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TXB0104: Additional PU limitations?

Andrew Ciezak
Prodigy 80 points
Part Number: TXB0104
Other Parts Discussed in Thread: TXU0304

Using TXB0104 to translate SPI from master 1.8V PHY to slave 3.3V Flash.  Turns out that the PHY PU does not have enough drive strength on one SPI bus lines for proper operation.  Placing a 1K PU on the 1.8V  line starts working,  while the other bus lines work without additional pull-ups.  Other than the decreased VOH and VOL from the PU,  are there any other concerns with such low pull-ups tied to the translator?

  • 0
    Guru 316690 points

    The TXB does not work with pull-up/-down resistors stronger than 50 kΩ.

    SPI uses fixed-direction signals. Use the TXU0304 instead.

  • 0
    TI__Guru 50346 points

    Hi Andrew,

    In addition to Clemens' feedback, please note that additional pull-ups would create a voltage divider with the internal pull-up resistor of the TXB. Hence, having pull-ups not above the 50K ohm recommendation would mean that the TXB will be driving the line and not the pull-ups. See Effects of pullup and pulldown resistors on TXS and TXB devices.

    In your case, I am presuming your VOH would be decreased by about 0.4V with the 1K ohm PU and would recommend to see section 6.5 of the datasheet for the recommended VOH/VOL levels.

    Please see below for implications to consider per decreased VOH and VOL levels:

    • Depending on how close to the threshold voltage between VIL and VIH; the device can be in an unknown state. Depending on how severe, the device could uncontrollably switch from low to high / high to low continuously in the form of oscillations. This could cause severe issues to the device as well as the system.
    • With the transistor partially conducting in an unknown state; increase in shoot-through current would have a high likelihood, as well as a high power consumption which could also potentially damage the device, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 80 points

    The affects of decreased VOH and VOL from a PU under 50K are understood.  Assuming all devised are in known states, are there any other TXB0104, reliability/life concerns?

  • +1 in reply to Andrew Ciezak
    TI__Guru 50346 points

    Hi Andrew,

    There are no other reliability concerns assuming the device is not in an unknown/undesired state per the datasheet recommendations while using the PU, thanks. 

    Best Regards,

    Michael.