Isolation and package fit for the BOM
The FODM8801C is a single-channel optoisolator from onsemi's OptoHit™ series, delivering 3750Vrms of galvanic isolation between a DC-input LED and a phototransistor output. That 3750Vrms rating covers basic isolation for industrial control, PLC digital inputs, and motor-drive feedback where the signal path needs to cross a safety barrier without ground loops. It comes in a 4-SOIC package (0.173-inch body height, 4.40mm width) with a 4-Mini-Flat supplier device code — a compact footprint that fits standard pick-and-place assembly and reflow profiles. The surface-mount package keeps the board-side height low, which matters when the opto sits between a backplane connector and a heatsink.
Current transfer ratio and switching speed
The current transfer ratio spans 200% minimum to 400% maximum at a forward current of 1mA. That 2:1 spread means the designer sizes the pull-up resistor and input threshold for the worst-case low CTR, not the typical — a common trap in phototransistor opto circuits that leads to marginal noise margin if ignored. Typical rise and fall times are 5µs and 5.5µs respectively, with turn-on/turn-off times both at 6µs. This puts the usable switching frequency around 10–20 kHz for clean edge detection — fine for 50/60 Hz line sync, relay coil sensing, or slow digital isolator channels, but not for high-speed serial links.
Temperature range and output ratings
The output transistor can handle up to 75V collector-emitter and 30mA continuous current, with a saturation voltage of 400mV max — enough to drive a 5V or 3.3V logic input through a pull-up resistor. Forward voltage is 1.35V typical at 20mA max DC forward current. That 1.35V drop is standard for an IR LED; the 20mA ceiling means the series resistor on the input side should be calculated for the available control voltage (e.g., 3.3V or 5V) to stay within the LED's safe operating area.