TLC556, TLC556Y
DUAL LinCMOS TIMERS
SLFS047B – FEBRUARY 1984 – REVISED SEPTEMBER 1997
10
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
0.1 µF
0.1 µF
C
T
C
L
R
L
Output
R
B
R
A
GND
TRIG
THRES
RESET
DISCH
CONT V
DD
OUT
V
DD
2/3 V
DD
1/3 V
DD
GND
t
PHL
t
PLH
t
H
t
L
CIRCUIT TRIGGER
AND
THRESHOLD
VOLTAGE
WAVEFORM
TLC556
Figure 3. Astable Operation
Connecting the trigger input to the threshold input, 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
H
) and low during the discharge cycle (t
L
). The duty cycle is
controlled by the values of R
A
, and R
B
, and C
T
, as shown in the equations below.
t
H
C
T
(R
A
R
B
) In 2 (In 2 0.693)
t
L
C
T
R
B
In 2
Period t
H
t
L
C
T
(R
A
2R
B
)In2
Output driver duty cycle
t
L
t
H
t
L
1
R
B
R
A
2R
B
Output waveform duty cycle
t
H
t
H
t
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 from the trigger and threshold inputs to the
discharge output. These delay times add directly to the period and create differences between calculated and
actual values that increase with frequency. In addition, the discharge output resistance r
on
adds to R
B
to provide
another source of 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.
t
H
C
T
(R
A
R
B
)In 3 exp
t
PLH
C
T
(R
B
r
on
)
t
PHL
t
L
C
T
(R
B
r
on
)In 3 exp
t
PHL
C
T
(R
A
R
B
)
t
PLH
