Electronic Design

RS-485 Transceivers Boast Switchable Termination, High ESD Protection

Two new families of RS-485 transceivers are aimed at systems with 3.3-V power supplies. The LTC2854/LTC2855 feature switchable, integrated termination, while the LTC2850/TC2851/LTC2852 offer industry-standard pinouts and support data rates to 20 Mbits/s. Enhanced ESD protection allows the interface pins of the LTC2854 to withstand up to ±25 kV (Human Body Model) without latchup or damage. The other devices withstand up to ±15 kV. All the receivers have a one-eighth unit load, supporting up to 256 devices on the bus, while all drivers maintain high output impedance over the entire common-mode range when disabled or when the supply is removed. Current limiting on all outputs and thermal shutdown prevent excessive power dissipation caused by bus contention or a fault.

The LTC2854 and LTC2855 complement the existing LTC2859/LTC2861 transceivers, which operate from 5-V supplies. RS-485 networks require users to install 120-Ω termination resistors, based on the physical layout of the twisted-pair wires and the placement of the nodes. If the bus is extended or reconfigured, the end user must change the termination again. By switching the integrated termination on or off under software control, the new transceivers allows the proper nodes to be terminated without the end user’s physical intervention.

The LTC2850 operates in half-duplex mode, and the LTC2851/LTC2852 support full-duplex operation. Packages include 8-lead and 10-lead 3- by 3-mm DFN, 8-lead and 10-lead MSOP, and 8-lead and 14-lead SO packages. The LTC2854 operates in half-duplex mode and comes in a 10-lead 3- by 3-mm DFN package. The LTC2855 supports full-duplex operation and is offered in 12-lead 4- by 3-mm DFN and 16-lead SSOP packages. The devices are specified over the commercial and industrial temperature ranges. They are available from stock, with prices starting at $1.45 each in 1,000-piece quantities.

Check out www.linear.com.

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