LMC555
Application Information
(Continued)
FREQUENCY DIVIDER
The monostable circuit of
can be used as a fre-
quency divider by adjusting the length of the timing cycle.
shows the waveforms generated in a divide by three
circuit.
00866912
V
CC
= 5V
TIME = 20 µs/Div.
R
A
= 3.9 kΩ
R
B
= 9 kΩ
C = 0.01 µF
Top Trace: Output 5 V/Div.
Bottom Trace: Capacitor Voltage 1 V/Div.
00866914
V
CC
= 5V
TIME = 20 µs/Div.
R
A
= 9.1 kΩ
C = 0.01 µF
Top Trace: Input 4 V/Div.
Middle Trace: Output 2 V/Div.
Bottom Trace: Capacitor 2 V/Div.
FIGURE 5. Astable Waveforms
The charge time (output high) is given by
t
1
= 0.693 (R
A
+ R
B
)C
And the discharge time (output low) by:
t
2
= 0.693 (R
B
)C
Thus the total period is:
T = t
1
+ t
2
= 0.693 (R
A
+ R
B
)C
The frequency of oscillation is:
FIGURE 7. Frequency Divider Waveforms
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.
shows the circuit, and in
are some waveform examples.
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:
00866920
FIGURE 8. Pulse Width Modulator
00866913
FIGURE 6. Free Running Frequency
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