UM3352E_15 Datasheet, PDF (9/13 Pages) Union Semiconductor, Inc. – ESD-Protected, Slew-Rate-Limited, Fail-Safe, True RS-485 Transceivers

UM3352E
Applications Information
256 Transceivers on the Bus
The standard RS-485 receiver input impedance is 12kΩ (one unit load), and the standard driver can
drive up to 32 unit loads. The Union family of transceivers have a 1/8 unit load receiver input
impedance (96kΩ), allowing up to 256 transceivers to be connected in parallel on one
communication line. Any combination of these devices, as well as other RS-485 transceivers with a
total of 32 unit loads or fewer, can be connected to the line.
Reduced EMI and Reflections
The UM3352E is slew-rate limited, minimizing EMI and reducing reflections caused by improperly
terminated cables. In general, a transmitter’s rise time relates directly to the length of an
unterminated stub, which can be driven with only minor waveform reflections. The following
equation expresses this relationship conservatively:
Length=t_RISE/(10×1.5ns/ft)
Where t_RISE is the transmitter’s rise time.
A system can work well with longer unterminated stubs, even with severe reflections, if the
waveform settles out before the receiver’s UART samples them.
Low-Power Shutdown Mode
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Low-power shutdown mode is initiated by bringing both R E high and DE low. In shutdown, the
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device typically draws only 2µA of supply current. R E and DE may be driven simultaneously; the
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parts are guaranteed not to enter shutdown if R E is high and DE is low for less than 50ns. If the
inputs are in this state for at least 600ns, the parts are guaranteed to enter shutdown. Enable times t_ZH
and t_ZL in the Switching Characteristics tables assume the part was not in a low-power shutdown
state. Enable times t_ZH(SHDN) and t_ZL(SHDN) assume the parts were shut down. It takes drivers and
receivers longer to become enabled from low-power shutdown mode (t_ZH(SHDN), t_ZL(SHDN)) than from
driver/receiver-disable mode (t_ZH, t_ZL).
Driver Output Protection
Two mechanisms prevent excessive output current and power dissipation caused by faults or by bus
contention. The first, a foldback current limit on the output stage, provides immediate protection
against short circuits over the whole common-mode voltage range. The second, a thermal shutdown
circuit, forces the driver outputs into a high-impedance state if the die temperature becomes
excessive.
Line Length vs. Data Rate
The RS-485/RS-422 standard covers line lengths up to 4000 feet. For line lengths greater than 4000
feet, repeater is required.
ESD Protection
All pins on these devices include class 2 (>2kV) Human Body Model (HBM) ESD protection
structures, but the RS-485 pins (driver outputs and receiver inputs) incorporate advanced structures
allowing them to survive ESD events in excess of ±15kV HBM, ±15kV IEC61000-4-2 Air
Discharge Mode and ±8kV IEC61000-4-2 Contact Discharge Mode. The RS-485 pins are
particularly vulnerable to ESD strikes because they typically connect to an exposed port on the
exterior of the finished product. Simply touching the port pins, or connecting a cable, can cause an
ESD event that might destroy unprotected ICs.
These new ESD structures protect the device whether or not it is powered up, and without degrading
the RS-485 common mode range of -7V to +12V. This built-in ESD protection cuts the need for
board level protection structures (e.g., transient suppression diodes), and the associated, undesirable
capacitive load they present.
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http://www.union-ic.com Rev.07 Sep.2015
9/13