BESCHREIBUNG

The LTC2854 and LTC2855 are low power, 20Mbps RS485/RS422 transceivers operating on 3.3V supplies. The receiver includes a logic-selectable 120Ω termination, one-eighth unit load supporting up to 256 nodes per bus (C-, I-Grade), and a failsafe feature that guarantees a high output state under conditions of floating or shorted inputs.

The driver maintains a high output impedance over the entire common mode range when disabled or when the supply is removed. Excessive power dissipation caused by bus contention or a fault is prevented by current limiting all outputs and by a thermal shutdown.

Enhanced ESD protection allows the LTC2854 to withstand ±25kV (human body model) and the LTC2855 to withstand ±15kV on the transceiver interface pins without latchup or damage.

 

FEATURES

Integrated, Logic-Selectable 120Ω Termination Resistor

3.3V Supply Voltage

20Mbps Maximum Data Rate

No Damage or Latchup Up to ±25kV HBM

High Input Impedance Supports 256 Nodes (C-, I-Grade)

Operation Up to 125°C (H-Grade)

Guaranteed Failsafe Receiver Operation Over the Entire Common Mode Range

Current Limited Drivers and Thermal Shutdown

Delayed Micropower Shutdown: 5µA Maximum (C-, I-Grade)

Power Up/Down Glitch-Free Driver Outputs

Low Operating Current: 370µA Typical in Receive Mode

Compatible with TIA/EIA-485-A Specifications

Available in 10-Pin 3mm × 3mm DFN, 12-Pin 4mm × 3mm DFN and 16-Pin SSOP Packages

 

ANWENDUNGEN

Low Power RS485/RS422 Transceiver

Level Translator

Backplane Transceiver

 

ANWENDUNGSINFORMATIONEN

Driver

The driver provides full RS485/RS422 compatibility. When enabled, if DI is high, Y-Z is positive for the full-duplex device (LTC2855) and A-B is positive for the half-duplex device (LTC2854).

When the driver is disabled, both outputs are high- impedance. For the full-duplex LTC2855, the leakage on the driver output pins is guaranteed to be less than 10µA over the entire common mode range of –7V to +12V. On the half-duplex LTC2854, the impedance is dominated by the receiver input resistance, RIN.

Driver Overvoltage and Overcurrent Protection

The driver outputs are protected from short-circuits to any voltage within the Absolute Maximum range of (VCC –15V) to +15V. The typical peak current in this condition does not exceed 180mA.

If a high driver output is shorted to a voltage just above VCC, a reverse current will flow into the supply. When this voltage exceeds VCC by about 1.4V, the reverse current turns off. Preventing the driver from turning off with outputs shorted to output voltages just above VCC keeps the driver active even for receiver loads that have a positive common mode with respect to the driver—a valid condition.

The worst-case peak reverse short-circuit current can be as high as 300mA in extreme cold conditions. If this current cannot be absorbed by the supply, a 3.6V Zener diode can be added in parallel with the supply to sink this current.

All devices also feature thermal shutdown protection that disables the driver and receiver in case of excessive power dissipation.

Receiver and Failsafe

With the receiver enabled, when the absolute value of the differential voltage between the A and B pins is greater than 200mV, the state of RO will reflect the polarity of (A-B).

The LTC2854/LTC2855 have a failsafe feature that guarantees the receiver output to be in a logic-high state when the inputs are either shorted, left open, or terminated (externally or internally), but not driven. This failsafe feature is guaranteed to work for inputs spanning the entire common mode range of –7V to +12V.

Switchable Termination

Proper cable termination is very important for good signal fidelity. If the cable is not terminated with its characteristic impedance, reflections will result in distorted waveforms.

The LTC2854/LTC2855 are the first 3.3V RS485/RS422 transceivers to offer integrated switchable termination resistors on the receiver input pins. This provides the advantage of being able to easily change, through logic control, the line termination for optimal performance when configuring transceiver networks.