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TS3V555
8/10
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 and 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 4 :
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 :
The duty cycle is given by :
Figure 5 :
VCC
Reset
Out
3
4
8
7
5
1
R
C
2
6
R
A
B
Control
Voltage
0.01 F
TS3V555
f =
1
T
--- =
1.44
(
RA
2
RB
)C
+
--------------------------------------
D =
RB
RA
2
RB
+
---------------------------
t = 0.5 ms / div
OUTPUT VOLTAGE = 5.0V/div
CAPACITOR VOLTAGE = 1.0V/div
L
R = R = 4.8 k , C = 0.1 F , R = 1.0k
AB
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