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SN74LVC4245A: Power-Up Consideration

ttd
Mastermind 8600 points
Part Number: SN74LVC4245A
Other Parts Discussed in Thread: SN74LVC8T245

Tool/software:

Dear Technical Support Team,

SN74LVC4245A has the following power up consideration on the datasheet.

Most current circuits do not install pullup of OE at VccA as described in "3. Tie OE to VCCA with a pullup resistor so that it ramps with VCCA".

In most cases, there is no pullup or pulluped at VccB.

I haven't faced any specific problem, but I'd like to know what the possible adverse effects could be.

Also, SN74LVC8T245 doesn't have this description, but can I assume that the power-on sequence is irrelevan?

Best Regards,

ttd

  • 0
    Guru 316940 points

    When /OE is low, then the outputs are active. During power-up, when the supply voltage is below the recommended minimum, the DIR or input pins might not be read correctly and have the wrong value.

  • 0
    TI__Mastermind 22830 points

    Hello ttd,

    I agree with Clemens. The issue we want to prevent is false logic level from appearing on the bus. This happens when /OE is pulled low from the start and one supply (typically the internal output buffer) is powered up first. This sends a spike on the output.

    When /OE is pulled to VCCA, it forces I/Os to be in a Hi-Z state which minimizes spikes/glitches. I would recommend /OE pulled-up to VCCA for all level translators.

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Mastermind 8600 points

    Hi Josh and Clemens,

    Thank you for your reply. I got /OE should  be pulled to VCCA.

    Josh said:

    When /OE is pulled to VCCA, it forces I/Os to be in a Hi-Z state which minimizes spikes/glitches. I would recommend /OE pulled-up to VCCA for all level translators.

    Do you have application note or is there explanation on datasheet about this for SN74LVC8T245 ?

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Mastermind 22830 points

    Hello ttd,

    Under the SN74LVC8T245 description section it notes: 

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Mastermind 8600 points

    Hi Josh,

    Thank you for you reply.

    I have aditional questions about SN74LVC4245A  and SN74LVC8T245 .

    Is it correct to assume that when OE is PullUp with VCCB, the same phenomenon occurs as when there is no PullUp?

    Also, if VCCA falls before VCCB when the power is turned off, is there any possibility of device damage other than the effect on output?

    Regards,

    ttd

  • 0 in reply to ttd
    TI__Mastermind 22830 points

    Hello ttd,

    No device damage if VCCA falls before VCCB.

    Is it correct to assume that when OE is PullUp with VCCB, the same phenomenon occurs as when there is no PullUp?

    Not necessarily. The control circuitry (OE and DIR) are referenced to VCCA. As long as OE and DIR voltages fall within VCCA VIH/VIL, you can pull-up OE to VCCB. 

    Regards,

    Josh

  • 0 in reply to Joshua Salinas
    Mastermind 8600 points

    Hi Josh,

    Thank you for your reply.

    Josh said:

    Not necessarily. The control circuitry (OE and DIR) are referenced to VCCA. As long as OE and DIR voltages fall within VCCA VIH/VIL, you can pull-up OE to VCCB. 

    Dose that mean that already input voltage of VIH level for OE such as 3.3V to prevent the spike on the output before ramp-up VCCA(= 0V)?

    SN74LVC8T245  has ioff function on the datasheet however it seems there is no description of ioff for SN74LVC4245A.

     

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Mastermind 22830 points

    Hello ttd,

    SN74LVC4245A does not have Ioff (meaning it does not go into Hi-Z when either supply goes to 0V). It does, however, have a Ioz parameter. This means IOs will enter Hi-Z when /OE goes high.

    Even if you apply 3.3V at /OE during power-up, it's not going to force I/Os into Hi-Z because the circuitry of OE is turned-off. Only when VCCA is ramping-up, I/Os will become Hi-Z.

    Regards,

    Josh