Galvanic isolation in a compact 4-SOIC
The Vishay TCMT1109 is a single-channel optocoupler with a DC-input phototransistor output, rated for 3750Vrms isolation voltage. It comes in a 4-SOIC surface-mount package, 4.40mm wide, suited for industrial and consumer applications where galvanic isolation is needed between low-voltage control logic and higher-voltage loads. The current transfer ratio (CTR) is specified at 200% minimum and 400% maximum when the LED forward current is 5mA, giving a well-defined window for logic-level interface design.
CTR range and switching timing
With a CTR spanning 200% to 400% at 5mA forward current, the TCMT1109 provides consistent gain for driving digital inputs or low-current loads. The output transistor can handle up to 50mA per channel and a maximum collector-emitter voltage of 70V. Typical turn-on time is 9.5µs and turn-off is 8.5µs, with rise and fall times of 5.5µs and 7µs respectively — fast enough for 100 kHz-ish switching or isolated serial data at moderate baud rates. The 300mV Vce saturation at max rated current keeps power dissipation low in the output stage.
Temperature range and rework considerations
The 4-SOIC package is straightforward to rework with hot air — low thermal mass, no exposed pad, and the 0.173" body width means it lifts cleanly without cooking adjacent parts.
For a production BOM, this part presents no near-term obsolescence risk.