LMC555 Datasheet, PDF (8/21 Pages) National Semiconductor (TI) – CMOS Timer

LMC555
SNAS558J – FEBRUARY 2000 – REVISED MARCH 2013
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V_CC = 5V
TIME = 20 µs/Div.
R_A = 3.9 kΩ
R_B = 9 kΩ
C = 0.01 µF
Top Trace: Output 5 V/Div.
Bottom Trace: Capacitor Voltage 1 V/Div.
Figure 9. Astable Waveforms
The charge time (output high) is given by
t₁ = 0.693 (R_A + R_B)C
(1)
And the discharge time (output low) by:
t₂ = 0.693 (R_B)C
(2)
Thus the total period is:
T = t₁ + t₂ = 0.693 (R_A + 2R_B)C
(3)
The frequency of oscillation is:
(4)
Figure 10 may be used for quick determination of these RC Values. The duty cycle, as a fraction of total period
that the output is low, is:
(5)
Figure 10. Free Running Frequency
8
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