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Question about SN74LV14A input capacitance

Takumi Suzuki1
Mastermind 8510 points

Hi,

I would like to ask you a question about the LV14A.

Are there anything restrictions about input capacitor?

For example, around 2uF capacitor will affect the device quality or movement?

I would like get your comment about this.

Thank you for your support.

Best Regards, 

  • 0
    TI__Guru 59070 points
    Hi Takumi San ,

    The LV14A should be schmitt trigger devices . do you have a reason for having 2uF caps specifically , which will slow down the input rise and fall time significantly ? The ST devices do not have input rise /fall rate spec but the drawback would be higher Icc current consumption when it slowly rise and falls turning on both transistors briefly .
  • 0 in reply to ShreyasRao
    Mastermind 8510 points

    Hi Shreyas-san,

    Thank you for your comment !

    The 2uF capacitor is for removing noise.  The "photo-couplar" (SW to short GND) is placed in front of LV14A.

    In fact, the phenomenon that input of LV14A is broken on the circuit has happened at my customer's product.

    From your comment, I understood that the slow rise/falls might have a possibility to cause turn on both low and high side transisitors at the same time but briefly.

    Is my understanding correct?

    And do you have any data of the rise/fall time to cause?

    Best Regards, 

  • 0 in reply to Takumi Suzuki1
    TI__Guru 59070 points
    Hi Takumi San ,

    your understanding is right about the schmitt trigger and the higher current due to slow rise and fall times. The schmitt trigger devices are built to be tolerant for the slow rising inputs , so the broken LV14A on the board could be from different issue but need more info on the fail case and events going on . Are there any higher input transients above the recommended conditions ?
  • 0 in reply to ShreyasRao
    Mastermind 8510 points

    Hi Shreyas-san,

    Thank you for your reply! Sorry for my late response.

    I understand that the slow transition would not be a root cause to be broken.

    Probably there are any other reasons.

    I will try to get more information from the customer.

    Best Regards,