reflections. Short networks using the
115kbps 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 specification requires that
drivers survive worst case bus contentions undamaged.
These 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 specification, 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, 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 re-enable after the die temperature
drops about 15°. If the co ntention persists, the thermal
shutdown/re-enable cycle repeats until the fault is cleared.
Receivers stay operational during thermal shutdown.
Low Power Shutdown Mode
These CMOS transceivers all use a fraction of the power
required by their bipolar counterparts, but they also include a
shutdown feature - except for the ISL83084E - that reduces
the already low quiescent I
devices enter shutdown whenever the receiver and driver
are simultaneously disabled (RE = V
a period of at least 600ns. Disabling both the driver and the
receiver for less than 60ns guarantees that the transceiver
will not enter shutdown.
CC
to a 70nA trickle. These
CC
and DE = GND) for
February 27, 2008
FN6085.8