ISL3293E Datasheet, PDF (10/17 Pages) Intersil Corporation – ±16.5kV ESD Protected, +125°C, 3.0V to 5.5V, SOT-23/TDFN Packaged, Low Power, RS-485/RS-422 Transmitters

ISL3293E, ISL3294E, ISL3295E, ISL3296E, ISL3297E, ISL3298E
Application Information
RS-485 and RS-422 are differential (balanced) data
transmission standards for use in long haul or noisy
environments. RS-422 is a subset of RS-485, so RS-485
transmitters and receivers are also RS-422 compliant.
RS-422 is a point-to-multipoint (multidrop) standard, which
allows only one driver and up to 10 (assuming one unit load
devices) receivers on each bus. RS-485 is a true multipoint
standard, which allows up to 32 one unit load devices (any
combination of drivers and receivers) on each bus. To allow
for multipoint operation, the RS-485 specification requires
that drivers must handle bus contention without sustaining
any damage.
Another important advantage of RS-485 is the extended
common mode range (CMR), which specifies that the driver
outputs and receiver inputs withstand signals that range from
+12V to -7V. RS-422 and RS-485 are intended for runs as
long as 4000’, so the wide CMR is necessary to handle
ground potential differences, as well as voltages induced in
the cable by external fields.
Driver Features
These RS-485/RS-422 drivers are differential output devices
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 width, and to
minimize EMI.
All drivers are tri-statable via the active high DE input. If the
Tx enable function isn’t needed, tie DE to V_CC (or V_L)
through a 1kΩ to 3kΩ resistor.
The 250kbps and 500kbps driver outputs are slew rate
limited to minimize EMI, and to reduce reflections in
unterminated or improperly terminated networks. Outputs of
the ISL3295E and ISL3298E drivers are not limited, so faster
output transition times allow data rates of at least 20Mbps.
Wide Supply Range
The ISL3293E through ISL3298E are optimized for 3.3V
operation, but can be operated with supply voltages as high
as 5.5V. These devices meet the RS-422 and RS-485
specifications for supply voltages less than 4V, and are
RS-422 and RS-485 compatible for supplies greater than
4V. Operation at +125°C requires V_CC ≤ 3.6V, while 5V
operation requires adding output current limiting resistors
(as described in the “Driver Overload Protection” on
page 11) if output short circuits (e.g., from bus contention)
are a possibility.
5.5V Tolerant Logic Pins
Logic input pins (DI, DE) contain no ESD nor parasitic
diodes to V_CC (nor to V_L), so they withstand input voltages
exceeding 5.5V regardless of the V_CC and V_L voltages.
V_CC = +3.3V
V_CC = +2V
GND
V_IH ≥ 2V
DI
T_XD
V_OH ≤ 2V
V_IH ≥ 2V
DE
DEN
V_OH ≤ 2V
GND
ISL3293E
V_CC = +3.3V
UART/PROCESSOR
V_CC = +2V
GND
V_L
DI ^{VIH = 1.4V}
T_XD
V_OH ≤ 2V
V_IH = 1.4V
DE
DEN
V_OH ≤ 2V
GND
ISL3296E
UART/PROCESSOR
FIGURE 5. USING V_L PIN TO ADJUST LOGIC LEVELS
Logic Supply (V_L Pin, ISL3296E through ISL3298E)
Note: Power-up V_CC before powering up the V_L supply.
The ISL3296E through ISL3298E include a V_L pin that
powers the logic inputs (DI and DE). These pins interface
with “logic” devices such as UARTs, ASICs, and µcontrollers,
and today most of these devices use power supplies
significantly lower than 3.3V. Thus, the logic device’s low
V_OH might not exceed the V_IH of a 3.3V or 5V powered DI or
DE input. Connecting the V_L pin to the power supply of the
logic device (as shown in Figure 5) reduces the DI and DE
input switching points to values compatible with the logic
device’s output levels. Tailoring the logic pin input switching
points and output levels to the supply voltage of the UART,
ASIC, or µcontroller eliminates the need for a level
shifter/translator between the two ICs.
V_L can be anywhere from V_CC down to 1.35V, but the input
switching points may not provide enough noise margin, and
20Mbps data rates may not be achievable, when V_L < 1.5V.
Table 2 indicates typical V_IH and V_IL values for various V_L
settings so the user can ascertain whether or not a particular
V_L voltage meets his needs.
10
FN6544.0
September 19, 2007