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IC555

Single Timer

Fairchild Semiconductor 

The equivalent circuit for discharging capacitor C1, when timer output is low is, as follows:

RB

C1 VC1(0-)=2Vcc/3

RD

C1 d----v-d---C-t---1--

+

-----------1------------

RA + RB

VC1

=

0

- ------------------t------------------

VC1(t)

=

2--

3

VCCe

(RA

+

RD)C1

(6)

(7)

Since the duration of the timer output low state (tL) is the amount of time it takes for the VC1(t) to reach VCC/3,

13-- VCC

=

23---

V

C

-(---R----A------+---t--RL----D-----)---C----1--

Ce

(8)

tL = C1(RB + RD)In2 = 0.693(RB + RD)C1

(9)

Since RD is normally RB>>RD although related to the size of discharging transistor,

tL = 0.693RBC1

(10)

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

't = tH+tL = 0.693(RA+RB)C1+0.693RBC1 = 0.693(RA+2RB)C1'

because the period is the sum of the charge time and discharge time. Since frequency is the reciprocal of the period,

the following applies:

frequency,

f

=

1--

t

=

(---R----A------+--1---2.--4--R-4---B-----)--C-----1-

( 11 )

© 2002 Fairchild Semiconductor Corporation

LM555 Rev. 1.1.0

7

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