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Original question:

SN74LV221A: SN74LV221A Shorter pulse than expected on output

SN74LV221A: Replacing SN74HCT221A

Harry McNally
Intellectual 420 points
Part Number: SN74LV221A
Other Parts Discussed in Thread: CD74HCT221

Hello

We have a product using the SN74HCT221A and evaluating whether the SN74LV221A will be a drop in replacement.

This thread describes a difference in the LV part:

SN74LV221A: SN74LV221A Shorter pulse than expected on output

e2e.ti.com/.../836072

Our circuit has a 510R and 200pF RC LPF then an VHCT125 buffer prior to the HCT221A inputs. The RC has been noted as a revision to remove input noise. Our input circuit is being changed and I am wanting to also delete the RC LPF and VHCT125.

Has TI evaluation from the thread above determined that the re-triggering issue is present only on the LV part ?

Also, has examination confirmed that the re-triggering failure only occurs during the capacitor discharge period as Nicolas' tests indicate ?

We have a 3.3 volt supply, Ct 470pF and Rt 1k (for 630nS output pulses) and our slow rise-times on a new opto-isolator at the inputs are unlikely to let through pulses as Nicolas demonstrated.

Thank you
Harry 

  • 0
    TI__Mastermind 24805 points

    Hello Harry, 

    This has been assigned to the appropriate product expert, who will help you with your request when we return from the US holiday.

    Best,
    Michael

  • 0
    TI__Expert 6680 points

    Hi Harry,

    Is there a reason you're switching from the CD74HCT221 to the SN74LV221A?

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    Intellectual 420 points

    Very sorry Chad, I mis-typed the part we are using (working too late in the day).

    We are replacing the Fairchild (On Semi) 74VHC221AM which is obsolete with the TI SN74LV221A.

    The filter that was installed in our circuit was to deal with ground bounce noise but I'm trying to determine if the re-triggering (and shortening of the output pulse) was actually what the RC in our circuit was solving (in the Fairchild VHC part).

    In other words, if the LV part has this issue then the 74VHC221AM might be similar.

    As I said previously, the rise time on the new opto in the input side will probably prevent this but I will now test. I'm just keen to understand more as I explore the issue. If I know the re-trigger is only let through during the capacitor discharge period then I have a good handle on what to test for.

    Sorry about the part confusion.

    Thank you
    Harry

  • 0 in reply to Harry McNally
    TI__Mastermind 44610 points

    Hey Harry,

    The LV221 is the only 221 function that has exhibited this issue. Any time the input was triggered during the capacitor discharge cycle, it would cause the output pulse to be cut short. If the edge came after the capacitor was fully discharged, the output pulse would work as normal. With the smaller sized cap compared to the one Nicholas and I used, you will have a shorter discharge time making it less likely for you to have this issue. 

  • 0 in reply to Dylan Hubbard
    Intellectual 420 points

    Thanks Dylan

    Knowing the LV221A is the only part with the behaviour clarifies it for me.

    All the best
    Harry