Electronic Design

Transformer-Isolated RS-485/422 MEMS Transceivers Integrate DC-DC Converters, Terminations

There are other RS-485/422 transceivers with inductive isolation available, but Linear Technology has taken a step forward. Its LTM2881 µModule includes its own complete, isolated dc-dc power supply, including the transformer, eliminating the need for any external components.

The integrated power supply powers the transceiver and provides an isolated 5-V supply output for powering any supporting external components. (ADI’s ADM2485 has an on-chip oscillator, but requires an external transformer, diodes, and passives to complete a power supply.) There are versions for 3.3- and 5-V input power.

There’s no need for external termination resistors with the LTM2881 either, thanks to integrated selectable termination. It’s possible to add or remove termination anywhere on the bus via a software switch.

The isolated transceiver guards against large groundto- ground differentials and common-mode transients. In industrial-control systems, ground potentials vary widely from node to node. The LTM2881 breaks ground loops by isolating the logic level interface and line transceiver. Inductive signal isolation consumes less power and supports faster data rates than opto-isolators. It also allows for a large commonmode voltage range and commonmode rejection better than 30 kV/µs (see the figure). Galvanic isolation is 2500 VRMS, and electrostatic discharge (ESD) protection is 15,000 V.

The transceiver operates at 20 Mbits/s or, for low electromagnetic interference (EMI), 250 kbits/s, in half- or full-duplex modes. The driver portion maintains high impedance over the common-mode range and provides a short-circuit current limit feature, as well as slew rate control for minimizing EMI. The receiver portion allows up to 256 devices to be connected on the same lines and defaults to a logic “1” if the receiver inputs are open, shorted, or terminated but not driven. This eliminates the need to pre-bias the network.

The device also features thermal shutdown protection, disabling the transceiver outputs in case of excessive power dissipation. The LTM2881 will be followed by RS-232c, CAN, and other versions for popular industrial-control buses. Packaging options comprise low-profile, 11.25- by 15- by 2.8-mm surface-mount land-grid and ball-grid arrays. Pricing starts at $8.45 each in 1000-piece quantities.



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