Applications Information (Continued)
Figure 5 shows the waveforms generated in this mode of
operation.
DS007851-11
V
= 5V
Top Trace: Input 4V/Div.
TIME = 20µs/DIV. Middle Trace: Output 2V/Div.
= 9.1kΩ Bottom Trace: Capacitor 2V/Div.
CC
R
A
C = 0.01µF
DS007851-9
FIGURE 7. Frequency Divider
V
= 5V
Top Trace: Output 5V/Div.
CC
TIME = 20µs/DIV.
Bottom Trace: Capacitor Voltage 1V/Div.
R
R
= 3.9kΩ
= 3kΩ
A
PULSE WIDTH MODULATOR
B
C = 0.01µF
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.
FIGURE 5. Astable Waveforms
The charge time (output high) is given by:
t1 = 0.693 (RA + RB) C
And the discharge time (output low) by:
t2 = 0.693 (RB) C
Thus the total period is:
T = t1 + 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:
DS007851-12
FIGURE 8. Pulse Width Modulator
DS007851-13
V
= 5V
Top Trace: Modulation 1V/Div.
TIME = 0.2 ms/DIV. Bottom Trace: Output Voltage 2V/Div.
= 9.1kΩ
CC
DS007851-10
FIGURE 6. Free Running Frequency
R
A
C = 0.01µF
FREQUENCY DIVIDER
FIGURE 9. Pulse Width Modulator
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.
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