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SN74LVC1G3157: Input transition rise or fall rate specification

Laszlo Gaspar
Prodigy 30 points
Part Number: SN74LVC1G3157
Other Parts Discussed in Thread: SN74LVC1G17, TLV810E, TMUX1219

Tool/software:

Could you please clarify a discrepancy between specifications in the data sheet and the typical application circuit.

The data sheet for the SN74LVC1G3157 shows that the recommended operating conditions include a 20 ns/V maximum rise and fall rates for the control input.

In addition, it also draws attention to the Application Report titled "Implications of Slow or Floating CMOS Inputs".

I understand why slow rise and fall times should be avoided with non-Schmitt-trigger CMOS inputs.

But then Section 15.2 of the data sheet (Typical Application) shows a schematic that definitely violates the maximum rise and fall conditions, as it shows an RC delay circuit driving the logic input that would certainly have rise and fall times in the range of tens of milliseconds in any practical application.

So what is the situation? Are slow rise and fall times acceptable on the logic input of the SN74LVC1G3157, or not? 

(By the way, I intend to use the SN74LVC1G3157 driven from an open-drain device; the fall time will certainly be fast but the rise time will be in the hundreds of nsec region.)

  • 0
    Guru 316690 points

    Yes, that schematic is wrong. A Schmitt-trigger buffer like the SN74LVC1G17 would be necessary (or a proper voltage supervisor like the TLV810E).

  • 0 in reply to Clemens Ladisch
    TI__Mastermind 24440 points

    Hi Laszlo,

    I agree with clemens

    I would also say that if you would like to replace the 3157 with a multiplexer that reduce the affects of a slow input control signal, then you could look into the TMUX1219 which has a feature called 1.8V logic support. 

    Essentially muxes with this feature have a smaller hysteresis range on the control inputs that limits the affects of slow inputs.

    Here is another good FAQ to read on this topic.

    I wanted to offer this suggestion as it is another option you could look into.

    Regards,

    Kameron

  • 0 in reply to Kameron Hill
    Prodigy 30 points

    Hi Kameron,

    I hear what you are saying... and don't disagree. 

    But I have a few remarks:

    1.) I find it hard to believe that Texas Instruments would publish a typical application circuit, as part of the data sheet, that is so wrong.

    2.) I like the SN74LVC1G3157 because it is multiple sourced (not only TI). 

    3.) The rise and fall conditions are recommended rather than absolute limits.

    4.) With regard to the FAQ about the multiplexer shoot-through: OK, during the input transition the supply current will show a 650 uA spike - so what? At 5 Volt supply that will result in a few mW of dissipation.

  • 0 in reply to Laszlo Gaspar
    Guru 316690 points

    2. Other 74LVC1G3157 devices have similar Δt/Δv limits.

    3. When you go outside the recommended operating conditions, the electrical characteristics or even correct operation are no longer guaranteed. In this case, the selection signal might oscillate while it is near the switching threshold.