TLC555_16 Datasheet, PDF (10/25 Pages) Texas Instruments – LinCMOSE TIMERS

TLC555
LinCMOS TIMER
SLFS043F − SEPTEMBER 1983 − REVISED FEBRUARY 2005
APPLICATION INFORMATION
Connecting TRIG to THRES, as shown in Figure 3, causes the timer to run as a multivibrator. The capacitor C_T
charges through R_A and R_B to the threshold voltage level (approximately 0.67 V_DD) and then discharges through R_B
only to the value of the trigger voltage level (approximately 0.33 V_DD). The output is high during the charging cycle
(t_c(H)) and low during the discharge cycle (t_c(L)). The duty cycle is controlled by the values of R_A, R_B, and C_T as shown
in the equations below.
tc(H) [ CT (RA ) RB) In 2 (In 2 + 0.693)
tc(L) [ CT RB In 2
Period + tc(H) ) tc(L) [ CT (RA ) 2RB) In 2
Output driver duty cycle
+
tc(L)
tc(H) ) tc(L)
[
1–
RA
RB
) 2RB
Output waveform duty cycle
+
tc(H)
tc(H) ) tc(L)
[
RA
RB
) 2RB
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 r_on during discharge adds to R_B to provide
another source of timing error in the calculation when R_B is very low or r_on is very high.
The equations below provide better agreement with measured values.
ƪ ǒ Ǔ ƫ tc(H) + CT (RA ) RB) In
3 –exp
– tPLH
CT (RB ) ron)
) tPHL
ƪ ǒ Ǔ ƫ tc(L) + CT (RB ) ron) In
3 –exp
– tPHL
CT (RA ) RB)
) tPLH
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% ^tc(H)
tc(H) ) tc(L)
require that ^tc(H)
tc(L)
< 1 and possibly R_A ≤ r_on. 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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