ISL3298EMRTEP Datasheet, PDF (5/9 Pages) Intersil Corporation – 9RS-485/RS-422 Transmitter

ISL3298EMRTEP
Applications 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
This 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 width, and to minimize EMI.
The driver is 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 ISL3298EMRTEP’s output transition times allow data rates
of at least 16Mbps.
Wide Supply Range
The ISL3298EMRTEP is optimized for 3.3V operation, but can
be operated with supply voltages as high as 5.5V. This device
meets the RS-422 and RS-485 specifications for supply
voltages less than 4V, and is 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 6) 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 (see Figure 5).
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)
Note: Power-up V_CC before powering up the V_L supply.
The ISL3298EMRTEP includes 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
16Mbps data rates may not be achievable, when V_L < 1.5V. The
E.C. table in the SMD 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/her needs.
The V_L supply current (I_L) is typically much less than 20µA, as
shown in Figure 9, when DE and DI are above/below V_IH/V_IL.
5
FN7607.1
July 28, 2011