SA555 datasheet(7/18 Pages) ARTSCHIP

7 / 18 page

NE555,SA555,SE555

PRECISION TIMERS

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APPLICATION INFORMATION

Astable operation

As shown in Figure 12, adding a second resistor, RB, to the circuit of Figure 9 and connecting the trigger input to the threshold

input causes the timer to self-trigger and run as a multivibrator. The capacitor C charges through RA and RB and then discharges

through RB only. Therefore, the duty cycle is controlled by the values of RA and RB.

This astable connection results in capacitor C charging and discharging between the threshold-Voltage level (≈0.67 x Vcc) and the

trigger-voltage level (≈0.33 x Vcc). As in the monostable circuit, charge and discharge times (and, therefore, the frequency and

duty cycle) are independent of the supply voltage.

Pin numbers shown are for the D, JG, P, PS, and PW packages.

Note A: Decoupling CONT voltage to ground with a capacitor

can improve operation. This should be evaluated for individual

applications.

Figure 12. Circuit for Astable Operation

Time – 0.5ms/div

Figure 13.

Typical Astable Waveforms

Astable Operation (Continued)

Figure 13 shows typical waveforms generated during astable operation. The output high-level duration tH and low-level duration tL

can be calculated as follows:

tH=0.693 (RA+RB)C

tL=0.693 (RB)C

Other useful relationships are shown below.

Period = tH+ tL =0.693 (RA + 2RB) C

Frequency ≈ 1.44/ (RA + 2RB) C

Output driver duty cycle = tL / (tH + tL)=RB / (RA + 2RB)

Output waveform duty cycle

=tH /( tH + tL) = 1- RB / (RA + 2RB)

Low – to – high ratio = tL /tH =RB / (RA + RB)

C – Capacitance -µF

Figure 14. Free-Running Frequency

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