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NE555 Datasheet(PDF) 8 Page - STMicroelectronics

Part No. NE555
Description  GENERAL PURPOSE SINGLE BIPOLAR TIMERS
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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LINEAR RAMP
When the pullup resistor, RA, in the monostable cir-
cuit is replaced by a constant current source, a linear
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
VCC
0.01
µF
R2
R1
R
2N4250
or equiv.
Figure 17.
Out
R A
C
NE55
2
4
3
1
5
6
7
8
VCC
51k
RB
22k
0.01
µF
VCC
0.01
µF
Figure 19 : 50% Duty Cycle Oscillator.
Figure 18 shows waveforms generator by the linear
ramp.
The time interval is given by :
T
= (
2/3 VCC RE
(R1+ R2) C
R1 VCC
− VBE (R1+ R2)
VBE = 0.6V
Figure 18 : Linear Ramp.
VCC = 5V
Top trace : input 3V/DIV
Time = 20
µs/DIV
Middle trace : output 5V/DIV
R1 = 47k
Bottom trace : output 5V/DIV
R2 = 100k
Bottom trace : capacitor voltage
RE = 2.7k
1V/DIV
C = 0.01
µF
50% DUTY CYCLE OSCILLATOR
For a 50% duty cycle the resistors RA and RE may
be connected as in figure19. The time preriod for the
output high is the same as previous,
t1 = 0.693 RA C.
For the output low it is t2 =
[
(RARB) ⁄ (RA + RB)] CLn
RB − 2RA
2RB
− RA
Thus the frequency of oscillation is f
=
1
t1
+ t2
Note that this circuit will not oscillate if RB is greater
than 1/2 RA because the junction of RA and RB can-
not bring pin 2 down to 1/3 VCC and trigger the lower
comparator.
ADDITIONAL INFORMATION
Adequate power supply bypassing is necessary to
protect associated circuitry. Minimum recom-
mended is 0.1
µF in parallel with 1µF electrolytic.
NE555/SA555/SE555
8/10




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