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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
B
charges through R and R to the threshold voltage level (approximately 0.67 V ) and then discharges through R
A
B
DD
only to the value of the trigger voltage level (approximately 0.33 V ). The output is high during the charging cycle
DD
(t
) and low during the discharge cycle (t
). The duty cycle is controlled by the values of R , R , and C as shown
c(H)
c(L) A B T
in the equations below.
t
[ C (R ) R ) In 2 (In 2 + 0.693)
c(H)
T
A
B
t
[ C R In 2
c(L)
T
B
Period + t
) t
[ C (R ) 2R ) In 2
c(H)
c(L)
T
A
c(L)
) t
B
t
R
B
Output driver duty cycle +
[ 1 –
t
R
R
) 2R
c(H)
c(L)
A
A
B
B
t
R
B
) 2R
c(H)
) t
Output waveform duty cycle +
[
t
c(H)
c(L)
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 during discharge adds to R to provide
on
B
another source of timing error in the calculation when R is very low or r is very high.
B
on
The equations below provide better agreement with measured values.
–t
PLH
t
t
+ C (R ) R ) In 3 –expǒ Ǔ ) t
ƪ
ƪ
ƫ
ƫ
c(H)
c(L)
T
A
B
PHL
C (R ) r
)
on
T
B
–t
PHL
+ C (R ) r ) In 3 –expǒ Ǔ ) t
on
T
B
PLH
C (R ) R )
T
A
B
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
t
t
c(H)
) t
c(H)
with good results. Duty cycles less than 50%
require that
<1 and possibly R ≤ r . These
A on
t
t
c(H)
c(L)
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.
10
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