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SN74LVTH16245A: Propagation Delay Too Fast

Robert Annis
Prodigy 50 points
Part Number: SN74LVTH16245A
Other Parts Discussed in Thread: SN74AHCT16245, SN74ALVCHR162245

Hi,

I'm looking for a transceiver with the same form factor as the SN74LVTH16245ADGG, but with a slower propagation delay (> 1.5ns Min.).  I saw the 74LVCR16245A, which has the same Min t_pd, but a slower Max.  Does anyone know of another with a slower Min t_pd?

Thanks in advance.

-Robert

  • 0
    TI__Guru* 96711 points
    Hi Robert,
    I've never heard of someone needing a 0.2ns slower minimum prop delay... can you share the application that requires such a small change?

    The easiest way to add a small amount of propagation delay is to add some trace on the PCB -- 1 inch equals about 165 ps... so adding about 1.2 inches of trace to the output will guarantee a minimum of 1.5ns for the SN74LVTH16245A.

    If that's not an option, you can also add output capacitance. The device is rated at 50pF, so doubling that would significantly slow the part.

    There's also weaker output drive devices like the SN74AHCT16245, however this device does not have the bus-hold feature of the part you are currently using. Is that a requirement for you?
  • 0 in reply to Emrys Maier
    Prodigy 50 points
    Hi Emrys,
    Thanks for your answer. In talking with one of my coworkers, I realized that I had my post wrong: I don't necessarily need a longer prop delay, but a more gradual rise/fall time. I'm not sure where I could get information on those specs, though.

    Sorry for the confusion, thanks for the suggestions.
  • 0 in reply to Robert Annis
    TI__Guru* 96711 points
    If you get a weaker driver (ie low output current) then you will see a reduction in the output edge rate. The AHCT device I suggested above will definintely have slower edges than the LVT driver, but it might not be as slow as you want.

    Are you trying to eliminate ringing by adjusting the output edge rate, or is there a device further down the line that's sensitive to the slew rate?
  • 0 in reply to Emrys Maier
    Prodigy 50 points
    Trying to eliminate the ringing in an existing design, so I'm trying to find a drop-in replacement before thinking about respinning the board. The AHCT looks good, but the current design is for 3.3V.
  • 0 in reply to Robert Annis
    TI__Guru* 96711 points
    The AHCT device will likely function at 3.3V, but it won't be guaranteed by TI since we don't spec it down there.
  • 0 in reply to Emrys Maier
    TI__Guru* 96711 points
    You could also try this: SN74ALVCHR162245

    It has the built-in damping resistors to limit ringing.
  • 0 in reply to Emrys Maier
    Prodigy 50 points

    I'll take a look.  Thank you for all your help.