XR3070-78X Datasheet, PDF (17/24 Pages) Exar Corporation – LOW POWER 18V TOLERANT RS-485/RS-422 +3.3V TRANSCEIVERS

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PRODUCT DESCRIPTION
XR3070-78X
LOW POWER 18V TOLERANT RS-485/RS-422 +3.3V TRANSCEIVERS
The XR3070-78X RS-485/422 devices are part of Exar’s X Series high performance serial interface product
line. The analog bus pins can survive direct shorts up to ±18V, and are protected against ESD events up to
±15kV. The high output differential driver delivers 10% higher SNR than other RS-485/422 devices, affording
additional noise margin or extended cable lengths.
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 XR3070-78X 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 XR3070-78X family interprets 0V differential as a logic high with
a minimum 50mV noise margin while maintaining compliance with the EIA/TIA-485 standard of ±200mV.
Although the XR3070-78X family does not need failsafe biasing resistors, it can operate without issue if biasing
is used.
RECEIVER INPUT FILTERING
XR3070-75X 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 XR3076X, XR3077X and XR3078X devices do not have
this input filtering.
HOT-SWAP CAPABILITY
When V_CC is first applied the XR3070-78X devices with DE and RE pins (70, 72, 73, 75, 76, & 78) hold the
driver enable and receiver enable inactive for approximately 10 microseconds. 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 these devices 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.
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