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2500VRMS Isolated RS485+RS232 Transceiver with Power

Brian Rippie - Associate Design Engineer Nov 23rd 2016

In this circuit, the LTM2887-3S µModule isolator provides isolated signaling and power to the LTC2873 for a complete, 3.3V robust, galvanically isolated dual-protocol transceiver. RS232 (transmit and receive) or RS485 (half-duplex) can be selected for transmission on a single pair of wires using the 485/232 control. Mode-specific termination is automatically selected by connecting the LTC2873 TE485 pin as shown. Alternately, TE485 can be tied high to the LTC2873 VL pin if RS485 termination is not desired.

The VCC2 output of the LTM2887-3S provides an isolated 5V supply that is used to power the LTC2873. In this application VCC2 is connected to a digital return channel on the LTM2887-3S with output on PWR_MON for a convenient way to determine when VCC2 is up. The LTM2887-5S can be used in place of the LTM2887-3S if a 5V input supply is required.

This robust solution provides protection from ESD to ±26kV on the bus pins and ±10kV across the isolation barrier (human body model), both unpowered and powered.

LTM2887-3S module isolator