LM555/NE555/SA555
H
-
– ------------------------------------
(
R A
+
R B
)C1
2
2
-
-
V
(
t
)
= --
V
=
V
1
– --
e
C1
CC
3
3
CC
t
(
4
)
(
5
)
t
H
=
C
(
R
+
R
)In2
=
0.693
(
R
+
R
)C
A
B 1
1 A
B
The equivalent circuit for discharging capacitor C1, when timer output is low is, as follows:
R
B
C1
V
C1
(0-)=2Vcc/3
R
D
dv
1
C1
-
C 1
-------------- + ----------------------
V C1
=
0
R
+
R
dt
A
B
2
-
V C1
(
t
)
= --
V
3 CC e
t
-
-
------------------------------------
(
R A
+
R D
)C1
(
6
)
(
7
)
Since the duration of the timer output low state(
t
L
) is the amount of time it takes for the V
C1
(t) to reach Vcc/3,
-
-
------------------------------------
(
R
+
R
)C1
tL
A
D
1
2
--
V
-
-
= --
V
(
8
)
3 CC 3 CC e
t
=
C
(
R
+
R
)In2
=
0.693
(
R
+
R
)C
B
D 1
L
1 B
D
(
9
)
Since R
D
is normally R
B
>>R
D
although related to the size of discharging Tr.,
t
L
=0.693R
B
C
1
(10)
Consequently, 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. And since frequency is the reciprocal of the period, the following applies.
1
1.44
-
f
= -- = ---------------------------------------
-
T
(
R
+
2R
)C
A
B 1
frequency,
(
11
)
3. Frequency divider
By adjusting the length of the timing cycle, the basic circuit of Figure 1 can be made to operate as a frequency divider. Figure
8. illustrates a divide-by-three circuit that makes use of the fact that retriggering cannot occur during the timing cycle.
7
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
