ISL3330_14 Datasheet, PDF (15/25 Pages) Intersil Corporation – 3.3V, ±15kV ESD Protected, Dual Protocol (RS-232/RS-485) Transceivers

ISL3330, ISL3331
Each RS-485/422 port includes a single receiver (RA), and
the unused Rx output (RB) is disabled.
Worst case receiver input currents are 20% lower than the
1 “unit load” (1mA) RS-485 limit, which translates to a 15kΩ
minimum input resistance.
These receivers include a “full fail-safe” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating), shorted together, or if the bus is
terminated but undriven (i.e., differential voltage collapses to
near zero due to termination). Failsafe with shorted or
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.
All the Rx outputs are short circuit protected, and are
tri-statable via the active high RXEN pin, or when the IC is
shutdown (see Tables 2 and 3, and the “Low Power
Shutdown (SHDN) Mode” on page 16). ISL3331 (QFN)
receiver outputs are also tri-statable via an active low RXEN
input (see “ISL3331 (QFN Package) Special Features” on
page 17).
For the ISL3331 (QFN), when using the active high RXEN
function, the RXEN pin may be left floating (internally pulled
high), or should be connected to V_CC through a 1kΩ resistor.
If using the active low RXEN, then the RXEN pin must be
connected to GND.
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 specification
requires that drivers survive worst case bus contentions
undamaged. The ISL333x 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 specification, 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°C. If the
contention persists, the thermal shutdown/re-enable cycle
repeats until the fault is cleared. Receivers stay operational
during thermal shutdown.
The RS-485 multi-driver operation also requires drivers to
include tri-state functionality, where the port has a DEN 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
DEN pin should connect to V_CC through a 1kΩ resistor.
Drivers are also tri-stated when the IC is in SHDN, or when
the 3.3V power supply is off.
SPEED OPTIONS
The ISL3330 (SSOP) features two speed options that are
user selectable via the SLEW pin: a high slew rate setting
optimized for 20Mbps data rates (Fast), and a slew rate
limited option for operation up to 460kbps (Med). The
ISL3331 (QFN) offers an additional, more slew rate limited,
option for data rates up to 115kbps (Slow). See the “Data
Rate“ and “RS-485 Slew Rate Limited Data Rates” on
page 17 for more information.
Receiver performance is the same for all three speed
options.
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 lengths of 20’ to
30’ (6m to 9m), while devices operating at or below 115kbps
can operate at the maximum length of 4000’ (1220m).
Higher data rates require faster edges, so both the ISL333x
versions offer an edge rate capable of 20Mbps data rates.
They both have a second option for 460kbps, but the
ISL3331 also offers another, very slew rate limited, edge rate
to minimize problems at slow 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” on
page 17 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 to 500 (107 to 152)
MED
100 to 150 (30.5 to 46)
FAST
1 to 3 (0.3 to 0.9)
15
FN6361.0
May 20, 2008