TS555CD datasheet(9/12 Pages) STMICROELECTRONICS

9 / 12 page

ASTABLE OPERATION

When the circuit is connected as shown in figure 4

(pin 2 and 6 connected) it triggers itself and free

runs as a multivibrator. The external capacitor

charges through RA and RB an d discharges

through RB only. Thus the duty cycle may be pre-

cisely set by the ratio of these two resistors.

In the astable mode of operation, C charges and

discharges between 1/3 VCC and 2/3 VCC.As in the

triggered mode, the charge and discharge times

and therefore frequency, are independent of the

supply voltage.

Figure 5 shows actual waveforms generated in this

mode of operation.

The charge time (output HIGH) is given by :

t1 = 0.693 (RA +RB)C

and the discharge time (output LOW) by :

t2 = 0.693 (RB)C

Thus the total period T is given by :

T = t1 + t2 = 0.693 (RA +2RB)C

The frequency of oscillation is then :

= 1

1.44

(RA + 2RB)C

The duty cycle is given by : D

RA + 2RB

VCC

Reset

Out

TS555

Control

Voltage

0.01 F

Figure 4

Ω

t = 0.5 ms / div

OUTPUT VOLTAGE = 5.0V/div

CAPACITOR VOLTAGE = 1.0V/div

R = R = 4.8 k , C = 0.1 F , R = 1.0k

Figure 5

TS555C,I,M

9/12

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