SA555DT Datasheet, PDF (10/20 Pages) STMicroelectronics – General-purpose single bipolar timers

Application information
Figure 15. Typical schematics in astable operation
V_CC = 5 to 15V
NE555 - SA555 - SE555
R1
4
8
Output
3
7
NE555
R2
Control
Voltage
5
0.01μF
1
6
2
C1
Figure 16 shows the actual waveforms generated in this mode of operation.
The charge time (output HIGH) is given by:
t1 = 0.693 (R₁ + R₂) C₁
and the discharge time (output LOW) by:
t2 = 0.693 (R₂) C₁
Thus the total period T is given by:
T = t1 + t2 = 0.693 (R1 + 2R2) C1
The frequency of oscillation is then:
f
=
-¹--
T
=
--------------¹----^.-⁴----⁴---------------
(R1 + 2R2)C1
It can easily be found from Figure 17.
The duty cycle is given by:
---------^t--¹---------- = -----⁽--^R-----¹-----+-----^R-----²---⁾-----
(t1 + t2) (R1 + 2 • R2)
= 1–
(^---R^-----1^--R--+---2--R^-----2^----)
10/20
Doc ID 2182 Rev 6