ISL33334E Datasheet, PDF (19/23 Pages) Intersil Corporation – Operates from a single 3.3V supply

ISL33334E, ISL33337E
terminated and undriven inputs is accomplished by setting the
Rx upper switching point at -40mV, thereby ensuring that the
Rx recognizes a 0V differential as a high level.
The Rx output is short-circuit protected and is tri-stated when
the RE485 input is driven low or when the IC is forced into
SHDN via a logic low on the SHDN pin.
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
2V across a 100Ω load (RS-422). The drivers feature low
propagation delay skew to maximize bit widths and to
minimize EMI.
To allow multiple drivers on a bus, the RS-485 spec requires
that drivers survive worst case bus contentions undamaged.
The ISL3333XE drivers meet this requirement via driver output
short-circuit current limits and on-chip thermal shutdown
circuitry. The output stages incorporate current limiting
circuitry that ensures that the output current never exceeds the
RS-485 spec, even at the common-mode voltage range
extremes of 12V and -7V. 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 degrees. If the contention persists,
the thermal shutdown/re-enable cycle repeats until the fault is
cleared. Receivers stay operational during thermal shutdown.
RS-485 multidriver operation also requires drivers to include
tri-state functionality, so port 2 has a DE485 pin to control this
function. If the driver is used in an RS-422 network, such that
driver tri-state isn’t required, then the DE485 pin should be
connected to V_CC, through a 1kΩ resistor, to keep the Tx in the
enabled state. Drivers are also tri-stated when the IC is in
SHDN, or when the 3.3V power supply is off.
Speed Options
The ISL3333XE offer two RS-485 speed options selectable via
the SLOW485 pin; “Fast” mode (SLOW485 = 1) selects high
slew rate driver outputs optimized for 20Mbps data rates while
“Slow” mode uses slew rate limiting designed for 115kbps
operation. See the “RS-485 Slew Rate Limited Data Rates”
and “Data Rate, Cables and Terminations” sections for more
information.
Receiver performance is the same for both speed options.
RS-485 Slew Rate Limited Data Rates
These ICs allow the user to select fast Tx output transitions
optimized for a 20Mbps data rate, or slew rate limited
transitions optimized for a data rate of 115kbps. The 20Mbps
fast edges may increase EMI and reflection issues, even
though fast transitions aren’t required at the lower data rates
used by many applications. Choosing the slew limited edges
for lower data rates permits longer unterminated networks, or
longer stubs off terminated busses and helps minimize EMI
and reflections.
Nevertheless, for the best jitter performance when driving long
cables, the faster speed option may be preferable, even at
lower data rates. The faster output transitions deliver less
variability (jitter) when loaded with the large capacitance
associated with long cables. Of course, faster transitions
require more attention to ensuring short stub lengths and
quality terminations, so there are trade-offs to be made.
Assuming a jitter budget of 10%, it is likely better to go with
the slow speed option for data rates of 115kbps or less, to
minimize fast edge effects. For higher data rates, or when the
absolute best jitter is required, use the high speed option. The
data rate selection pertains to only port 2.
Data Rate, Cables and Terminations
RS-485/RS-422 are intended for network lengths up to 4000’
(1220m), but the maximum system data rate decreases as the
transmission length increases. Devices operating at the
maximum data rate of 20Mbps are limited to maximum
lengths of 20-100’ (6-31m), while devices operating at or
below 115kbps can operate at the maximum length of 4000’
(1220m).
Higher data rates require faster edges, so both of the
ISL3333XE versions offer an edge rate capable of 20Mbps
data rates. These ICs also offer a slew rate limited option to
minimize problems at slower data rates. Nevertheless, for the
best jitter performance when driving long cables, the faster
speed settings may be preferable, even at low data rates. See
the “RS-485 Slew Rate Limited Data Rates” section for details.
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.
The preferred cable connection technique is “daisy-chaining”,
where the cable runs from the connector of one device directly
to the connector of the next device, such that cable stub
lengths are negligible. A “backbone” structure, where stubs
run from the main backbone cable to each device’s connector,
is the next best choice, but care must be taken to ensure that
each stub is electrically “short”. See Table 4 for recommended
maximum stub lengths for each speed option.
TABLE 4. RECOMMENDED STUB LENGTHS
SPEED OPTION
MAXIMUM STUB LENGTH
ft (m)
SLOW
350-500 (107-152)
FAST
1-3 (0.3 - 0.9)
Proper termination is imperative to minimize reflections when
using the 20Mbps speed option. Short networks using the slow
speed option need not be terminated, but terminations are
recommended unless power dissipation is an overriding
concern. Note that the RS-485 spec allows a maximum of two
terminations on a network, otherwise the Tx output voltage
may not meet the required V_OD.
In point-to-point, or point-to-multireceiver (RS-422) networks,
the main cable should be terminated in its characteristic
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FN8776.1
October 21, 2016