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TLC555: TLC555 wrong timing

ZODIAC AEROSPACE
Prodigy 60 points
Part Number: TLC555

Hello,

I use a TLC555 as a monostable. In the worst case, when I take into account the TCR (15%) and tolerance (10%) for capacitors X7R, this monostable shall provide a pulse with a duration between 3.65s and 6.2s. In some case, at ambient temperature, I measure a pulse higher than 6.2s (about 6.5s or 6.7s). anybody can help me understand why or Do you know if anybody already has a similar problem?  already meet an equivalent problem.

  • 0
    TI__Guru*** 149986 points
    Zodiac,

    For an accurate time period it is important for the VCC (5V in this case) to be consistent throughout the timing period. There appears to be significant E1_AGND current so check to see if the difference between C1609/1610 and pin 1 GND is very low for output high and low.
    Is there a capacitor leakage specification? Is this leakage maximum significant compared to the average charge current, 3uA
    Last and most important, does this issue follow the timing capacitors in A-B-A swaps?

    You could also check threshold voltage ramp, however be careful not the change the ramp by measuiterning it.
  • 0 in reply to Ron Michallick
    TI__Guru*** 149986 points
    Zodiac,

    It is also best for control pin to charge to at least 1V (just after power up) before the trigger pulse [E1_COUNT_WD] arrives.