What is the CTR classification by rank?

The current transfer ratio (CTR) of transistor couplers appear as a multiplication of main three factors which are the degree of LED light emission, light sensitivity of the photodetector device and degree of amplification by the device, and degree of optical coupling when LED chip and photodetector chip are placed in a three-dimensional environment. Each factor has manufacture variations, and as a result, the CTR value shows widely distributed variations. From a practical viewpoint, Toshiba thus provides these by classifying them into several types. For example, for Toshiba's typical transistor coupler TLP385, the rank classification in the following table is set.