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TLC555: Output frequency not fix

Ankit Gupta6
Intellectual 565 points
Part Number: TLC555
Other Parts Discussed in Thread: SN74LVC1GX04

Hello,

I have a really basic question.

I designed an Astable 555 timer circuit with an output frequency of 100kHz. 

However, even though I set its output frequency to 100kHz by varying the resistors, it never remains fixed and changes by 4-5% during operation.

I thought that the problem was with the timing capacitors I was using. So I replaced them with class I ceramic caps. But still, I have the same issue.

Even after i reset the TLC555 IC it continues to operate at the wrong frequency.

Can someone please guide me to what can the possible error?

or can suggest some new circuit with a more robust output.

-Ankit

  • 0
    TI__Guru**** 164656 points
    Ankit,

    What values are you using for RA, RB, and C? Do you have a capacitor on control pin to ground? What is the Vdd voltage?

    The frequency drift you see is higher than I would expect. Can you link the frequency change to something else in the system that changes in the same way or at the same time?
  • 0 in reply to Ron Michallick
    Intellectual 565 points

    Schematic Prints.pdfHi Ron,

    Kindly find attached a schematic pdf file which has details about my circuit and the value of different capacitors and resistors used.

    R11 is a variable resistor (potentiometer Bourn 3296)
    R10 is a fixed 10K resistor with 805 packages.
    Timing Capacitor C6 is a 150pF Class I capacitor.
    Vdd is 3.3V.

    the Vdd is constant, reset signal comes from the DSP through a PNP transistor logic.

    The frequency keeps drifting in the negative direction (reduces with time). Even after giving it a reset signal, the frequency starts from the same value it was having before the reset. I think it has something to do with the RC circuit.
    What are your thoughts?

    -Ankit

  • 0 in reply to Ankit Gupta6
    TI__Guru**** 164656 points

    Ankit,

    First try adding a 100nF (or similar) capacitor from CONT pin to ground. Pins 5 to 1. If that doesn't help, the problem may be temperature change. Is the TLC555 in a pace where the temperature changes a lot? The TLC555, both resistors, capacitor, and diode change with temperature. 

    If getting as close to 100 Khz as possible is important, then you could lose the potentiometer and switch to a crystal. The cost of the crystal may be more than offset by not having to buy and adjust the potentiometer. try SN74LVC1GX04 with a 100 kHz crystal. Pulling X1 pin to ground would "reset" the SN74LVC1GX04 to an output low state 

  • 0 in reply to Ron Michallick
    Intellectual 565 points
    Thank you Ron,

    Adding a 100nF capacitor between pin 1 and 5 helped.
    Now the variation is less than 0.5%. I think i can work with that.

    Also, thanks for the suggestion you provided of using a 100kHz crystal.

    -Ankit