White LED Voltage Booster Uses 555 Timer IC

Posted on Feb 6, 2014

This voltage-booster circuit for driving one or more white LEDs uses a 555 timer as its main element ( see the figure ). The timer, IC1, functions as a resettable astable multivibrator where R1, R2, and C2 are the timing components. When the supply voltage, VS, is first applied, D1 conducts and reservoir capacitor C1 charges to a voltage just less

White LED Voltage Booster Uses 555 Timer IC
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than VS. Initially, transistor Q2 is off, IC1`s RESET input is high, and the OUTPUT pin goes high, allowing C2 to charge up via R1. During this time, the DISCHARGE pin is pulled up by R4, which turns on Q1, and the current (IL) in inductor L1 starts to ramp up. Because Q1 is saturated, both D3 and the LED are reverse-biased. When the voltage on C2 crosses the threshold voltage at pin 6 of IC1, both the OUTPUT and DISCHARGE pins go low, and Q1 turns off. The resulting "back EMF" generated across L1 instantaneously raises the LED`s anode voltage (VA) above VS, thereby illuminating the LED. Diode D3 is now forward-biased and pulls up IC1`s supply voltage (V+) to a level some 2 to 4 V higher than VS. C2 is now quickly discharged via D2 and R2, ready for the next cycle. Provided the values of R5 and R6 are chosen correctly, Q2 turns on while the LED is RESET input. When the energy stored in L1 is exhausted, the LED and D3 again become reverse-biased, and VA falls to a low level. Q2 now turns off, allowing IC1 to commence another cycle, and C2 again begins to charge via R1. The process repeats thousands of times a second, so the LED appears to be continually illuminated. The circuit uses three "tricks" to optimize performance. First, the bootstrapping provided by D3 boosts the timer`s supply voltage, enabling the circuit to continue working even when VS drops below 1 V. In addition, it provides enhanced base drive for Q1 via R4....

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