XR33152 Datasheet, PDF (15/19 Pages) Exar Corporation – TIA-485/TIA-422 Transceivers

XR33152/56/58
Applications Information
The XR33152/56/58 TIA-485/TIA-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 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
XR33152/56/58 family 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 XR33152/56/58 family 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 XR33152/56/58 family
does not need failsafe biasing resistors, it can operate
without issue if biasing is used.
Receiver Input Filtering
The XR33152 receivers incorporate 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. The high speed XR33156 and
XR33158 devices do not have this input filtering.
Hot Swap Capability
When V_CC is first applied the XR33152/56/58 family
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 XR33152/56/58 family 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 and 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 XR33152/56/58 family 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 XR33152/56/58 keep operating
without latch up or damage.
ESD protection can be tested in various ways. The transmitter
outputs and receiver inputs of the XR33152/56/58 are
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 XR33152 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
XR33156/58 transceivers have a 1/2.5 unit load receiver
input impedance of 30kΩ, allowing up to 80 transceivers
to be connected in parallel on a communication line. Any
combination of these devices and other TIA-485 transceivers
up to a total of 32 unit loads may be connected to the line.
REV1A
15/19