UM3491 Datasheet, PDF (9/14 Pages) Union Semiconductor, Inc. – +3.3V Low Power，±15kV ESD-Protected, Fail-Safe, RS-422 Transceivers in SOP14/DIP14

UM3491
Detailed Description
The UM3491 high-speed transceivers for RS-422 communication contain one driver and one
receiver. The device features fail-safe circuitry, which guarantees a logic-high receiver output
when the receiver inputs are open or shorted, or when they are connected to a terminated
transmission line with all drivers disabled.
The UM3491 offer higher driver output slew-rate limits, allowing transmit speeds up to 10Mbps.
The UM3491 is full-duplex transceiver. It operates from a single +3.3V supply. Drivers are output
short-circuit current limited. Thermal shutdown circuitry protects drivers against excessive power
dissipation. When activated, the thermal shutdown circuitry places the driver outputs into a high-
impedance state.
Receiver Input Filtering
The receivers of the UM3491 incorporate input filtering in addition to input hysteresis. This
filtering enhances noise immunity with differential signals that have very slow rise and fall times.
Receiver propagation delay increases by 20% due to this filtering.
Fail-Safe
The UM3491 guarantees a logic-high receiver output when the receiver inputs are shorted or open,
or when they are connected to a terminated transmission line with all drivers disabled. This is
done by setting the receiver threshold between -50mV and -200mV. If the differential receiver
input voltage (A-B) is greater than or equal to -50mV, RO is logic high. If A-B is less than or
equal to -200mV, RO is logic low. In the case of a terminated bus with all transmitters disabled,
the receiver’s differential input voltage is pulled to 0V by the termination. With the receiver
thresholds of the UM3491, this results in a logic high with a 50mV minimum noise margin.
Unlike previous fail-safe devices, the -50mV to -200mV threshold complies with the ±200mV
EIA/TIA-422 standard.
±15kV ESD Protection
As with all Union devices, 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 UM3491 have extra protection against static electricity. Union’s engineers
have developed state-of-the-art structures to protect these pins against ESD of ±15kV without
damage.
The ESD-protected pins are tested with reference to the ground pin in a powered-down condition.
They are tested to ±15kV using the Human Body Model.
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http://www.union-ic.com Rev.02 Jun.2012
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