ISL33354E Datasheet, PDF (20/23 Pages) Intersil Corporation – Full fail-safe (open/short) RS-485/422 port 2 Rx

ISL33354E, ISL33357E
impedance (typically 120Ω) at the end farthest from the driver.
In multireceiver applications, stubs connecting receivers to the
main cable should be kept as short as possible, but definitely
shorter than the limits shown in Table 4. Multipoint (RS-485)
systems require that the main cable be terminated in its
characteristic impedance at both ends. Again, keep stubs
connecting a transceiver to the main cable as short as possible
and refer to Table 4. Avoid “star” and other configurations,
where there are many “ends” which would require more than
the two allowed terminations to prevent reflections.
Active Low Rx Enable (RE485)
In many RS-485 applications, especially half duplex
configurations, users like to implement “echo suppression” by
disabling the corresponding receiver while its driver is
transmitting data. This function is available on the ISL3335XE
parts via an active low RE485 pin for port 2. The active low
function simplifies direction control, by allowing a single GPIO
line to provide a Tx/Rx direction control signal. Figure 34
details the advantage of using the RE485 pin.
+5V
ISL3335XE
RO
RE485
Tx/Rx
DE485
DI
V_CC
RA
B
Z
D
Y
GND
+
0.1µF
FIGURE 34. USING ACTIVE LOW RX ENABLE
Additional Features
High ESD
All pins on the ISL3335XE include ESD protection structures
rated at ±4kV (HBM), which is good enough to survive ESD
events commonly seen during manufacturing. But the bus pins
(Tx outputs and Rx inputs) are particularly vulnerable to ESD
events because they connect to an exposed port on the
exterior of the finished product. Simply touching the port pins,
or connecting a cable, can destroy an unprotected port.
ISL3335XE bus pins are fitted with advanced structures that
deliver ESD protection in excess of ±15kV (HBM), without
interfering with any signal in the RS-485 or the RS-232 range.
This high level of protection may eliminate the need for board
level protection, or at the very least will increase the
robustness of any board level scheme.
Small Packages
Many competing dual protocol devices are only available in
monstrously large 24 to 28 Ld SOIC packages. The ISL33354’s
28 Ld SSOP is 50% smaller than even a 24 Ld SOIC, and the
ISL33357E’s small 6x6mm QFN footprint is 80% smaller than
a 28 Ld SOIC.
Flow-Through Pinouts
Even the ISL3335XE pinouts are features, in that the true
flow-through design simplifies board layout. Having the bus
pins all on one side of the package for easy routing to a cable
connector, and the Rx outputs and Tx inputs (logic pins) on the
other side for easy connection to a UART, avoids costly and
problematic crossovers. Competing “flow-through” pinouts mix
logic and bus pin inputs on one side of the package, and logic
and bus pin outputs on the other side. This forces the designer
to route four traces from the right side of the IC around the IC
to the cable connector. Figure 35 illustrates the flow-through
nature of the ISL3335XE’s pinout.
ISL33354E
R1IN
R2IN
T1OUT
T2OUT
Y
Z
A
B
D
R
R2OUT
R1OUT
T2IN
T1IN
UART
OR
ASIC
DI
OR
RO
µCONTROLLER
FIGURE 35. ILLUSTRATION OF FLOW-THROUGH PINOUT
Low Power Shutdown (SHDN) Mode
The SHDN pin is driven low to place the IC (both ports) in the
SHDN mode, and the already low supply current drops to as
low as 55µA. If this functionality isn’t desired, the SHDN pin
should be connected to V_CC through a 1kΩ resistor. SHDN
disables the Tx and Rx outputs, and disables the charge
pumps, so V+ collapses to V_CC, and V- collapses to GND.
All but 5µA of SHDN supply current is due to control input
(SHDN, SLOW485, DE485) pull-up resistors (~17µA/resistor
when the input = 0V), so SHDN supply current varies
depending on the ISL3335XE configuration. For example, the
RS-485 drivers are disabled in SHDN, so driving the DE485 pin
high during this time reduces the SHDN supply current. The
spec table indicates the SHDN I_CC for the worst case
configuration.
When enabling from SHDN, allow at least 20µs for the charge
pumps to stabilize before transmitting RS-232 data. The
charge pumps aren’t used by the RS-485 port, so the
transceiver is ready to send or receive data in less than 1µs,
which is much faster than competing devices that require the
charge pump for all modes of operation.
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FN8775.1
October 21, 2016