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PS2651L2 Datasheet(PDF) 4 Page - NEC |
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PS2651L2 Datasheet(HTML) 4 Page - NEC |
4 / 38 page 3 There are two kinds of optocouplers (a light emitting diode (LED) as an input and a phototransistor as an output) according to the type of output transistor: Single transistor type and Darlington-transistor type. The single-transistor type optocouplers are used to perform high-speed switching (with high-speed response). The Darlington- transistor type optocouplers are used to obtain a large output current by utilizing a small input current (independently of switching speeds). Designing the circuits properly will improve the PS2601 optocoupler (Single Transistor type) by having a base pin in terms of switching speed, elimination of noise in input signals, and output leakage current (collector dark current, and application to high- voltage circuits). APPLICATIONS OF OPTOCOUPLER BASE PINS INCREASING SWITCHING SPEED The switching speed of an optocoupler with a base pin can be increased by inserting a resistor between the base and the emitter of its phototransistor even when the optocoupler is applied to a large load resistance. Generally, the phototransistor of an optocoupler such as the PS2601 has a large photo-sensitive area on it. Accordingly, the junction capacitance (CC-B) between the collector and the base of the phototransistor is great (up to 20 pF) and as a result its response speed (turn-off time toff) is low. The relationship between turn-off time toff and collector-base capacitance CC-B is expressed by: toff × CC-B x hFE x RL ................(1) where toff : Turn-off time (See Fig. 2-2.) CC-B : Collector-base capacitance hFE : D.C. current amplification factor RL : Load resistance Cc-B RL Figure 2-1. Collector-Base Capacitance CC-B of Phototransistor |
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