© 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:

(10)

Consquently, if the timer operates in astable, the period is the same with

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

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

---

VCC

2

3

---

V

CC

e

-

tL

RA RD

+

()C1

-------------------------------------

=

8

()

tL

C1 RB RD

+

()In2 0.693 R

B

RD

+

()C

1

=

=

9

()

frequency,

f

1

t

---

1.44

R

A

2R

B

+

()C

1

----------------------------------------

==

11

()