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TXB0108: Clarification on power-up sequencing

Calvin Field
Prodigy 180 points
Part Number: TXB0108

The TXB0108 datasheet gives the following guidance on power sequencing: 

"During operation, ensure that VCCA ≤ VCCB at all times. During power-up sequencing, VCCA ≥ VCCB does not
damage the device, so any power supply can be ramped up first."

We are in a system in which we do not control power sequencing. We know the supplies come up at roughly the same time but there is the possibility there is some small amount of time (a few ms) where VCCA > VCCB, Is this time delay an issue and could you provide some guidance on when this might be a problem? For reference OE is tied directly to VCCA. 

  • 0
    TI__Genius 16735 points
    Hello Calvin,

    There is no issue in powering up the VCCA and VCCB in a slight delay to each other. As long as you are not attempting to transmit signal during power up there should not be an issue. If you are trying to power up the device in 3-State to avoid any signal transfer at power up then you can tie the OE pin to ground through a pull down resistor.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 180 points
    Karan thank you for your quick response. Just to clarify with respect to the statement "As long as you are not attempting to transmit signal during power up", there are some discrete command and status signals that will transition to their default state as power is ramped up. Would these transitions cause a problem and if so could you describe the implications of such behavior?

    Thanks,
    Calvin
  • 0 in reply to Calvin Field
    TI__Genius 16735 points
    Hi Calvin,

    So the TXB device has VCC isolation which means that when either input is at 0V, it is in 3-State. Now depending on how slow the power up sequence is, there is a possibility that a signal can be translated incorrectly while the device is powering up. Power up is typically not that slow and so there is a low chance of this happening. As I mentioned, the device allows for the OE pin to be tied to GND through a resistor during power up to put the outputs in 3-State and avoid this exact situation.

    There is no damage caused to the TXB device but we cannot guarantee with certainty that an incorrect signal will not be transmitted when the signals transition to their default state.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 180 points
    Thanks again. Just to clarify these questions stem from an existing design that has been in production for several years. We have received reports of signals being held low during initialization and wanted to make sure that the TXB device wasn't being put in a mode where an input inadvertently determined it was an output or vice versa in some kind of latch-up like condition that could not be cleared.

    Regards,

    Calvin
  • 0 in reply to Calvin Field
    TI__Genius 16735 points
    Hello Calvin,

    I have not yet heard of the TXB device getting latched up. Our translation expert is currently out but I will confirm and let you know if any such issues are possible.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    TI__Genius 16735 points
    Hello Calvin,

    So as mentioned in this app note www.ti.com/.../scea060.pdf
    There is a diode that can be forward biased when Vccb is less than Vcca. I am not sure what problem would occur if this diode is forward biased.
    Can you find out for me with certainty if Vcca > Vccb condition occurs at power up and exactly for how long this occurs. This is the only way we would know if in fact the TXB device is getting damaged.

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 180 points

    VCC25 and VDD33.pdf

    The attached file contains a scope plot showing power sequencing between VCCA (2.5V) Yellow and VCCB (3.3V) Blue.  VCCA start leads VCCB by 9 mS.

    Regards, 


    Calvin

  • 0 in reply to Calvin Field
    TI__Genius 16735 points
    Hi Calvin,

    So the TXB device has VCC isolation, this means that during those 9ms the device should be in a 3-state mode. However the diode between VCCA and VCCB will start getting impacted once VCCB starts to ramp up. However it doesn't seem like it takes that long for VCCB to ramp up fully. I need much more information to make sure that the TXB device is actually getting damaged.
    Can you provide me with a schematic showing what is providing the power?
    Can you also show me which signal lines are being held low?
    Do the lines stay low or do they start working properly after initialization? Because if this damages the TXB then I don't think the TXB device would start to work again.
    How often have you seen this errors?

    Thanks!
    -Karan
  • 0 in reply to Karan Kotadia
    Prodigy 180 points
    Hi Karan,

    This design is five years old and we have close to a 1000 units in the field and we just heard about this issue last week so I don't believe it is a problem. We just want to make sure we haven't missed something and clarify the power sequencing note from the data sheet. The problem is intermittent on a few units and is likely due to other causes.

    Regards,
    Calvin