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Original question:

SA555: R, C value considerations

SA555: How to calculate propagation delay

Hiromu Susami
Intellectual 1120 points

Part Number: SA555
Other Parts Discussed in Thread: TLC555,

Hi expart,

In previous thread, I asked about propagation delay.

 SA555: R, C value considerations - Clock & timing forum - Clock & timing - TI E2E support forums

Ron exprained "Propagation delay for 16kHz will introduce roughly a 4% error"

I calculated it but the results was <0.1% Delay. Could you tell me how to calculate?
Propagation delay calculator.xlsx

Followings from TLC555 datasheet.

image.png

image.png

Regards,
Hiromu

 

  • +1
    TI__Guru**** 170196 points

    Hiromu-san,

    TLC555 is much faster than SA555, so the data is not helpful. The tc(h) and tc(l) formulas need the effective propagation delay that is greater than a charted propagation delay. Charted propagation delay is a large signal step function setup. In real astable circuits, the input is a slow resistor capacitor charge curve. Therefore the over drive varies with time at a rate set by CT, RA, RB, ron. For too much data for a simple chart. Calculation yields an simplified approximate result. Additionally most modern capacitors vary based on operating point (example voltage modulation). 

    I interpolated 4% based on empirical real tests. It is also an estimation. The circuit has to be built to know the exact result.