Datasheet

LM555 — Single Timer
© 2002 Fairchild Semiconductor Corporation www.fairchildsemi.com
LM555 Rev. 1.1.0 10
In Figure 14, current source is created by PNP transistor Q1 and resistor R1, R2, and R
E
.
For example, if V
CC
= 15 V, R
E
= 20 kΩ, R1 = 5 kΩ, R2 = 10 kΩ, and V
BE
= 0.7 V,
V
E
=0.7 V+10 V=10.7 V, and
I
C
=(15-10.7) / 20 k=0.215 mA.
When the trigger starts in a timer configured as shown in Figure 14, 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 15. The gradient S of the linear ramp function is defined as
follows:
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:
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 0.215 mA and the capacitance is 0.02 μF, the gradient of the ramp
function at both ends of the capacitor is S = 0.215 m / 0.022 μ = 9.77 V/ms.
I
C
V
CC
V
E
–
R
E
---------------------------=12()
Here, V
E is
V
E
V
BE
R
2
R
1
R
2
+
----------------------V
CC
+=13()
S
V
pp–
t
----------------=14()
V
t
----
Q
t§
C
-------------=16()