TLC555

LinCMOS TIMER

SLFS043F − SEPTEMBER 1983 − REVISED FEBRUARY 2005

10

POST OFFICE BOX 655303

APPLICATION INFORMATION

in the equations below.

t

c(H) [

C

T

(R

A )

R

B

)In2 (In2

+ 0.693)

t

c(L) [

C

T

R

B

In 2

Period

+ t

c(H) )

t

c(L) [

C

T

(R

A )

2R

B

)In2

Output driver duty cycle

+

t

c(L)

t

c(H) )

t

c(L)

[ 1–

R

B

R

A )

2R

B

Output waveform duty cycle

+

t

c(H)

t

c(H) )

t

c(L)

[

R

B

R

A )

2R

B

The 0.1-

µF capacitor at CONT in Figure 3 decreases the period by about 10%.

The formulas shown above do not allow for any propagation delay times from the TRIG and THRES inputs to DISCH.

These delay times add directly to the period and create differences between calculated and actual values that

The equations below provide better agreement with measured values.

t

c(H) +

C

T

(R

A )

R

B

)In

3– exp

–t

PLH

C

T

(R

B )

ron)

) t

PHL

t

c(L) +

C

T

(R

B )

ron)In

3– exp

–t

PHL

C

T

(R

A )

R

B

)

) t

PLH

These equations and those given earlier are similar in that a time constant is multiplied by the logarithm of a number

or function. The limit values of the logarithmic terms must be between In 2 at low frequencies and In 3 at extremely

high frequencies. For a duty cycle close to 50%, an appropriate constant for the logarithmic terms can be substituted

with good results. Duty cycles less than 50%

t

c(H)

t

c(H) )

t

c(L)

require that

t

c(H)

t

c(L)

conditions can be difficult to obtain.

In monostable applications, the trip point on TRIG can be set by a voltage applied to CONT. An input voltage between

10% and 80% of the supply voltage from a resistor divider with at least 500-

µA bias provides good results.