© 2002 Fairchild Semiconductor Corporation

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LM555 Rev. 1.1.0

9

5. Pulse Position Modulation

If the modulating signal is applied to the control terminal while the timer is connected for the astable operation, as in

Figure 12, the timer becomes a pulse position modulator.

In the pulse position modulator, the reference of the timer's internal comparators is modulated, which modulates the

timer output according to the modulation signal applied to the control terminal.

Figure 13 illustrates a sine wave for modulation signal and the resulting output pulse position modulation; however, any

wave shape be used

.

6. Linear Ramp

When the pull-up resistor RA in the monostable circuit shown in Figure 2 is replaced with constant current source, the

trates the generated linear ramp waveforms.

Figure 12. Circuit for Pulse Position Modluation

Figure 13. Wafeforms of pulse position modulation

Figure 14. Circuit for Linear Ramp

Figure 15. Waveforms of Linear Ramp

8

4

7

1

2

3

5

6

CONT

GND

Vcc

DISCH

THRES

RESET

TRIG

OUT

+Vcc

R

A

C

R

B

Modulation

Output

1

5

6

7

8

4

2

3

RESET

Vcc

DISCH

THRES

CONT

GND

OUT

TRIG

+Vcc

C2

R1

R2

C1

Q1

Output