LM555/NE555/SA555
+Vcc
R
R
A
B
4
8
Vcc
RESET
7
6
5
DISCH
TRIG
2
3
THRES
CONT
Output
OUT
Modulation
C
GND
1
Figure 12. Waveforms of pulse position modulation
Figure 11. Circuit for 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
R1
R
E
4
8
RESET
Vcc
7
6
DISCH
Q1
TRIG
OUT
2
3
R2
THRES
CONT
Output
C1
5
GND
C2
1
Figure 14. Waveforms of Linear Ramp
In Figure 13, current source is created by PNP transistor Q1 and resistor R1, R2, and R .
Figure 13. Circuit for Linear Ramp
E
V
– V
CC
R
E
I
= --------------------------
(12)
C
E
Here, V
E is
R
2
---------------------
V
= V
+
V
(13)
E
BE
CC
R
+ R
1
2
For example, if Vcc=15V, R =20kΩ, R1=5kW, R2=10kΩ, and V =0.7V,
BE
E
V =0.7V+10V=10.7V
E
Ic=(15-10.7)/20k=0.215mA
9
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
