XR33156IDEVB Datasheet, PDF (16/19 Pages) Exar Corporation – 60V Fault Tolerant 3.0V to 5.5V RS-485/RS-422 Transceivers

XR33152/55/56/58
Applications Information (Continued)
The XR3315x 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
XR3315x 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 XR3315x family 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 XR3315x family 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 XR3315x 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
XR3315x 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 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 XR3315x 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 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 RS-485 transceivers
up to a total of 32 unit loads may be connected to the line.
Low Power Shutdown Mode
The XR33156 has a low-power shutdown mode that is
initiated by bringing both RE high and DE low simultaneously.
While in shutdown the XR33156 draws 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.
XR33156 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 part is 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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