LM555
Applications Information
(Continued)
PULSE POSITION MODULATOR
This application uses the timer connected for astable opera-
tion, as in
Figure 10,
with a modulating signal again applied
to the control voltage terminal. The pulse position varies with
the modulating signal, since the threshold voltage and hence
the time delay is varied.
Figure 11
shows the waveforms
generated for a triangle wave modulation signal.
DS007851-16
FIGURE 12.
Figure 13
shows waveforms generated by the linear ramp.
The time interval is given by:
DS007851-14
FIGURE 10. Pulse Position Modulator
V
BE
≅
0.6V
DS007851-17
DS007851-15
V
CC
= 5V
TIME = 0.1 ms/DIV.
R
A
= 3.9kΩ
R
B
= 3kΩ
C = 0.01µF
Top Trace: Modulation Input 1V/Div.
Bottom Trace: Output 2V/Div.
V
CC
= 5V
Top Trace: Input 3V/Div.
TIME = 20µs/DIV. Middle Trace: Output 5V/Div.
Bottom Trace: Capacitor Voltage 1V/Div.
R
1
= 47kΩ
R
2
= 100kΩ
R
E
= 2.7 kΩ
C = 0.01 µF
FIGURE 13. Linear Ramp
FIGURE 11. Pulse Position Modulator
LINEAR RAMP
When the pullup resistor, R
A
, in the monostable circuit is re-
placed by a constant current source, a linear ramp is gener-
ated.
Figure 12
shows a circuit configuration that will perform
this function.
9
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