TLC555CDBLE datasheet(10/19 Pages) TI

10 / 19 page

TLC555

LinCMOS TIMER

SLFS043F − SEPTEMBER 1983 − REVISED FEBRUARY 2005

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

Connecting TRIG to THRES, as shown in Figure 3, causes the timer to run as a multivibrator. The capacitor CT

charges through RA and RB to the threshold voltage level (approximately 0.67 VDD) and then discharges through RB

only to the value of the trigger voltage level (approximately 0.33 VDD). The output is high during the charging cycle

(tc(H)) and low during the discharge cycle (tc(L)). The duty cycle is controlled by the values of RA, RB, and CT as shown

in the equations below.

c(H) [

A )

)In2 (In2

+ 0.693)

c(L) [

In 2

Period

+ t

c(H) )

c(L) [

A )

)In2

Output driver duty cycle

c(L)

c(H) )

c(L)

[ 1–

A )

Output waveform duty cycle

c(H)

c(H) )

c(L)

[

A )

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

increase with frequency. In addition, the internal on-state resistance ron during discharge adds to RB to provide

another source of timing error in the calculation when RB is very low or ron is very high.

The equations below provide better agreement with measured values.

c(H) +

A )

)In

3– exp

–t

PLH

B )

ron)

) t

PHL

c(L) +

B )

ron)In

3– exp

–t

PHL

A )

)

) 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%

c(H)

c(H) )

c(L)

require that

c(H)

c(L)

< 1 and possibly RA ≤ ron. These

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.

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