Datasheet

LM555
SNAS548B –MAY 2004–REVISED JULY 2006 www.ti.com
Figure 5. Astable
In this mode of operation, the capacitor 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 the frequency are independent of the supply
voltage.
Figure 6 shows the waveforms generated in this mode of operation.
V
CC
= 5V Top Trace: Output 5V/Div.
TIME = 20μs/DIV. Bottom Trace: Capacitor Voltage 1V/Div.
R
A
= 3.9kΩ
R
B
= 3kΩ
C = 0.01μF
Figure 6. Astable Waveforms
The charge time (output high) is given by:
t
1
= 0.693 (R
A
+ R
B
) C (1)
And the discharge time (output low) by:
t
2
= 0.693 (R
B
) C (2)
Thus the total period is:
T = t
1
+ t
2
= 0.693 (R
A
+2R
B
) C (3)
The frequency of oscillation is:
(4)
Figure 7 may be used for quick determination of these RC values.
The duty cycle is:
(5)
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