CTR range and forward voltage — what they mean for the drive circuit
The FOD2741C: Current transfer ratio (CTR) is specified at 100% minimum to 200% maximum, tested at IF=10mA — this means the output transistor can sink at least as much current as the LED drives, with a 2:1 spread that the designer must budget for across temperature and device tolerance. Forward voltage Vf is 1.5V max at the same 10mA forward current, so the input-side series resistor must drop the remaining drive voltage; the 30V output max and 50mA output current per channel suit 5V or 24V logic-level loads.
onsemi lists the FOD2741C as obsolete.
Industrial temperature range and DC input
The input type is DC — the internal LED is driven by a DC forward current, not an AC signal, so the part is suited for on/off or PWM signalling from a microcontroller or logic gate.