LM555QML datasheet(10/16 Pages) TI1

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Applications Information (Continued)

Figure 5 shows the waveforms generated in this mode of

operation.

The charge time (output high) is given by:

1 = 0.693 (RA +RB)C

And the discharge time (output low) by:

2 = 0.693 (RB)C

Thus the total period is:

T= t

1 +t2 = 0.693 (RA +2RB)C

The frequency of oscillation is:

Figure 6 may be used for quick determination of these RC

values.

The duty cycle is:

FREQUENCY DIVIDER

The monostable circuit of Figure 1 can be used as a fre-

quency divider by adjusting the length of the timing cycle.

Figure 7 shows the waveforms generated in a divide by three

circuit.

PULSE WIDTH MODULATOR

When the timer is connected in the monostable mode and

triggered with a continuous pulse train, the output pulse

width can be modulated by a signal applied to pin 5. Figure

8 shows the circuit, and in Figure 9 are some waveform

examples.

20149809

VCC = 5V

Top Trace: Output 5V/Div.

TIME = 20µs/DIV.

Bottom Trace: Capacitor Voltage 1V/Div.

RA = 3.9kΩ

RB =3kΩ

C = 0.01µF

FIGURE 5. Astable Waveforms

20149810

FIGURE 6. Free Running Frequency

20149811

VCC = 5V

Top Trace: Input 4V/Div.

TIME = 20µs/DIV.

Middle Trace: Output 2V/Div.

RA = 9.1kΩ

Bottom Trace: Capacitor 2V/Div.

C = 0.01µF

FIGURE 7. Frequency Divider

20149812

FIGURE 8. Pulse Width Modulator

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