Datasheet

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 R
A
and R
B
and
discharges through R
B
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 V
CC
and 2/3 V
CC
.
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 (R
A
+ R
B
) C
The discharge time (output LOW) by:
t2 = 0.693 x R
B
x C
Thus the total period T is given by:
T = t1 + t2 = 0.693 (R
A
+ 2R
B
) C
The frequency of oscillation is then:
The duty cycle is given by:
Figure 7. Timing diagram
9
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--- =
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--------------------------------------
D =
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RA 2RB+
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