ISL32470E Datasheet, PDF (12/19 Pages) Intersil Corporation – Fault Protected, Extended Common Mode Range, RS-485/RS-422 Transceivers

ISL32470E, ISL32472E, ISL32475E, ISL32478E
Receivers easily meet the data rates supported by the corresponding
driver, and all receiver outputs are three-statable via the active low
RE input.
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 ISL32478E drivers
are not limited; thus, faster output transition times allow data rates
of at least 15Mbps.
High Overvoltage (Fault) Protection
Increases Ruggedness
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 ISL32470E, ISL32472E,
ISL32475E, ISL32478E 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 ISL32470E, ISL32472E,
ISL32475E, ISL32478E 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.
Wide 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 ISL32470E, ISL32472E, ISL32475E,
ISL32478E has extended ±15V CMR, which allows for 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.
High V_OD Improves Noise Immunity and
Flexibility
The ISL32470E, ISL32472E, ISL32475E, ISL32478E driver design
delivers larger differential output voltages (V_OD) than the RS-485
standard requires or than most RS-485 transmitters can deliver. The
typical ±2.5V V_OD provides more noise immunity than networks built
using many other transceivers.
Another advantage of the large V_OD is the ability to drive more than
two bus terminations, which allows for utilizing the ISL32470E,
ISL32472E, ISL32475E, ISL32478E in “star” and other
multi-terminated, nonstandard network topologies. Figure 10 details
the transmitter’s V_OD vs I_OUT characteristic and includes load lines
for four (30Ω) and six (20Ω) 120Ω terminations. Figure 10 shows
that the driver typically delivers ±1.3V into six terminations, and the
“Electrical Specifications” table guarantees a V_OD of ±0.8V at 21Ω
over the full temperature range. The RS-485 standard requires a
minimum 1.5V V_OD into two terminations, but the ISL32470E,
ISL32472E, ISL32475E, ISL32478E delivers RS-485 voltage levels
with two to three times the number of terminations.
Hot Plug Function
When a piece of equipment powers up, there is a period of time in
which 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 ISL32470E, ISL32472E, ISL32475E, ISL32478E
devices incorporate a “Hot Plug” function. Circuitry monitoring V_CC
ensures that, during power-up and power-down, the Tx and Rx outputs
remain disabled, regardless of the state of DE and RE, if V_CC is less 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 ISL32470E, ISL32472E,
ISL32475E, ISL32478E compared to an RS-485 IC without the Hot Plug
feature.
V_CC
3.5V
2.8V
5.0
2.5
A/Y
0
ISL83088E
ISL3247xE
RO
ISL3247xE
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 (ISL3247xE) 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, which allows them to survive ESD events in
excess of ±16.5kV HBM (±15kV for full-duplex versions). 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,
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FN7784.0
January 21, 2011