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Original question:

SN74VMEH22501A: undershoot

SN74VMEH22501A: undershoot for around 10ns

Hung Ching Hsu
Expert 3086 points
Part Number: SN74VMEH22501A
Other Parts Discussed in Thread: SN74LV14A

Hi Team, 

My customer is using SN74VMEH22501A and SN74LV14A. 

He has question to undershoot of VI. The undershoot may be -0.7V in customer's application, which exceeding abs. max rating of VI. 

Datasheet says "(2) The input and output negative-voltage ratings may be exceeded if the input and output current ratings are observed.", 

So I believe both device can exceed -0.5V as long as IIK is lower than 50mA(SN74VMEH22501A) and 20mA(SN74LV14A) respectively, is my understanding correct?

If so, could you advise dampening resistor value to suppress the current for each device? 

Thanks.

Regards,

Jo

  • 0
    TI__Genius 14210 points

    Hi Hung,

    Hung Ching Hsu said:
    So I believe both device can exceed -0.5V as long as IIK is lower than 50mA(SN74VMEH22501A) and 20mA(SN74LV14A) respectively, is my understanding correct?

    You are correct. Can you tell me the voltages you are applying on the input of each device? 

    Regards,

    Sebastian 

  • 0 in reply to Sebastian Muriel
    TI__Expert 3086 points

    Hi Sebastian, 

    Now customer have difficulty to measure IIK since they have to add a lead which may cause the transient performance change, and also the current probe is difficult to precisely measure 20mA or 50mA. 

    Customer is wondering if there's any measure that can estimate IIK based on following information, could you kindly help? Thanks. Or any idea that customer can check IIK?

    Following is customer's config. There are two types. Undershoot happens on input pin of both receive side. 

    Transmit side and receive side is on different board and the cable length is around 20cm. 

    1. Transmit side (SN74VMEH22501A) -> Receive side (SN74VMEH22501A)

    2. Transmit side (SN74VMEH22501A) -> Receive side (SN74LV14A)

    For SN74VMEH22501A, Vpeak of undershoot = -0.66V. The time period below 0V is 12ns, and time period below abs. max rating(-0.5V) is 8ns~10ns. 

    For SN74LV14A, Vpeak of undershoot = -0.54V. The time period below 0V is 3~4ns, and time period below abs. max rating(-0.5V) is 1ns. 

    Thanks.

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    Guru 256660 points

    This could be estimated from the IBIS models.

    sn74vmeh22501a.ibs says that for a VMEH22501A_IO_3A pin, at −0.665 V, the worst-case current is 2.1 mA.

    lv14a.ibs says that for a LV14A_IN_50 pin, at −0.5 V, the worst-case current is 0.353 mA.

  • 0 in reply to Clemens Ladisch
    TI__Expert 3086 points

    Hi Clemens, 

    Thank you for the help. 

    Based on your comment, since current doesn't exceed respective abs. max rating, it won't cause issue for customer's usage above, correct? 

    Also, no damping resistor needed to add to input pin, right? 

    Thanks.

    Regards,

    Jo

  • +1 in reply to Hung Ching Hsu
    TI__Genius 14210 points

    Hi Hung,

    As long as the input current remains under 50mA(SN74VMEH22501A) and 20mA(SN74LV14A), the damping resistor is not needed on the input and there shouldn't be any issues .

    Regards,

    Sebastian