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TLC555: Some questions to get 1MHz PWM

daniel
Genius 5095 points
Part Number: TLC555


Hi Team,

We want to use TLC555I (3.3VDD) to get 1MHz PWM, please kindly help on below questions. Thanks.

1. Should I need another 1MHz OSC as a clock input on TRIG pin?

2. What is the voltage range of the OSC? Is it also 3.3V?

3. If I use 1.65V as a modulation input, would the PWM duty become 50%?

4. What is the voltage range of the modulation input?

5. Would you recommend RL,RA,C value?

Thanks

  • 0
    TI__Guru 64125 points

    Hello Daniel,

    Our TLC555 experts are looking in to your question and will respond tomorrow.

  • 0
    TI__Guru**** 166116 points
    Daniel,

    Let's look at this circuit. Output goes high just after "clock input" and it will stay high until just after timing capacitor voltage exceeds "modulation input". At that point output will go low if "clock" input is high. Therefore the negative pulse width of the "clock input" must be less than the lowest positive pulse width needed. Also the rise in capacitor voltage is not linear it is exponential. Therefore the positive pulse width vs "modulation input" is not linear. Does it need to be linear?

    What is the minimum and maximum duty cycle or pulse width is required?

    1) Yes, input 1MHz
    2) Clock input low level must be less than 1.1V ideally near 0V, high level must be greater than 1.1V; near 3.3V is fine.
    3) No not 50% (however there is a way to improve linearity)
    4) Pin 5 input range is about 0.5V to near 3.3V
    5) I will provide these later.
  • 0 in reply to Ron Michallick
    Genius 5095 points
    Dear Michallick,
    Thanks for your help.
    So how to adjust the duty cycle period of PWM wave? The method of calculating the duty cycle period?

    Thanks
  • 0 in reply to daniel
    TI__Guru**** 166116 points
    Daniel,

    1 MHz is close to the maximum frequency of TLC555 so I will need to test this so see how well it works.
    I should have results by the end of the week.
  • 0 in reply to Ron Michallick
    TI__Guru**** 166116 points

    Daniel,

    The TLC555 did not perform well in this application's frequency request.

    Input was a 1 MHz 50% duty cycle clock. VCC = 3,3V; Control voltage tested from 0V to 3.35V

    The missing section of the curves indicates the output waveform was inconsistent (flaky) at low control voltage and 500 kHz output at high control voltage.

    There was about 160ns propagation delay from trigger input to output going high. So TLC555 is not fast enough.

    The results indicate that TLC555 is not a good fit for this application need.

    I suggest using a triangle wave generator and a high speed comparator to get a 1 MHz PWM instead of TLC555

  • 0 in reply to Ron Michallick
    Genius 5095 points
    Dear Michallick,

    Many thanks for your help.
    I will discuss with customer about this limitation.

    Thanks