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Original question:

SN74LV221A: PULSE CUT SHORT, RETRIGGERS WHEN IT SHOULD NOT

SN74LV221A: SN74LV221A Issue Resolution and CD74HCT221 Recommendation

Obi Musturu
Prodigy 35 points
Part Number: SN74LV221A
Other Parts Discussed in Thread: CD74HCT221, CD74HC221

Tool/software:

Dear Texas Instruments Support Team,

I’m designing a trigger box for an MR scanner requiring reliable 5ms pulse stretching. I’ve noted issues with the SN74LV221A on E2E forums (e.g., pulse shortening/retriggering during capacitor discharge, threads: e2e.ti.com/support/logic/f/836072 and e2e.ti.com/support/logic/f/956110). My questions are:

  1. Have these issues been resolved in newer SN74LV221ASN74LV221A revisions?

  2. If not, do you recommend the CD74HCT221 for my 5V, 5ms pulse application (RC: ~71.5kΩ/100nF)?

Reliability is critical for medical use. I can provide further setup details if needed.

Thank you,

  • 0
    TI__Genius 12891 points

    Hi Obi,

    1. I can't find a satisfactory answer to if these problems were solved.

    2. That device or the CD74HC221 can work fine for this application. The main difference between the two will be the trigger level.

    Is there a reason you chose the 221 function? The reason I ask is we are releasing a new family of parts to match similar functionality to older monostable-multivibrators. Currently we've already released the TPUL2G123 which the main difference between the functions will be the retriggerable feature. We've done a lot of testing on these new devices, so they have much more reliability and consistency numbers within the datasheet than the older devices. If the non-retriggerable is necessary we are planning to release more functions in this family, and depending on your timelines those might be better for you.

    Regards,

    Owen

  • 0 in reply to Owen Westfall
    Prodigy 35 points

    Dear Owen,

    Thank you for your response regarding the SN74LV221A issues and alternative options.

    My application requires:

    1. Reliable 5ms pulse stretching from very short input pulses (100ns to 10ms)

    2. Non-retriggerable operation to ensure consistent output pulse width

    3. Multiple input sources (FOC via AFBR-2528 fiber optic receiver, BNC isolated inputs, and DSUB BIT16)

    4. Rising-edge triggering capability with noise immunity

    Can the CD74HC221 meet these requirements?

    I'm particularly concerned about the reliability in a medical application and whether the CD74HC221 will properly handle the rising-edge triggering from my FOC signal path.

    While I'm interested in the upcoming non-retriggerable TPUL family devices you mentioned, could let me know planned release time ?

    Thank you for your assistance.

    Best regards,
    Obi

  • +1 in reply to Obi Musturu
    TI__Genius 12891 points

    Hi Obi,

    Yes it can meet those requirements, and I'll have someone from our team reach out to you about the schedule of those TPUL devices.

    I did note the AFBR2528 has TTL outputs, so it may be best to use the CD74HCT221 as you originally asked as that is tuned to the TTL levels.

    Regards,

    Owen