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TXB0108: TXB0108

Santosh Gurral
Prodigy 110 points
Part Number: TXB0108

Hi Team

TXB0108 output is showing low voltage when a high is driven from a microcontroller pin (pin set to 50Kohm pull up mode in the controller)

but when the microcontroller pin is configured as push-pull it works (means when a high is driven, the output of txb is also high) why is this behavior? when I set the controller pin to 50kohm pull mode why it does not work?

what difference does the 50Kohm on the input side make?

Please see the attached image for my setup/configuration

Thanks

Santosh

  • 0
    Guru 243220 points

    The TXB cannot be used with pull-up/-down resistors. See sections 8.3.2/3//5 of the datasheet.

  • 0 in reply to Clemens Ladisch
    Prodigy 110 points

    it says we can have pull ups or downs of 50k and above

  • 0 in reply to Santosh Gurral
    Guru 243220 points

    It says that 50 kΩ pullups do not contend with the output drivers. In other words, when the pin on the A side drives low, the pull-up is too weak to make the signal high.

    That pullup is allowed, but it does not have any effect. As described in section 8.3.2, the line must be pulled high with at least 2 mA.

  • 0 in reply to Clemens Ladisch
    Prodigy 110 points

    Hi Clemen

    I am not able to understand this at least 2mA requirement...plz help me understand this.

    so if I make the 50K value small enough like 500 ohms which is 1.8V/500=3.6mA then it will be considereed as high right?

    but again I am not allowed to put a strong pull up (pull up value must be higher than 50Kohm)? why is this pull-up higher than the 50K ohm requirement?

    what happens if I put a very strong pull-up and pass more than 2mA through the pullup?

  • 0 in reply to Santosh Gurral
    Guru 243220 points

    500 Ω is not enough; the 2 mA must be provided at the switching threshold near 0.9 V.

    If you have a strong pull-up, the signal will always be high.

    The TXB is a bidirectional translator. It does not detect the direction in any way; the output drivers on both sides are always active, and they are deliberately weak so that other devices can override the voltage. If you do not have bidirectional signals, do not use the TXB.