LM555/NE555/SA555
+Vcc
4
RESET
8
Vcc
DISCH
R
A
7
R
B
6
2
TRIG
THRES
Output
3
OUT
GND
CONT
Modulation
5
C
1
Figure 11. Circuit for Pulse Position Modulation
Figure 12. Waveforms of pulse position modulation
6. Linear Ramp
When the pull-up resistor RA in the monostable circuit shown in Figure 1 is replaced with constant current source, the V
C1
increases linearly, generating a linear ramp. Figure 13 shows the linear ramp generating circuit and Figure 14 illustrates the
generated linear ramp waveforms.
+Vcc
R
E
R1
4
RESET
8
Vcc
DISCH
7
Q1
2
Output
TRIG
THRES
6
C1
C2
R2
3
OUT
GND
CONT
5
1
Figure 13. Circuit for Linear Ramp
Figure 14. Waveforms of Linear Ramp
In Figure 13, current source is created by PNP transistor Q1 and resistor R1, R2, and R
E
.
V
–
V
CC
E
= --------------------------
-
C
R
E
Here, V
E is
R2
-
+ ---------------------
V
BE R
+
R CC
1
2
I
(
12
)
V
E
=
V
(
13
)
For example, if Vcc=15V, R
E
=20kΩ, R1=5kW, R2=10kΩ, and V
BE
=0.7V,
V
E
=0.7V+10V=10.7V
Ic=(15-10.7)/20k=0.215mA
9
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
