LMC555
Application Info
(Continued)
DS008669-14
DS008669-12
V
CC
= 5V
TIME = 20 µs/Div.
R
A
= 3.9kΩ
R
B
= 9kΩ
C = 0.01µF
Top Trace: Output 5V/Div.
Bottom Trace: Capacitor Voltage 1V/Div.
V
CC
= 5V
Top Trace: Input 4V/Div.
TIME = 20 µs/Div. Middle Trace: Output 2V/Div.
Bottom Trace: Capacitor 2V/Div.
R
A
= 9.1 kΩ
C = 0.01µF
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.
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
t
1
= Ln2 (R
A
+ R
B
)C
And the discharge time (output low) by:
t
2
= Ln2 (R
B
)C
Thus the total period is:
T = t
1
+ t
2
= Ln2 (R
A
+ R
B
)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:
DS008669-20
FIGURE 8. Pulse Width Modulator
DS008669-13
DS008669-15
FIGURE 6. Free Running Frequency
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.
V
CC
= 5V
Top Trace: Modulation 1V/Div.
TIME = 0.2 ms/Div. Bottom Trace: Output Voltage 2V/Div.
R
A
= 9.1 kΩ
C = 0.01µF
FIGURE 9. Pulse Width Modulator Waveforms
PULSE POSITION MODULATOR
This application uses the timer connected for astable opera-
tion, as in
Figure 10,
with a modulating signal again applied
to the control voltage terminal. The pulse position varies with
www.national.com
6
NSC [ NATIONAL SEMICONDUCTOR ]
