SE555 datasheet(8/10 Pages) STMICROELECTRONICS

8 / 10 page

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

NE555

VCC

0.01

µF

2N4250

or equiv.

Figure 17.

Out

R A

NE55

VCC

51k

Ω

22k

Ω

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 :

= (

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

Ω

Bottom trace : output 5V/DIV

R2 = 100k

Ω

Bottom trace : capacitor voltage

RE = 2.7k

Ω

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

+ 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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