Datasheet NE555, SA555, NA555 (Diodes) - 8

NE555/SA555/NA555 PRECISION TIMERS Typical Applications Characteristics (cont.) Astable Operation
As shown in Figure 4, adding a second resistor, RB, to the circuit of Figure 1 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. 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 (≉0.67VCC) and the trigger-voltage level (≉0.33VCC). As in the monostable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are independent of the supply voltage.
T
  VCC
UC
(5V to 15V) 0.01µF Open RA (See Note A) R 5 L 8 CONT V 4 CC RESET
NEW PROD
7 DISCH Output 3 R OUT B 6 THRES 2 TRIG C GND 1 Decoupling CONT voltage to ground with a capacitor can improve operation. This should be evaluated for individual applications.
Fig. 5 Typical Astable Waveforms Fig. 4 Circuit for Astable Operation
Figure 5 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 equations are: 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)
Fig. 6 Free Running Frequency
NE555/SA555/NA555 8 of 14 February 2012 Document number: DS35112 Rev. 4 - 2
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