XR33052 Datasheet, PDF (15/18 Pages) Exar Corporation – 60V Fault Tolerant 3.0V to 5.5V RS-485/RS-422 Transceivers

XR33052/53/55/58
Applications Information (Continued)
The XR3305x RS-485/RS-422 devices are part of Exar’s
high performance serial interface product line. The analog
bus pins can survive direct shorts up to ±60V and are
protected against ESD events up to ±15kV.
Enhanced Failsafe
Ordinary RS-485 differential receivers will be in an
indeterminate state whenever the data bus is not being
actively driven. The enhanced failsafe feature of the
XR3305x guarantees a logic-high receiver output when
the receiver inputs are open, shorted or when they are
connected to a terminated transmission line with all drivers
disabled. In a terminated bus with all transmitters disabled,
the receivers’ differential input voltage is pulled to 0V by the
termination. The XR3305x interprets 0V differential as a logic
high with a minimum 50mV noise margin while maintaining
compliance with the RS-485 standard of ±200mV. Although
the XR3305x does not need failsafe biasing resistors, it can
operate without issue if biasing is used.
Hot Swap Capability
When V_CC is first applied, the XR3305x holds the driver
enable and receiver enable inactive for approximately
10μs. During power ramp-up, other system ICs may drive
unpredictable values or tristated lines may be influenced
by stray capacitance. The hot swap feature prevents the
XR3305x from driving any output signal until power has
stabilized. After the initial 10μs, the driver and receiver
enable pins are weakly pulled to their disabled states (low
for DE, high for RE) until the first transition. After the first
transition, the DE and RE pins operate as high impedance
inputs.
If circuit boards are inserted into an energized backplane
(commonly called “live insertion” or “hot swap”) power
may suddenly be applied to all circuits. Without the hot
swap capability, this situation could improperly enable the
transceiver’s driver or receiver, driving invalid data onto
shared buses and possibly causing driver contention or
device damage.
Driver Output Protection
Two mechanisms prevent excessive output current and
power dissipation caused by faults or by bus contention.
First, a driver current limit on the output stage provides
immediate protection against short circuits over the whole
common-mode voltage range. Second, a thermal shutdown
circuit forces the driver outputs into a high-impedance state
if junction temperature becomes excessive.
Line Length
The RS-485/RS-422 standard covers line lengths up to
4000ft. Maximum achievable line length is a function of
signal attenuation and noise. Termination prevents signal
reflections by eliminating the impedance mismatches on
a transmission line. Line termination is generally used if
rise and fall times are shorter than the round-trip signal
propagation time. Higher output drivers may allow longer
cables to be used.
±15kV HBM ESD Protection (Unpowered Part)
ESD protection structures are incorporated on all pins to
protect against electrostatic discharges encountered during
handling and assembly. The driver outputs and receiver
inputs of the XR3305x family have extra protection against
static electricity. Exar uses state-of-the-art structures to
protect these pins against ESD damage:
■■ ±15kV HBM for bus pins to GND
■■ ±4kV HBM for all other pins
ESD Test Conditions
ESD performance depends on a variety of conditions.
Contact Exar for a reliability report that documents test
setup, methodology and results.
Maximum Number of Transceivers on the Bus
The standard RS-485 receiver input impedance is 12kΩ (1
unit load). A standard driver can drive up to 32 unit loads.
The XR33052/53/55 transceiver has a 1/10th unit load
receiver input impedance of 120kΩ, allowing up to 320
transceivers to be connected in parallel on a communication
line. The XR33058 receiver input impedance is a least 30KΩ
(1/2.5 unit load), allowing more than 80 devices on the
bus. Any combination of the XR3305x’s and other RS-485
transceivers up to a total of 32 unit loads may be connected
to the line.
Low Power Shutdown Mode
Low power shutdown mode is initiated by bringing both RE
high and DE low simultaneously. While in shutdown devices
draw less than 1μA of supply current. DE and RE may be
tied together and driven by a single control signal. Devices
are guaranteed not to enter shutdown if RE is high and DE
is low for less than 50ns. If the inputs are in this state for at
least 600ns, the parts will enter shutdown.
Enable times t_ZH and t_ZL apply when the part is not in low
power shutdown state. Enable times t_ZH(SHDN) and t_ZL(SHDN)
apply when the parts are shutdown. The driver and receiver
take longer to become enabled from low power shutdown
t_ZH(SHDN) and t_ZL(SHDN) than from driver or receiver disable
mode (t_ZH and t_ZL).
REV2A
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