NA555, NE555, SA555, SE555
PRECISION TIMERS
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SLFS022F–SEPTEMBER 1973–REVISED JUNE 2006
APPLICATION INFORMATION (continued)
Pulse-Position Modulation
As shown in Figure 20, any of these timers can be used as a pulse-position modulator. This application
modulates the threshold voltage and, thereby, the time delay, of a free-running oscillator. Figure 21 shows a
triangular-wave modulation signal for such a circuit; however, any wave shape could be used.
R
A
R
B
= 3 kΩ
= 500 Ω
V
CC
(5 V to 15 V)
R = 1 kΩ
L
See Figure 20
R
L
R
A
4
8
RESET
TRIG
V
CC
3
OUT
Output
2
5
Modulation Input Voltage
7
6
DISCH
THRES
Modulation
Input
R
B
CONT
(see Note A)
GND
Output Voltage
C
Pin numbers shown are for the D, JG, P, PS, and PW packages.
NOTE A: The modulating signal can be direct or capacitively coupled
to CONT. For direct coupling, the effects of modulation
source voltage and impedance on the bias of the timer
should be considered.
Capacitor Voltage
Time − 0.1 ms/div
Figure 20. Circuit for Pulse-Position Modulation
Figure 21. Pulse-Position-Modulation Waveforms
14
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