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Why are there two current transfer ratios: CTR and CTR (Saturation)?

Coupling characteristics

The current transfer ratio (CTR) has LED current IF and transistor collector-emitter voltage VCE dependencies. For typical transistor-output photocouplers, CTR is specified under the conditions IF=5mA and VCE=5V. However, VCE=5V is the condition for the non-saturated area. In applications using photocoulers, ON and OFF digital signals are transmitted very often. In such a case, in the ON state, photocouplers go into the saturated area. Therefore, the saturated condition (In Toshiba's case, VCE=0.4V or 0.3V) may be also set in the CTR requirements, and the CTR under this condition is defined as CTR (Saturation).

Example: Case of transistor-output photocoupler TLP385

IC-VCE characteristics example