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SN74LV244A: Undershoots of SN74LV00APWR, SN74LV14APWR, SN74LV32APWR, and SN74LV244APWR

?? ??
Prodigy 10 points
Part Number: SN74LV244A

Tool/software:

Hi Team,
We use the following devices. All devices have exceeded the undershoot.
The duration is within 1 ns.


SN74LV00APWR -0.700mV
SN74LV14APWR -0.700mV
SN74LV32APWR -0.660mV
SN74LV244APWR -0.780mV

So I believe all devices can exceed -0.5V as long as IIK is lower than 20mA respectively, is my understanding correct?

Based on the above, could you tell me how many volts I can use?
Or can I check the description in the IBIS model?
For example, the description after "Model LV 14 A _ IN _ 50" in "lv 14 a.ibs". The applied voltage is typ5V (4.75 V ~ 5.25 V).

Thanks.

Regards,

  • 0
    Guru 279750 points

    The clamping diode is a plain silicon diode. At about −0.7 V, the current increases greatly.

    The SN74LV244A IBIS model says that at −0.7 V, the worst-case current is about −13.6 mA, but at −0.78 V, it's −25 mA.

    How exactly did you measure these undershoots? If you did not use an oscilloscope probe with a ground spring, then it's likely that you also got some ringing in the loop formed by the probe's ground clip.

    If you actually have undershppts, then you should properly terminate the signal lines. Usually, the easiest way is source termination; you could also add external clamping diodes at the receiver.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thank you for your response.

    As with "SN74LV244A", does the current on other devices exceed 20 mA when it exceeds -0.7 V?

    Or is the threshold around -0.75 V?

    I measured using an active probe.The length of the ground lead is short.

  • 0 in reply to ?? ??
    Prodigy 10 points

    I add questions.

    Can't -20 mA or -0.5 V exceed even 1 ns? Is there a time limit?

  • 0 in reply to ?? ??
    Guru 279750 points

    All LV-A inputs behave the same.

    The absolute maximum ratings must not be exceeded, not even for a short time. (But it's not clear if they actually get exceeded.)