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SN74AVCB164245 when both Vcca and Vccb are at 0V

Jamaal_Charles
Genius 3245 points
Other Parts Discussed in Thread: SN74AVCB164245

Hello,

We have a couple quick questions about the SN74AVCB164245.  We are using them in a scenario where we don't really care about the state during power up and down, and are low on pins.  So, our plan is to just tie OE to ground, so that it is always active.  However, we are curious about a couple things.  Could this damage the part since we'd be violating the recommended power sequencing?  Also, the datasheet states that if either Vcca or Vccb are at 0V, the outputs will be at High Impedance.  I want to verify that if both Vcca and Vccb are 0V, that the outputs are still guaranteed to be Hi-Z, even though OE is grounded?

Thanks

  • 0
    TI__Guru 71526 points

    A good reference is the AVC Logic Family Technology and Applications: http://www.ti.com/litv/pdf/scea006a .  As shown, the output circuits have blocking back to Vccx when the respective power supply is 0v (see pages 3 and 5).   Suggest also this App Note ( http://www.ti.com/litv/pdf/scea030a ), as it addresses the 'AVCx16245 directly.   You'll also note that on  page 6 in the datasheet, for Ioff conditions, when either VCCA or VCCB is specified at 0v, the other supply ranges from 0 to 3.6v, so both supplies at 0v is indeed a valid condition.

    And the question about OE being grounded during power-up is described on page 13 of the App Note SCEA030A; recommended is to use a pull-up to VCCA, although description in last paragraph on page 13 has OE tied to GND during power-up.  Datasheet page 1 also states OE to VCCB to ensure high-Z during power-up.    

  • 0 in reply to LeonardEllis
    Genius 3245 points

    Thank you,

    So when both power supplies are at ground or turned off,  what is the resistance of the A and B ports? 

    I thought Ioff is a reference to leakage current between side a and b of the device? 

    Thanks again

  • 0 in reply to Jamaal_Charles
    TI__Guru 71526 points

    The Ioff circuitry disables the outputs.  As stated on page 1 of the datasheet, "If either Vcc is at GND, both ports are in the high-impedance state."  Same information is in the SCEA030a App Note pg 12 and graphic Fig 8d on page 13; this shows all the possible VCC combinations and the associated currents and output driver status. 

    Also note that there is a difference between a power supply that is OFF, and one that is floating.  Read about floating Vcc on page 12 of the App Note.   If this is a design where the '16245 circuitry is powered by a backplane connector or pluggable cable, it is important to ensure the GND is the first connection made.  

    -Leonard

     

  • 0 in reply to LeonardEllis
    Genius 3245 points

    Thanks Leonard,

    It might be nice to add the Vcca = 0 and Vccb = 0 scenario to the tables in the datasheet just to make it clear that they will be in Hi-Z in that case just to eliminate any confusion.

    Thanks though

     

  • 0 in reply to Jamaal_Charles
    TI__Guru 71526 points

    Yes, it could be clearer, and I've passed that request on to the Product Group; thanks for pointing this out.  I like Fig 8 from the App Note, it shows the various scenarios, perhaps we can get that added to the datasheet.