3750Vrms isolation in a 4-SOIC — what it buys the BOM
The Toshiba TLP291(BL-TP,SE is a single-channel DC-input, transistor-output optocoupler rated for 3750Vrms galvanic isolation. It comes in a 4-SOIC surface-mount package (4.55 mm width) and is specified over the full -55°C to 110°C operating range. The BL suffix denotes a current transfer ratio (CTR) bin of 200% to 600% at a forward current of 5 mA, which gives the designer a predictable gain window for driving logic inputs or low-current loads up to 50 mA per channel. The output transistor can block up to 80 V, making it suitable for 24 V or 48 V industrial bus interfaces.
CTR bin and switching speed — sizing the drive margin
The BL CTR bin (200% to 600% at 5 mA) sits between the lower GR rank and the higher GB rank in the TLP291 family. At 5 mA forward current, the minimum CTR of 200% guarantees the output transistor can saturate with a collector current of at least 10 mA, which is adequate for most TTL/CMOS logic inputs or relay-driver stages. The 300 mV maximum Vce saturation at the rated output current keeps power dissipation low in the output stage.