LM555/NE555/SA555
When the trigger starts in a timer configured as shown in Figure 13, the current flowing through capacitor C1 becomes a
constant current generated by PNP transistor and resistors.
Hence, the V
C
is a linear ramp function as shown in Figure 14. The gradient S of the linear ramp function is defined as
follows:
Vp
–
p
S
= ----------------
T
(
14
)
Here the Vp-p is the peak-to-peak voltage.
If the electric charge amount accumulated in the capacitor is divided by the capacitance, the V
C
comes out as follows:
V=Q/C
(15)
The above equation divided on both sides by T gives us
V
Q
⁄
T
--- = -----------
-
-
T
C
(
16
)
and may be simplified into the following equation.
S=I/C
(17)
In other words, the gradient of the linear ramp function appearing across the capacitor can be obtained by using the constant
current flowing through the capacitor.
If the constant current flow through the capacitor is 0.215mA and the capacitance is 0.02µF, the gradient of the ramp function
at both ends of the capacitor is S = 0.215m/0.022µ = 9.77V/ms.
10
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
