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TXB0106: pull up/down resistor for different application requirement

Arthur Huang
Expert 6065 points
Part Number: TXB0106

Hi Team,

customer design the TXB0106 for their system serial IRQ(open drain signal that required pull high resistor) and Reset signals level shift, they found when used 8.2K pull up resistor on serial IRQ signal, the signal H/L level is unstable like attached captured waveform, however, when changed it from 8.2k to 47k, the problem got improved. Per TXB0106 data sheet, it said Pull up resistors are not required on both sides for Logic I/O and If pull up or pull down resistors are needed, the resistor value must be over 50 kΩ. So, customer confused how to judge whether the pull up resistor is needed or not and what is the proper pull high value for the open drain signals or system Reset signals that may connect to multiple devices(3 ~ 6) for power on reset?

Please advise. Thanks.

Regards,

Arthur

  • 0
    Guru 281640 points

    For an open-drain signal, pull-up resistors are needed.

    The TXB's auto-direction-sensing feature is implemented with an output driver that is very weak so that the external driver can override it. Therefore, the pull-up resistor must also be weak (at least 50 kΩ) so that the TXB's driver is able to pull the line down.

    If auto direction sensing is not needed, consider using unidirectional or direction-controlled translators.

  • 0
    TI__Mastermind 26925 points

    Hello Arthur,

    Pull up resistors should be avoided with TXB translators. Because of the 4k buffer driving the output, pull ups will create a voltage divider at the output which will negatively impact your Vol levels. To estimate the resulting Vol you can use the following equation:

    Vol = 4k/(4k + Rpu) * Vcco

    Rpu is the pull up value and Vcco is the Vcc voltage at the output.

    If pull up resistors are required I would recommend using the TXS instead.