ISL33354E Datasheet, PDF (18/23 Pages) Intersil Corporation – Full fail-safe (open/short) RS-485/422 port 2 Rx

ISL33354E, ISL33357E
ISL3335XE Advantages
These dual protocol ICs offer many parametric improvements
vs those offered on competing dual protocol devices. Some of
the major improvements are:
• 15kV Bus Pin ESD - Eases board level requirements
• Full Fail-Safe RS-485 Rx - Eliminates bus biasing
• Selectable RS-485 Data Rate - Up to 20Mbps, or slew rate
limited for low EMI and fewer termination issues
• High RS-232 Data Rate - >460kbps
• Lower Tx and Rx Skews - Wider, consistent bit widths
• Lower I_CC - Max I_CC is 2x to 4x lower than competition
• Flow-Through Pinouts - Tx, Rx bus pins on one side, logic pins
on the other, for easy routing to connector/UART
• Packaging - Smaller (QFN) and Pb-free
RS-232 Mode (Port 1)
Rx Features
RS-232 receivers invert and convert RS-232 input levels (±3V
to ±25V) to the standard TTL/CMOS levels required by a UART,
ASIC, or µcontroller serial port. Receivers are designed to
operate at faster data rates than the drivers, and they feature
very low skews (10ns) so the receivers contribute negligibly to
bit width distortion. Inputs include the standards required 3kΩ
to 7kΩ pull-down resistor, so unused inputs may be left
unconnected. Rx inputs also have built-in hysteresis to
increase noise immunity and to decrease erroneous triggering
due to slowly transitioning input signals.
Rx outputs are short-circuit protected, and are only tri-statable
when the entire IC is shutdown via the SHDN pin (see Table 3
on page 5 and “Low Power Shutdown (SHDN) Mode” on
page 20 for more details).
Tx Features
RS-232 drivers invert and convert the standard TTL/CMOS
levels from a UART, or µcontroller serial port to RS-232
compliant levels (±5V minimum). The Tx delivers these
compliant output levels even at data rates of 650kbps, with
loads of 1000pF. The drivers are designed for low skew
(typically 12% of the 500kbps bit width), and are compliant to
the RS-232 slew rate spec (4 to 30V/µs) for a wide range of
load capacitances. Tx inputs float if left unconnected and may
cause I_CC increases. For the best results, connect unused
inputs to GND.
Tx outputs are short-circuit protected, and incorporate a
thermal shutdown feature to protect the IC in situations of
severe power dissipation - see the RS-485 “Tx Features” on
page 19 section for more details. Both Tx outputs disable
when the IC enters thermal shutdown.
Drivers also tri-state in SHDN mode (SHDN = 0), or when the
5V power supply is off (see Table 3 and “Low Power Shutdown
(SHDN) Mode” on page 20 for more details). SHDN is a global
function - affecting both ports - so it is useful for disabling the
RS-232 outputs only if both ports will always be disabled
together, and if it is acceptable for the Rx to be disabled as well.
Charge Pumps
The on-chip charge pumps create the RS-232 transmitter
power supplies (typically +6/-7V) from a single supply as low
as 4.5V, and are enabled all the time unless in SHDN via the
SHDN pin. The efficient design requires only four small 0.1µF
capacitors for the voltage doubler and inverter functions. By
operating discontinuously (i.e., turning off as soon as V+ and V-
pump up to the nominal values), the charge pump contribution
to I_CC reduces significantly.
Data Rates and Cabling
Drivers operate at data rates up to 650kbps and are
guaranteed for data rates up to 460kbps. The charge pumps
and drivers are designed such that both drivers in port 1 can
be operated at the rated load, and at 460kbps (see Figure 22
on page 14). Figure 22 also shows that drivers can easily drive
several thousand picofarads at data rates up to 250kbps,
while still delivering compliant ±5V output levels.
Receivers operate at data rates up to 2Mbps. They are
designed for a higher data rate to facilitate faster factory
downloading of software into the final product, thereby
improving the user’s manufacturing throughput.
Figures 25 and 26 illustrate driver and receiver waveforms at
250kbps, and 500kbps, respectively. For these graphs, both
drivers of port 1 drive the specified capacitive load and a
receiver in the port.
RS-232 doesn’t require anything special for cabling; just a
single bus wire per transmitter and receiver, and another wire
for GND. So an ISL3335XE RS-232 port uses a five conductor
cable for interconnection. Bus terminations are not required,
nor allowed, by the RS-232 standard.
RS-485 Mode (Port 2)
Rx Features
RS-485 receivers convert differential input signals as small as
200mV, as required by the RS-485 and RS-422 standards, to
TTL/CMOS output levels. The differential Rx provides
maximum sensitivity, noise immunity and common-mode
rejection. Per the RS-485 standard, receiver inputs function
with common-mode voltages from +12V to -7V, regardless of
supply voltage, making them ideal for long networks where
induced voltages are a realistic concern. The RS-485/RS-422
port includes a single receiver (RO).
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.
This receiver includes 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). Fail-safe 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.
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FN8775.1
October 21, 2016