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CD54HC4050: Output transition time for 20 pF load

Shashank Meti
Intellectual 950 points
Part Number: CD54HC4050

Hi,

For the device TI CDHC4050 (High speed Hex buffer) the transition time according to datasheet for 50 pF output load is 22 ns.

What will be the transition time for the output load capacitance of 20 pF?

Thanks & Regards,

Shashank  

  • 0
    TI__Guru 62930 points
    Shashank,
    We do not have a datapoint characterized for 20pF.
    I will try to perform a simulation using IBIS to estimate the change with smaller load. We do not have IBIS for the 4050, but do for the 4049, and it has same output and spec for trise/fall.
    Unfortunately, I do not have access to a license to simulate until later today, or early tomorrow. I will update after the simulation is completed.
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    TI__Guru 62930 points
    I was able to simulate a 50pf load, and a 20pf load.
    Unfortunately, the IBIS models is an empirical one, and only encompasses results for 1 device. It is not known if this device is "typical", or stronger or weaker with respect to process.
    However, this may still help you make a decision.
    The rising edge results were 5.65ns with 50pf and 3.25 with 20pf. This is a reduction of 42%.
    The falling edge results were 3.9ns with 50pf and 2.85 with 20pf. This is a 27% reduction.

    Being somewhat conservative, you could apply the worst case 22ns number with the 27% decrease to estimate this. I expect that this will be conservative, but uncertain due to the many parameters involved that cannot be easily accounted for.
    Using this approach, you could estimate a rise/fall worst case of 16ns with a 20pf load.

    If this answers your question, please click "Verify it as the answer"
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    TI__Intellectual 950 points

    Hi Wade,

    Can you please share the simulation file? So that we can further verify for different load conditions.

    Thanks,

    Shashank

  • 0 in reply to Shashank Meti
    TI__Guru 62930 points
    I used the IBIS model located here:
    www.ti.com/.../toolssoftware
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    TI__Intellectual 950 points
    Hi Wade,

    If the input signal is 24 MHz clock. Which among these is suitable?

    54HC244 – Tristate buffer, output transition time – 18 ns
    54HC4050 – 2-state buffer, output transition time – 22 ns

    Can you please suggest taking into consideration the rise & fall times as well?

    Thanks,
    Shashank
  • 0 in reply to Shashank Meti
    TI__Guru 62930 points
    You are looking for a 5v buffer (with or without tristate) that can drive 24Mhz to a high impedance input with 20pf load. Operating temp range -55 to 125c in hermetic (ceramic) package?
    Any other constraints?
    I can try to see what is the best available if you can confirm above.
    Regards,
    Wade
  • 0 in reply to Wade Vonbergen
    TI__Guru 62930 points
    Closing this out, as remaining questions handled via email.
    -Wade