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TXB0304: driver strength

Feng Wu
Prodigy 300 points
Part Number: TXB0304
Other Parts Discussed in Thread: SN74LVC125A, SN74AUC34

In customer use, they use a 3.3V for both VCCB and pull-up voltage. But if they use 4.7k pull-up resistor, the Voh just 0.9V while they use 1k pull-up resistor, the Voh=3.3V. So it looks the souring current of B I/O pin is limited. I want to check what's the maximum sourcing current of the B I/O pin, I'm not clearly with the description of datasheet. 

  • 0
    Guru 267950 points

    The sinking current is specified as the test condition for VOLx; it's 20 µA.

  • 0 in reply to Clemens Ladisch
    TI__Expert 6680 points

    Hi Feng,

    Check out section 8.4.2 of the datasheet. It goes into detail about pullup/pulldown resistor sizing.

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    TI__Prodigy 300 points

    Thanks Crosby, one more question, in datasheet I see For proper operation, the device driving the data I/Os of the TXB0304 must have drive strength of at least ±3 mA. If the input driver strength is not enough, could we add external pull-up to supply 3mA input current?

  • 0 in reply to Feng Wu
    Guru 267950 points

    How would you pull that signal low? Is the driver stronger when sinking current?

    For an unidirectional signal, a unidirectional or direction-controlled level shifter would be a more robust solution.

  • 0 in reply to Clemens Ladisch
    TI__Prodigy 300 points

    Thanks Ladisch, I need to clarify my question: In customer application, the direction is 3.3V to 1.8V, and they use pull up resistor in 3.3V side. the 3.3V is from DC/DC and both supply Vccb. If they use 4.7k, the VioH=0.9V while VioL=0.2V; If they use 1k, the VioH=3.3V while VioL=0.2V. So they want to check if there is any risk in the pull up resistor.

  • 0 in reply to Feng Wu
    Guru 267950 points

    Section 8.4.2 of the datasheet says:

    If pull-up or pull-down resistors are connected externally to the data I/Os, their values must be kept higher than 20 kΩ to ensure that they do not contend with the output drivers of the TXB0304.

    Do all four channels go from 3.3 V to 1.8 V?

  • 0 in reply to Clemens Ladisch
    TI__Prodigy 300 points

    yes , all four channels go from 3.3V to 1.8V. But the issue is found in input side. before connected TXB0304, the periperal I/O 3.3V is normal, but after connected with TXB0304 I/O(pull up resistor is 4.7k), the voltage is pulled down to 0.9V, it looks like the periperal I/O sourcing current is too low to drive I/O pin to 3.3V. After changing pull up resistor to 1k, the souring current into I/O increased so the voltage of I/O pin can go to 3.3V. So is it any risk in this usage? If it is dual-direction application, you mean it maybe compete with output driver, how about single direction applications,if the 3.3V is only used for input?

  • 0 in reply to Feng Wu
    Guru 267950 points

    The TXB is always bidirectional; its output drivers on both sides are always active. The 3 mA drive strength requirement is necessary to ensure that external devices can overpower the TXB outputs.

    For this application (unidirectional downtranslation), I would recommend a simple buffer with overvoltage-tolerant inputs, e.g., SN74LVC125A or SN74AUC34.