NA555, NE555, SA555, SE555
PRECISION TIMERS
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SLFS022F–SEPTEMBER 1973–REVISED JUNE 2006
APPLICATION INFORMATION (continued)
Figure 13 shows typical waveforms generated during astable operation. The output high-level duration tH and
low-level duration tL can be calculated as follows:
100 k
t
t
+ 0.693 (R ) R
C
H
L
A
B)
R
A
+ 2 R = 1 kΩ
B
+ 0.693 (R
C
R
A
+ 2 R = 10 kΩ
B
B)
10 k
1 k
Other useful relationships are shown below.
R
A
+ 2 R = 100 kΩ
B
period + t ) t + 0.693 (R ) 2R ) C
H
L
A
B
1.44
frequency [
(R ) 2R ) C
A
B
100
10
t
R
L
B
Output driver duty cycle +
+
t
) t
R
) 2R
H
L
A
B
Output waveform duty cycle
t
R
1
H
B
+
+ 1–
R
A
+ 2 R = 1 MΩ
B
t
) t
R
) 2R
H
L
A
B
R
R
A
+ 2 R = 10 MΩ
B
t
0.1
0.001
L
B
+
+
Low-to-high ratio
0.01
0.1
1
10
100
t
R
) R
H
A
B
C − Capacitance − µF
Figure 14. Free-Running Frequency
Missing-Pulse Detector
The circuit shown in Figure 15 can be used to detect a missing pulse or abnormally long spacing between
consecutive pulses in a train of pulses. The timing interval of the monostable circuit is retriggered continuously
by the input pulse train as long as the pulse spacing is less than the timing interval. A longer pulse spacing,
missing pulse, or terminated pulse train permits the timing interval to be completed, thereby generating an
output pulse as shown in Figure 16.
V
CC
(5 V to 15 V)
V
CC
= 5 V
R
A
= 1 kΩ
C = 0.1 µF
See Figure 15
R
L
R
A
4
8
RESET
V
CC
Input
3
Output
OUT
2
5
TRIG
Input Voltage
7
6
DISCH
CONT
THRES
0.01 µF
Output Voltage
GND
1
C
A5T3644
Capacitor Voltage
Time − 0.1 ms/div
Pin numbers shown are shown for the D, JG, P, PS, and PW packages.
Figure 15. Circuit for Missing-Pulse Detector
Figure 16. Completed Timing Waveforms for
Missing-Pulse Detector
11
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