MC3456
Astable Mode
In the astable mode the timer is connected so that it will
retrigger itself and cause the capacitor voltage to oscillate
discharge current (Pin 7 current) within the maximum rating
of the discharge transistor (200 mA).
between 1/3 V and 2/3 V (see Figure 17).
The minimum value of R is given by:
CC
CC
A
The external capacitor charges to 2/3 V through R and
CC
A
VCC (Vdc)
I7 (A)
VCC (Vdc)
≥
≥
RA
R and discharges to 1/3 V through R . By varying the
B
CC
B
0.2
ratio of these resistors the duty cycle can be varied. The
charge and discharge times are independent of the supply
voltage.
100
10
The charge time (output high) is given by:
t1 = 0.695 (RA+RB) C
The discharge time (output low) by:
1.0
0.1
t2 = 0.695 (RB) C
Thus the total period is given by:
T = t1 + t2 = 0.695 (RA + 2RB) C
0.01
1.44
(RA +2RB) C
1
T
=
The frequency of oscillation is then: f =
(R + 2 R )
A B
0.001
and may be easily found as shown in Figure 19.
0.1
1.0
10
100
1.0 k
f, FREE RUNNING FREQUENCY (Hz)
10 k
100
RB
The duty cycle is given by: DC =
RA +2RB
Figure 19. Free Running Frequency
To obtain the maximum duty cycle, R must be as small as
A
possible; but it must also be large enough to limit the
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