Datasheet

LINEAR RAMP
When the pullup resistor, R
A
, in the monostable cir-
cuit is replacedbya constantcurrentsource,alinear
ramp is generated. Figure 17 shows a circuit con-
figuration that will perform this function.
Trigger
Output
C
NE555
2
4
3
1
5
6
7
8
E
V
CC
0.01µF
R2
R1
R
2N4250
or equiv.
Figure 17.
Out
R
A
C
NE55
2
4
3
1
5
6
7
8
V
CC
51kΩ
R
B
22kΩ
0.01µF
V
CC
0.01µF
Figure 19 : 50%Duty Cycle Oscillator.
Figure 18 shows waveformsgeneratorby the linear
ramp.
The time interval is given by :
T =
(2/3 V
CC
R
E
(R
1+
R
2)
C
R
1
V
CC
− V
BE
(R
1+
R
2)
V
BE
= 0.6V
Figure 18 : Linear Ramp.
V
CC
= 5V Top trace : input 3V/DIV
Time = 20µs/DIV Middle trace : output 5V/DIV
R
1
= 47kΩ Bottom trace : output 5V/DIV
R
2
= 100kΩ Bottom trace : capacitor voltage
R
E
= 2.7kΩ 1V/DIV
C = 0.01µF
50% DUTY CYCLE OSCILLATOR
For a 50% duty cycle the resistors R
A
and R
E
may
beconnectedasin figure19. Thetime preriodforthe
output high is the same as previous,
t
1
= 0.693 R
A
C.
For the output low it is t
2
=
[(R
A
R
B
) ⁄ (R
A
+ R
B
)] CLn



R
B
− 2R
A
2R
B
− R
A



Thus the frequencyof oscillation is f
=
1
t
1
+ t
2
Note that this circuit will not oscillate if R
B
is greater
than 1/2 R
A
becausethe junction of R
A
and R
B
can-
notbring pin2 down to 1/3 V
CC
andtriggerthelower
comparator.
ADDITIONAL INFORMATION
Adequate power supply bypassing is necessary to
protect associated circuitry. Minimum recom-
mended is 0.1µF in parallel with 1µF electrolytic.
NE555/SA555/SE555
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