555an - INTEGRATED CIRCUITS AN170 NE555 and NE556...

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Philips Semiconductors AN170 NE555 and NE556 applications 1988 Dec INTEGRATED CIRCUITS
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Philips Semiconductors Application note AN170 NE555 and NE556 applications 2 1988 Dec INTRODUCTION In mid 1972, Philips Semiconductors introduced the 555 timer, a unique functional building block that has enjoyed unprecedented popularity. The timer’s success can be attributed to several inherent characteristics foremost of which are versatility, stability and low cost. There can be no doubt that the 555 timer has altered the course of the electronics industry with an impact not unlike that of the IC operational amplifier. The simplicity of the timer, in conjunction with its ability to produce long time delays in a variety of applications, has lured many designers from mechanical timers, op amps, and various discrete circuits into the ever increasing ranks of timer users. DESCRIPTION The 555 timer consists of two voltage comparators, a bistable flip-flop, a discharge transistor, and a resistor divider network. To understand the basic concept of the timer let’s first examine the timer in block form as in Figure 1. The resistive divider network is used to set the comparator levels. Since all three resistors are of equal value, the threshold comparator is referenced internally at 2/3 of supply voltage level and the trigger comparator is referenced at 1/3 of supply voltage. The outputs of the comparators are tied to the bistable flip-flop. When the trigger voltage is moved below 1/3 of the supply, the comparator changes state and sets the flip-flop driving the output to a high state. The threshold pin normally monitors the capacitor voltage of the RC timing network. When the FLIP FLOP COMP COMP 555 OR 1/2 556 DISCHARGE CONTROL VOLTAGE THRESHOLD TRIGGER RESET R R R V CC OUTPUT OUTPUT SL00954 Figure 1. 555/556 Timer Functional Block Diagram capacitor voltage exceeds 2/3 of the supply, the threshold comparator resets the flip-flop which in turn drives the output to a low state. When the output is in a low state, the discharge transistor is “on”, thereby discharging the external timing capacitor. Once the capacitor is discharged, the timer will await another trigger pulse, the timing cycle having been completed. The 555 and its complement, the 556 Dual Timer, exhibit a typical initial timing accuracy of 1% with a 50ppm/C timing drift with temperature. To operate the timer as a one-shot, only two external oscillator, only one additional resistor is necessary. By proper selection of external components, oscillating frequencies from one cycle per half hour to 500kHz can be realized. Duty cycles can be adjusted from less than one percent to 99 percent over the frequency spectrum. Voltage control of timing and oscillation functions is also available. Timer Circuitry
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This note was uploaded on 10/19/2008 for the course ECE 2B taught by Professor York during the Winter '07 term at UCSB.

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555an - INTEGRATED CIRCUITS AN170 NE555 and NE556...

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