Datasheet

LM555 — Single Timer
© 2002 Fairchild Semiconductor Corporation www.fairchildsemi.com
LM555 Rev. 1.1.0 7
The equivalent circuit for discharging capacitor C1, when timer output is low is, as follows:
Since the duration of the timer output low state (t
L
) is the amount of time it takes for the VC1(t) to reach V
CC
/3,
Since R
D
is normally R
B
>>R
D
although related to the size of discharging transistor,
t
L
= 0.693R
B
C
1
(10)
Consquently, if the timer operates in astable, the period is the same with
't = t
H
+t
L
= 0.693(RA+R
B
)C
1
+0.693R
B
C
1
= 0.693(R
A
+2R
B
)C
1
'
because the period is the sum of the charge time and discharge time. Since frequency is the reciprocal of the period,
the following applies:
C1
R
B
R
D
V
C1
(0-)=2Vcc/3
C
1
dv
C1
dt
---------------
1
R
A
R
B
+
----------------------- V
C1
0=+6()
V
C1
t()
2
3
---V
CC
e
-
t
R
A
R
D
+()C1
-------------------------------------
=7()
1
3
--- V
CC
2
3
--- V
CC
e
-
t
L
R
A
R
D
+()C1
-------------------------------------
= 8()
t
L
C
1
R
B
R
D
+()In2 0.693 R
B
R
D
+()C
1
== 9()
frequency, f
1
t
---
1.44
R
A
2R
B
+()C
1
----------------------------------------== 11()