TS556ID datasheet(9/11 Pages) STMICROELECTRONICS

9 / 11 page

ASTABLE OPERATION

When the circuit is connected as shown in figure 4

it triggers itself and free runs as a multivibrator. The

external capacitor charges through RA and RB and

discharges through RB only. Thus the duty cycle

may be precisely 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 :

1.44

(RA + 2RB)C

The duty cycle is given by : D

RA + 2RB

VCC

Reset

Out

Control

Voltage

0.01 F

1/2

TS556

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

TS556C,I,M

9/11

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