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TLC555: Astable oscillator startup problem

Jef Daems
Prodigy 50 points

Part Number: TLC555

Hi,

I am having problems with a TLC555I circuit in astable mode:

The circuit is designed to oscillate at about 700 kHz and drives a charge pump via a 2.2 uF ceramic capacitor on node "CP_SWITCH". 
The input voltage is 4.8 V, and the circuit works fine when I directly apply the input voltage by plugging it in directly. However, when slowly ramping up the input voltage, the oscillator doesn't seem to be able to start. The capacitor is discharged, but only once, and seems to stop discharging too late. This repeats every +-2 ms:

 

Yellow = pin 2/6 (trig/thres), green = pin 7 (discharge), red = pin 5 (Vcontrol), blue = pin 8 (VCC)

I already checked for noise on the reset pin, which seemed to be clean. Does anyone have a clue as to what might cause this behavior?

Kind regards,

Jef Daems

  • 0
    TI__Guru**** 173986 points
    Jef,

    This is very odd. Thank you for the waveforms, they answer most of my questions. However, I do have some left.
    Is this a new or old design?
    Do multiple TLC555 samples do this?
    Does the next output pulse occurrence sync up with something else in the system?
    I assume OUT is like green waveform except it rises much faster.

    Try scope measuring pin 1 directly at the TLC555 (longshot as faster 4.8V rise time did work)
    Does the same issue happen if output PNP and NPN are not installed?
  • 0 in reply to Ron Michallick
    Prodigy 50 points

    Hi Ron,

    thanks for the fast response.

    This is an old design, but it is incorporated on a new board that has a different input supply design. 
    As it was a new board fresh out of production, it didn't occur to me that the chip might be faulty. As per your suggestion, I swapped out the chips between the old (working) board and the new board and the problem moved with it. Other boards of the same batch (with TLC555I of the same lot) do not show the problem. I guess the IC was indeed faulty, and I'll have to make sure that the new input supply design did not cause this.

    For me this solves the problem.

    Thanks!