NE555/SA555/NA555
PRECISION TIMERS
Typical Applications Characteristics (cont.)
Pulse Width Modulation
The operation of the timer can be modified by modulating the internal threshold and trigger voltages, which is accomplished
by applying an external voltage (or current) to CONT. Figure 10 shows a circuit for pulse-width modulation. A continuous
input pulse train triggers the monostable circuit, and a control signal modulates the threshold voltage. Figure 11 shows the
resulting output pulse-width modulation. While a sine-wave modulation signal is shown, any wave shape could be used.
Fig 10. Circuit for Pulse width modulation
Pulse Position Modulation
Fig 11. Pulse width modulation timing diagrams
As shown in Figure 12, 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 13 shows a triangular-wave modulation
signal for such a circuit; however, any wave shape could be used.
Fig 13. Pulse position modulation timing diagrams
Fig 12. Circuit for pulse position modulation
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February 2012
© Diodes Incorporated
NE555/SA555/NA555
Document number: DS35112 Rev. 4 - 2
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