5000Vrms isolation — the safety barrier this part nails
The Renesas NEPOC PS2381-1Y-F3-AX is a single-channel phototransistor optocoupler in a 4-SMD gull-wing package.
Current transfer ratio spans 50% to 400% at 5mA forward current. That wide spread is typical for phototransistor couplers, but it means the LED drive resistor needs to be sized for the minimum CTR case to guarantee the output saturates at the load current. At the high end, a 400% CTR lets you run the LED at under 2mA and still get a clean logic-level output — useful in battery-powered or low-power sensor interfaces. The 300mV Vce saturation at 50mA keeps dissipation low enough that the 4-LSOP package doesn't need a heatsink.
The 4µs rise and 5µs fall times are adequate for 10 kHz to 50 kHz switching signals — think PWM feedback from a PFC stage or a slow serial data isolator. For faster edges, the PS2801C-4-V-A quad-channel sibling switches slightly slower (5µs, 7µs) and offers only 2500Vrms isolation, so the PS2381-1Y-F3-AX is the better pick where voltage withstand matters more than channel count.
The NEPOC series has been a stable Renesas opto line for years; replacement risk is low.