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Input circuit of the sn74LVTH244A

Anthony Monti
Intellectual 405 points

We have a board with the SN74LVTH244APWR. The inputs are tied to a VME bus and a 4.7k ohm pull-up to 5V_VME. The VCC is 3.3V. We have been informed the VME Power Supply may have an 11.18V spike for .5ms. Will the spike on the input blow the input? Can you supply me the input circuit for this part? (See U77 of attached schematic.) Thank You, Tony1340F101_Revision_X0 33.pdf

  • 0
    TI__Guru 61970 points

    Hi Anthony ,

    The input abs max voltage for the part can be seen from the datasheet abs max table which is 7V . 11V is well over and above the abs max . There will definitely be reliability concerns since 0.5ms is significant amount of time .The input may not blow up as such in an instant but eventually handling 11.2V is a concern .

    Although there are series resistors  which will limit the current into the inputs , the voltage over the process node can damage the internal transistors . Small 10nf cap on the line can dampen the overshoot . bypass caps on the VME is an option .

    I have attached an app note which explains the input structure of bushold circuits .

  • 0 in reply to ShreyasRao
    Intellectual 405 points

    Hi ShreyasRao, 

    Thank you for the response. I'll change the input to protect the device.

    Thank You,

    Tony