XR33053 Datasheet, PDF (13/16 Pages) Exar Corporation – TIA-485/TIA-422 Transceiver

XR33053
Applications Information
The XR33053 TIA-485/TIA-422 device is 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 TIA-485 differential receivers will be in an
indeterminate state whenever the data bus is not being
actively driven. The enhanced failsafe feature of the
XR33053 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 XR33053 interprets 0V differential as a logic
high with a minimum 50mV noise margin while maintaining
compliance with the TIA-485 standard of ±200mV. Although
the XR33053 does not need failsafe biasing resistors, it can
operate without issue if biasing is used.
Receiver Input Filtering
XR33053 receiver incorporates internal filtering in addition
to input hysteresis. This filtering enhances noise immunity
by ignoring signals that do not meet a minimum pulse width
of 30ns. Receiver propagation delay increases slightly due
to this filtering.
Hot Swap Capability
When V_CC is first applied, the XR33053 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
XR33053 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 TIA-485/TIA-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 ESD Protection
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 XR33053 has extra protection against static
electricity. Exar uses state-of-the-art structures to protect
these pins against ESD of ±15kV without damage. The
ESD structures withstand high ESD in all states: normal
operation, shutdown and powered down. After an ESD
event, the XR33053 keeps operating without latch-up or
damage.
ESD protection can be tested in various ways. The
transmitter outputs and receiver inputs of the XR33053 is
characterized for protection to the following limits:
■■ ±15kV using the Human Body Model, TIA-485
bus pins
■■ ±4kV using the Human Body Model, 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 TIA-485 receiver input impedance is 12kΩ (1
unit load). A standard driver can drive up to 32 unit loads.
The XR33053 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.
Any combination of the XR33053’s and other TIA-485
transceivers up to a total of 32 unit loads may be connected
to the line.
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