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SN74AVC4T774: Partial Power

Richard Katz
Intellectual 290 points
Part Number: SN74AVC4T774

Tool/software:

Hi,

I am diagnosing a problem in a board I was given.  And a question is, how does this part behave with partial power?

In particular, the A side is off (no VCCA), the A inputs are not driven, OE* is grounded, and the DIR inputs are not driven.

The B side is powered at 3.3V (VCCB).  

What will the B outputs do?  Is the behavior deterministic?

It has been observed that the B outputs are driving to 3.3V at least most of the time.

Thank you,

rich

  • 0
    TI__Genius 13620 points

    Hello Rich,

    During partial power down, the IOs become Hi-Z and and the maximum leakage into or out of any IO is given by the Ioff spec. For this feature to work, VCCA or VCCB cannot be left floating.

    We also need to make sure the A inputs are at GND and not floating. The AVC family does not have integrated pull-downs and not having them the inputs at GND can cause some glitches at the output. 

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Intellectual 290 points

    Hi Josh,

    Thank you for the response.

    As it turns out, for this application which I inherited, VCCA and all of the inputs are left floating (OE is grounded).

    So, under these conditions, if power is supplied to VCCB, what will appear on the "B" side outputs?  Measurements indicate (which were hard to make) that under these conditions if VCCB is driven to 3.3V, we're seeing 3.3V on the B side outputs. Is that how the part works?  Or does it just "depend" on some factors?

    Again, thanks,

    rich

  • 0 in reply to Richard Katz
    TI__Genius 13620 points

    Hello Rich,

    With the inputs floating, outputs will be in an unknown state. They could be either logic High or Low when VCCB is powered.

    Regards,

    Josh