Isolation barrier and working voltage
The FODM121V provides 3750Vrms isolation between input and output, rated for basic isolation in mains-referenced circuits up to 250V working voltage. That 3750Vrms is the test voltage, not the continuous working voltage — for reinforced isolation applications you need a part with a higher creepage package. Input side is DC-coupled, output is a single NPN phototransistor with a maximum collector-emitter voltage of 80V and continuous output current of 80mA per channel. The 400mV Vce(sat) at rated current keeps conduction loss low enough for logic-level interfaces.
CTR spread and drive margin
Current transfer ratio is specified at 5mA forward current: minimum 50%, maximum 600%. That 12:1 spread means the LED drive current must be chosen so the minimum CTR still guarantees the output saturates at the load current. At 5mA input, the output can sink 2.5mA minimum — enough for a 74HC gate input or a 5V relay coil with a series resistor. Rise and fall times are both 3µs typical, which limits the switching frequency to about 50 kHz for clean edges. Above that, the output transition becomes asymmetric and the pulse-width distortion grows.
Sourcing and lifecycle reality
onsemi lists the FODM121V as Active with ROHS3 compliance. Package is 4-SMD Gull Wing, also listed as Box — the Gull Wing leads are formed for surface-mount reflow, not through-hole. The narrow 4-pin SOIC-style body fits a 5.0mm × 4.4mm footprint with a 2.54mm lead pitch.
Temperature range and environment
Operating temperature spans -40°C to 110°C, which covers industrial enclosures, outdoor telecom cabinets, and under-hood automotive auxiliary circuits (non-AEC-Q100). The 110°C ceiling is the junction temperature limit — derate output current above 85°C ambient.