SE555 datasheet(14/20 Pages) STMICROELECTRONICS

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14 / 20 page

Application information

NE555 - SA555 - SE555

14/20

Figure 20 shows the waveforms generator by the linear ramp.

The time interval is given by:

Figure 20.

Linear ramp

4.5

50% duty cycle oscillator

For a 50% duty cycle, the resistors RA and RE can be connected as in figure 19. The time

period for the output high is the same as for astable operation (see Section 4.2 on page 10):

t1 = 0.693 RA C

For the output low it is

Thus the frequency of oscillation is:

T =

(2/3 Vcc RE (R1+R2) C

R1 Vcc - VBE (R1+R2)

---------------------------------------------------------------- VBE = 0.6V

VCC = 5V

Time:

20µs/DIV

R1 + 47kΩ

R2 = 100kΩ

RE = 2.7kΩ

C = 0.01µF

Top trace: input 3V/DIV

Middle trace: output 5V/DIV

Bottom trace: output 5V/DIV

Bottom trace: capacitor voltage 1V/DIV

[(R. RB)/(RA+RB)].C.Ln

2RA

–

2RB

–

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