Isolation barrier and signal transfer
The FOD2741A: Its current transfer ratio (CTR) is specified at 100% minimum and 200% maximum when the LED is driven at 10 mA, so the output transistor saturates reliably even with moderate LED current degradation over temperature or life — a key margin for designs that cannot afford a logic-level dropout at the receiver.
Output drive and saturation limits
The phototransistor can sink up to 50 mA continuous with a maximum Vce(sat) of 400 mV — the collector-emitter voltage stays low enough to drive a 5 V CMOS input directly without an external pull-up resistor, as long as the load current stays under that ceiling. Maximum output voltage is 30 V, so the part fits 24 V industrial logic rails and 12 V / 5 V signal levels without exceeding the breakdown rating.