3750Vrms isolation in a 4-SOIC footprint
The TLP291(SE is a single-channel optocoupler from Toshiba that provides 3750Vrms of galvanic isolation between a DC input LED and a phototransistor output. The 4-SOIC package (0.179" width, 4.55 mm) saves board space compared to DIP-4 alternatives while still meeting basic isolation requirements for industrial interfaces. With a current transfer ratio ranging from 50% to 600% at 5 mA forward current, the design must accommodate the full CTR spread — the minimum 50% guarantees the output transistor can saturate with a 5 mA LED drive, while the 600% maximum means the output can sink up to 50 mA without exceeding the 300 mV Vce(sat) ceiling.
Switching speed and temperature range
Rise and fall times are 2 µs and 3 µs typical, with turn-on and turn-off times both 3 µs typical. This suits low-frequency signal isolation (below ~50 kHz) such as PLC digital inputs, relay drivers, and status feedback loops — not for high-speed comms like I²C or CAN. The 80 V maximum output voltage and 50 mA continuous output current are adequate for driving logic gates, small relays, or optocoupler-isolated transistor stages.