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74avch20t245 Vcc isolation feature

James Hillman
Expert 1436 points

Hi There,

I have a design which uses the 74avch20t245 part. I got a review comment asking what happens if Vcc isn't exactly GND - floating due to leakage. Does the part have a specification for the isolation feature i.e. when the VCC is below the minimum (1V2) does it kick in?

Many thanks,

  • 0
    TI__Guru 62050 points
    Hi James,

    I will move this into voltage translation forum for now . This part has isolation feature as speced on the feature page with Hi-Z whenever the Vcc is at gnd .also , Ioff spec is defined for partial power down applications .
  • 0 in reply to ShreyasRao
    Expert 1436 points
    Hi Shreyas, So are you saying if VCC = 0.1V, that the isolation feature isn't active. If so, what does it do? Many thanks, James.
  • 0 in reply to James Hillman
    TI__Mastermind 23156 points
    Hello James,
    Let me try and make sure I understand the question. You are asking isolation, do you mean that the input and output are in tri-state or is disabled and not allowing signals to travel from VCCA voltage translation level to VCCB voltage translation level? It isn't specified clearly in the datasheet but assuming the /OE is referenced to VCCA and that level is above logic level high (which is >65% of VCCA) then it will be disabled. I will have to do a bit of research to see what is the maximum voltage that can be on either VCCA or VCCB to ensure that it is in a disabled state. It should be on the order of 0.5V to 0.7V but that is something that is technology and design dependent. I will have to get back to you on that.
    -Francis Houde
  • 0 in reply to fhoude
    Expert 1436 points
    Hi Francis,

    Thank you for looking into my question.

    In my circuit, VCCA may be unpowered, whilst VCCB remains powered (always) at 2.7V. I need to ensure that none of the signals on the B side are affected by the unpowered device on the A side. Additionally, I need to make sure that excessive current is drawn - ideally I would achieve the Ioff current specified.

    many thanks,
  • 0 in reply to James Hillman
    TI__Mastermind 23156 points
    Hello James,
    The part will be shut down if either supplies are held to ground (or very close to 0V) or if OE has been set to disable the part. If you satisfy any of these conditions you should be assured that no excessive current draw will take place. Are you seeing leakage that is causing your Vcca to be well above ground? How have you handled your OE, did you tie it to Vcca or is it controlled externally?
    -Francis Houde
  • 0 in reply to fhoude
    Expert 1436 points

    Hi Francis,

    Thanks for replying again.

    fhoude said:
    Are you seeing leakage that is causing your Vcca to be well above ground?

    In previous designs I haven't noticed this issue. Currently with this latest design, I am at the stage of layout.

    fhoude said:
    How have you handled your OE, did you tie it to Vcca or is it controlled externally?

    I have tied OE to VCCA through a resistor. VCCA is the side that becomes unpowered. Would this be sufficient to enter tri-state and prevent excessive current draw?

    Many thanks again,