ISL32490E Datasheet, PDF (13/21 Pages) Intersil Corporation – ±60V Fault Protected, 5V, RS-485/RS-422 Transceivers with ±25V CMR

ISL32490E, ISL32492E, ISL32493E, ISL32495E, ISL32496E, ISL32498E
than ≈3.5V. This gives the processor/ASIC a chance to stabilize and
drive the RS-485 control lines to the proper states. Figure 9
illustrates the power-up and power-down performance of the
ISL3249xE compared to an RS-485 IC without the Hot Plug feature.
3.5V
2.8V
V_CC
5.0
2.5
A/Y
0
ISL83088E
ISL3249xE
RO
ISL3249xE
DE, DI = V_CC
RE = GND
5.0
2.5
0
R_L = 1kΩ
5.0
R_L = 1kΩ
2.5
0
TIME (40µs/DIV)
FIGURE 9. HOT PLUG PERFORMANCE (ISL3249xE) vs
ISL83088E WITHOUT HOT PLUG CIRCUITRY
ESD Protection
All pins on these devices include class 3 (>8kV) Human Body
Model (HBM) ESD protection structures that are good enough to
survive ESD events commonly seen during manufacturing. Even
so, the RS-485 pins (driver outputs and receiver inputs)
incorporate more advanced structures, allowing them to survive
ESD events in excess of ±16.5kV HBM (±15kV for full-duplex
version). The RS-485 pins are particularly vulnerable to ESD
strikes 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, and without interfering with the
exceptional ±25V CMR. This built-in ESD protection minimizes
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 feet, but the maximum system data rate decreases as the
transmission length increases. Devices operating at 15Mbps
may be used at lengths up to 150 feet (46m), but the distance
can be increased to 328 feet (100m) by operating at 10Mbps.
The 1Mbps versions can operate at full data rates with lengths up
to 800 feet (244m). Jitter is the limiting parameter at these
faster data rates, so employing encoded data streams (e.g.,
Manchester coded or Return-to-Zero) may allow increased
transmission distances. The slow versions can operate at
115kbps or less at the full 4000-foot (1220m) distance or at
250kbps for lengths up to 3000 feet (915m). DC cable
attenuation is the limiting parameter, so using better quality
cables (e.g., 22 AWG) may allow increased transmission
distance.
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 15Mbps
devices, to minimize reflections. Short networks using the
250kbps versions need not be terminated; however, terminations
are recommended unless power dissipation is an overriding
concern.
In point-to-point or point-to-multipoint (single driver on bus, like
RS-422) 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 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
transceivers meet this requirement via driver output short circuit
current limits and on-chip thermal shutdown circuitry.
The driver output stages incorporate a double foldback, short
circuit current limiting scheme, which ensures that the output
current never exceeds the RS-485 specification, even at the
common mode and fault condition voltage range extremes. The
first foldback current level (≈70mA) is set to ensure that the
driver never folds back when driving loads with common mode
voltages up to ±25V. The very low second foldback current
setting (≈9mA) minimizes power dissipation if the Tx is enabled
when a fault occurs.
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°C. If the
contention persists, the thermal shutdown/re-enable cycle repeats
until the fault is cleared. Receivers stay operational during thermal
shutdown.
Low Power Shutdown Mode
These BiCMOS transceivers all use a fraction of the power
required by competitive devices, but they also include a
shutdown feature that reduces the already low quiescent I_CC to a
10µA trickle. These devices enter shutdown whenever the
receiver and driver are simultaneously disabled (RE = V_CC and
DE = GND) for 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.
Note that receiver and driver enable times increase when the
transceiver enables from shutdown. Refer to Notes 9, 10, 11, 12
and 13, at the end of the “Electrical Specifications” table on
page 9, for more information.
13
FN7786.0
January 18, 2011