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SN74AVC8T245: power sequencing VCCA before VCCB or it doesn't matter?

Xavier Queffelec
Intellectual 310 points
Part Number: SN74AVC8T245

Hello,

I am looking to use the SN74AVC8T245 for level shifting from 2.5V to 3.3V and I saw in the datasheet, that I need to care about the enable pin (both VCCA and VCCB must be present).

I am making my 2.5V from my 3.3V so the 2.5V will have a small delay. Actually I powered VCCA with the 2.5V and VCCB with 3.3V.

Is there any problem when I do this or should I change the direction VCCA with 3.3V and VCCB with 2.5V so that VCCA will come first.

Many Thank for your help.

Best Regards,

Xavier

  • 0
    TI__Guru* 97046 points
    Hi Xavier,

    The appropriate applications engineer has been notified and will be back with you soon.
  • 0 in reply to Emrys Maier
    TI__Guru 62160 points
    Hi Xavier ,

    There are many different conditions and combinations which can change the power sequencing scenarios. what is the status of OE pin?
    With I/Os grnded and outputs enabled, the Vcca & Vccb ramping simultaneously will be best.
    Else , Vcca leading Vccb is suggested.
  • 0 in reply to ShreyasRao
    Intellectual 310 points

    Dear Shreyas,

    Many Thanks for your answer.

    I am driving the OE pin with a signal that mean "interface active". So on the 3.3V side I "copy" the input from the 2.5V side.

    I am just using one direction from 2.5V to 3.3V and I take this chip dues to the low latency (less than 3ns).

    I don't have the control of the input side (2.5V), I just know when the data are valid and I will use this signal for the OE.

    I cannot assume to ramp VCCA & VCCB simultaneously, so if I correctly understand your answer:

    it will be better to power VCCA first then power VCCB before OE to be asserted low to assume proper operation condition.

    Can you just confirm me if it will be ok in this way?

    Many Thank for your help.

    Best Regards,

    Xavier

  • 0 in reply to ShreyasRao
    Intellectual 310 points
    Please can you confirm me that it will be working as I wrote you yesterday. Many Thanks, Xavier
  • 0 in reply to Xavier Queffelec
    TI__Guru 62160 points
    Hi Xavier ,

    Vcca powering up first and Vccb coming up later with outputs enabled (OE low).
    Again, this is the suggestion but please use diligence before setting up to avoid any issues.