SA555DT Datasheet, PDF (13/20 Pages) STMicroelectronics – General-purpose single bipolar timers

NE555 - SA555 - SE555
Application information
Figure 20 shows the waveforms generator by the linear ramp.
The time interval is given by:
T = --_R(--²---₁-^/-³---_V--^V--_c--^c-_c--^c--_----^R-_V---^E-_B----_E-⁽--^R---₍-¹-_R---⁺-₁--^R-₊---²-_R---⁾-₂--^C-₎-- VBE = 0.6V
Figure 20. Linear ramp
V_CC = 5 V
Time:
20 µs/DIV
R₁ + 47 kΩ
R₂ = 100 kΩ
R_E = 2.7 kΩ
C = 0.01 µF
Top trace: input 3 V/DIV
Middle trace: output 5 V/DIV
Bottom trace: output 5 V/DIV
Bottom trace: capacitor voltage 1 V/DIV
4.5
50% duty cycle oscillator
For a 50% duty cycle, the resistors R_A and R_B can be connected as in Figure 21. The time
period for the output high is the same as for astable operation (see Section 4.2 on page 9):
t1 = 0.693 R_A C
For the output low it is
t₂= [(R. RB)/(RA+RB)].C.Ln
R_-----B_{------–----}2_----R_-----A_--
2RB – RA
Thus the frequency of oscillation is:
f = -------¹---------
t1 + t2
Doc ID 2182 Rev 6
13/20