Application information

TS555

14/19

DocID4077 Rev 4

4.2

Astable operation

When the circuit is connected as shown in Figure 6 (pins 2 and 6 connected) it triggers itself

and runs as a multi-vibrator. The external capacitor charges through RA and RB and

discharges through RB only. Therefore, 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 6. Application schematic

Figure 7 shows actual waveforms generated in this mode of operation.

The charge time (output HIGH) is given by:

t1 = 0.693 (RA + RB) C

The discharge time (output LOW) by:

t2 = 0.693 x RB x C

Thus the total period T is given by:

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

The frequency of oscillation is then:

The duty cycle is given by:

Figure 7. Timing diagram

f = 1

T

--- =

1.44

(RA 2RB

)C

+

--------------------------------------

D =

RB

RA 2RB

+

---------------------------