LMC555CM datasheet(6/12 Pages) NSC

6 / 12 page

Application Information (Continued)

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 +RB)C

The frequency of oscillation is:

Figure 6 may be used for quick determination of these RC

Values. The duty cycle, as a fraction of total period that the

output is low, 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 the Control

Voltage Terminal. Figure 8 shows the circuit, and in Figure 9

are some waveform examples.

00866912

VCC = 5V

Top Trace: Output 5 V/Div.

TIME = 20 µs/Div.

Bottom Trace: Capacitor Voltage 1 V/Div.

RA = 3.9 kΩ

RB =9kΩ

C = 0.01 µF

FIGURE 5. Astable Waveforms

00866913

FIGURE 6. Free Running Frequency

00866914

VCC = 5V

Top Trace: Input 4 V/Div.

TIME = 20 µs/Div.

Middle Trace: Output 2 V/Div.

RA = 9.1 kΩ

Bottom Trace: Capacitor 2 V/Div.

C = 0.01 µF

FIGURE 7. Frequency Divider Waveforms

00866920

FIGURE 8. Pulse Width Modulator

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