3750Vrms isolation in a 4-SOIC package
The TLP291(GR-TP,SE is a single-channel optoisolator from Toshiba that couples a DC-input LED to a phototransistor output across a 3750Vrms galvanic barrier. That isolation rating suits it for basic functional isolation in industrial control interfaces, PLC digital inputs, and general-purpose signal isolation where you need to break ground loops or protect low-voltage logic from a higher-voltage field side. The output transistor can handle up to 80V collector-emitter and 50 mA continuous current, with a typical Vce(sat) of 300 mV at the rated forward current — low enough to drive a logic gate input or a small relay coil without an extra buffer stage.
CTR bin GR — 100% to 300% at 5 mA forward
The GR suffix bins the current transfer ratio between 100% and 300% at If=5 mA. That means with 5 mA through the LED, the output transistor can sink at least 5 mA (100% CTR) and up to 15 mA (300% CTR) — a wide margin that lets you design the input drive for a fixed 5 mA and still guarantee the output saturates even at the low end of the bin.