Isolation barrier and CTR — what they mean for the BOM
The CNY17F43S provides 5300Vrms galvanic isolation between input and output — this is a peak test voltage, not a working voltage rating, but it certifies the part for reinforced isolation in mains-referenced circuits per IEC 60950-1. The 1.35V typical forward voltage and 100 mA max forward current define the LED drive budget. Current transfer ratio (CTR) is 160% minimum and 320% maximum at 10 mA forward current — this range tells you the output transistor saturation margin across the full production spread. At the low end, 160% CTR means 16 mA collector current for 10 mA in; the 300 mV Vce(sat) max ensures the output pulls low enough for TTL or 3.3V logic inputs.
Switching speed and temperature range
Turn-on time is 2 µs typical, turn-off 3 µs typical, with rise/fall times of 1 µs and 2 µs respectively — this part is suited for 10-100 kHz signal isolation, not high-speed data links. The -55°C to 100°C operating range covers industrial and military temperature grades; the 70V maximum output voltage allows direct interface to 48V rails or 24V PLC inputs with margin.
Obsolete — supply posture
onsemi has marked the CNY17F43S as Obsolete. For a pin-compatible alternative within the same CNY17 family, the active variants (e.g., CNY17F-1, CNY17F-2, CNY17F-3) share the same 6-SMD gull wing footprint and 5300Vrms isolation but differ in CTR bin — the '43S' suffix corresponds to the 160-320% range. Verify the CTR requirement before substituting.