TLC556IDG4 Datasheet, PDF (10/22 Pages) Texas Instruments – DUAL LinCMOSE TIMERS

TLC556, TLC556Y
DUAL LinCMOS™ TIMERS
SLFS047B – FEBRUARY 1984 – REVISED SEPTEMBER 1997
APPLICATION INFORMATION
0.1 µF
RA
0.1 µF
CONT
RESET
VDD
TLC556
DISCH
RB
THRES
OUT
TRIG
GND
CT
VDD
RL
2/3 VDD
Output
CL ^{1/3 VDD}
GND
tL
tH
tPHL
tPLH
CIRCUIT
TRIGGER AND THRESHOLD VOLTAGE WAVEFORM
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.
[ ) + tH CT (RA RB) In 2 (In 2 0.693)
[ tL CT RB In 2
+ ) [ ) Period tH tL
CT (RA
2RB) In 2
+ ₎ [ * ₎ Output driver duty cycle
tL
tH tL
1
RB
RA 2RB
+ ₎ [ ₎ Output waveform duty cycle
tH
tH tL
RB
RA 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 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.
+ ) ƪ * ǒ ^* ₎ Ǔƫ ) tH
CT (RA
RB) In
3
exp
tPLH
CT (RB ron)
tPHL
+ ) ƪ * ǒ ^* ₎ Ǔƫ ) tL
CT (RB
ron) In
3
exp
tPHL
CT (RA RB)
tPLH
10
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