General question about standard 555 timer

Discussion in 'General Electronics Chat' started by steveb, Oct 5, 2009.

  1. steveb

    Thread Starter Senior Member

    Jul 3, 2008
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    I have a general question about standard 555 timers. I know there are a number of 555 experts here, including Bill M.

    What is the approximate maximum frequency that can be generated with a 555 timer producing a decent quality sawtooth wave?

    I'm not looking for anything too accurate here; just a best estimate. The quality of the sawtooth is not critical and need only look like a sawtooth to the eye.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    I've never approached it, but given the theory of operation, I'm betting around 1Mhz. I'd put 500Khz as a minimum for sure. I'm basing this number off this datasheet. I suspect the layout of the PCB (stray capacitance, inductance) will have a strong say in this region, so for maximum performance I'd go SMT.

    Given the non quality of my oscope, this is just an opinion.

    Just curious, why?
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    A few hundred kHz. The sawtooth porduced by a 555 is not a linear sawtooth; it is in fact composed of segments of an RC charge and discharge curve.
     
  4. Wendy

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    Actually, the definition of a sawtooth is the RC curve version. The linear version is a triangle wave.

    *************************************************************************

    Just looked up the TLC555 (the TI version of CMOS 555), it has double the frequency rating (2Mhz), so should be able to go higher on the sawtooth function. I wonder how many sawtooths degenerate to something approaching a sine wave before the chip stops working?
     
    Last edited: Oct 5, 2009
  5. steveb

    Thread Starter Senior Member

    Jul 3, 2008
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    Thank you both for your help. I'm basically trying to find out if a 1 MHz sawtooth can be generated with a 555 timer. The sawtooth needs to be approximately a linear shape and of the type shown in the attached figure. I would try to implement a constant current charging on the capacitor to create a linear shape, rather than the normal exponential (RC) shape.

    Based on the TI chip, perhaps this is possible, but I'm still not sure. If anyone has any recommended circuits and some experience with this, I'd appreciate any advice. Otherwise, I'll probably try it out experimentally.
     
  6. MikeML

    AAC Fanatic!

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    That doesn't jive with my definition. Both triangles and sawtooths have linear slopes. A triangle wave has slope symmetry. A sawtooth has two different slopes.
     
  7. Wendy

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    Mar 24, 2008
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    I looked it up on Wikipedia a little bit ago. I stand corrected.
     
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