NE555 Просмотр технического описания (PDF) - Diodes Incorporated.

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NE555

PRECISION TIMERS

Diodes Incorporated. 

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.

VCC

(5V to 15V)

0.01µF

Open

RA (See Note A) 5

4

CONT

RESET

7 DISCH

RB

6

THRES

2

TRIG

8

VCC

OUT 3

C

GND

1

RL

Output

Decoupling CONT voltage to ground with a capacitor can

improve operation. This should be evaluated for individual

applications.

Fig. 4 Circuit for Astable Operation

Fig. 5 Typical Astable Waveforms

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

Document number: DS35112 Rev. 4 - 2

8 of 14

www.diodes.com

February 2012

© Diodes Incorporated

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