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TLC555: Switching frequency generator with TLC555 - frequency tolerances/errors

Roman Heinisch
Prodigy 20 points
Part Number: TLC555
Other Parts Discussed in Thread: LMC555

Hi,

I am using the timer to generate a switching frequency for a high frquency transformer, see circuit diagram below. 2fw is connected to the clock input of a D-Flip-Flop where Qn is connected to its input D. So it halfs 2fsw to fsw.

I must know which switching frequency range is possible.

So I did the error calculation below,  including propagation delay times and VIT und VI(TRIG) tolerances from the datasheet.

The frequency range is quite big. The measured switching frequency at room temperature is 268.12kHz. So it is very near to the caculated value of 278kHz.

I am wondering if VIT can have the max. value of 3.9V while VI(TRIG) has its smallest value of 1.26V.

Or is VI(TRIG)=2.06V (MAX) when VI(TRIG)=3.9V (MAX)?

Thanks a lot for your help in advance.

  • 0
    TI__Mastermind 42273 points

    Hi Roman,

    Your pictures did not come through. Can you reattach using the "Insert/Edit Media" button:

    The min/max values for VIT and VI(TRIG) come from the potential mismatch in the internal resister divider. 

    So depending on the mismatch, it's possible that VI(TRIG) could be at its min value while VIT is at its max value.

  • 0 in reply to Katlynne Jones
    Prodigy 20 points

    Hi Katlynne,

    thanks a lot for you quick answer.

    I have to say the tolerances are quite big.

    V(TRIG) Trigger voltage Full range = 1.26..1.66..2.04V (+/-24%)

    VIT Threshold voltage Full range = 2.7..3.3..3.8V (+/-18%)

    This tolerances make the frequency range im my application very big 214..278..396kHz. So I can't use this IC, because my HF-transformer is not able for this range.

    Why is the resistance divider so inaccurate? Today resistors have 1% perhapy 5%, but not 24%?

    Greetings

    Roman

  • 0 in reply to Roman Heinisch
    TI__Mastermind 42273 points

    Hi Roman,

    I asked a colleague and he said the resistors (actually FET based resistors) are very well matched. You'd probably see values closer to the typical values, though unfortunately the min/max specs in the datasheet are wide. The LMC555 has a slightly tighter tolerance, though I suspect it still may be too wide for you. If accuracy is a big concern, we usually recommend nanotimers for lower frequency applications, and a crystal oscillator or logic multi-stage ripple counter for high frequency application.

    Let me know if I can help you with anything else.  

  • 0 in reply to Katlynne Jones
    TI__Mastermind 42273 points

    Hi Roman,

    I received an email from the field applications engineer working with you asking if the tolerances are from production variation or temperature drifts. I'll answer that question here.

    So the internal resistors are well matched and you probably won't see as much variation, but the specs given in the 25°C row do not include any temperature drift. So that would be solely from production variations. If you look at the row below, the min/max range for the full temperature range is given. I don't have a spec for the temperature drift, but you can see it's within 100mV of the 25°C value.  

    The LMC555 does give a timing shift with temperature value of 75ppm/°C and it's marketed to have excellent temperature stability.