© 2002 Fairchild Semiconductor Corporation

www.fairchildsemi.com

LM555 Rev. 1.1.0

10

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.

follows:

Here the Vp-p is the peak-to-peak voltage.

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

()