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当采用的电源电压不能满足或超过了LED的3V正向压降时,使用蓝色LED就会遇到麻烦。本设计实例说明如何用一节3V电池或其它电源驱动一支蓝色LED。图1电路采用了On Semiconductor的NCP1729电压变换器(IC_1),产生足以驱动蓝色LED D_1的电压。晶体管Q_1用作LED正向电流的恒流限流器。当通过LED和R_S的电流增加某个水平,即能提供足够的基射极电压使Q_1导通时,Q_1的集电极从R_1和R_2构成的分压器吸收电流,迫使IC_1关断。当R_3上的电压降低于Q_1的基射极导通
When using the power supply voltage can not meet or exceed the LED’s 3V forward voltage drop, the use of blue LED will encounter trouble. This design example shows how to drive a blue LED with a 3V battery or other power source. The circuit of Figure 1 incorporates On Semiconductor’s NCP1729 voltage converter (IC_1), which generates enough voltage to drive the blue LED D_1. Transistor Q_1 acts as a constant current limiter for the LED forward current. When the current through LEDs and R_S is increased by a certain level to provide enough base emitter voltage to turn Q_1 on, the collector of Q_1 sinks current from the voltage divider formed by R_1 and R_2, forcing IC_1 off. When R_3 drops below Q_1, the base emitter turns on