SNAS558J – FEBRUARY 2000 – REVISED MARCH 2013
V
CC
= 5V
TIME = 20 µs/Div.
R
A
= 3.9 kΩ
R
B
= 9 kΩ
C = 0.01 µF
Top Trace: Output 5 V/Div.
Bottom Trace: Capacitor Voltage 1 V/Div.
Figure 9. Astable Waveforms
The charge time (output high) is given by
t
1
= 0.693 (R
A
+ R
B
)C
(1)
(2)
(3)
And the discharge time (output low) by:
t
2
= 0.693 (R
B
)C
Thus the total period is:
T = t
1
+ t
2
= 0.693 (R
A
+ 2R
B
)C
The frequency of oscillation is:
(4)
may be used for quick determination of these RC Values. The duty cycle, as a fraction of total period
that the output is low, is:
(5)
Figure 10. Free Running Frequency
8
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