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LSF0108: Pull-up resistor value

Motoyasu Hattori
Genius 11605 points
Part Number: LSF0108
Other Parts Discussed in Thread: TXB0304

Hi,

I can find the calculated pull-up resistor value on "Table 9-3. Pull-Up Resistor Values".

Is this Minimum value?  My customer wants to add a few k-ohm.  Is it OK to add a few k-ohm?

Please advise us.

Thanks and best regards,
M.HATTORI.

  • 0
    TI__Genius 11605 points

    Hi,

    My customer wants to use LSF0108 between OSPI interface as 3.3V<->1.8V.  But speed is MAX 50MHz throughput.

    I think pull-up resistor value of Table 9-3 is minimum value.  However, if pull-up resistor is very large such as 100k ohm, I seems difficult to communicate 50MHz throughput.

    So could you please let us know how to decide pull-up resistor value under this condition?

    Thanks and best regards,
    M.HATTORI.

  • +1
    Guru 318700 points

    When translating up, the speed of rising edges is limited by the low-pass filter formed by the pull-up resistors and the trace/device capacitances. You cannot easily change the capacitance, so you have to adjust the resistors. (The minimum resistor value is limited by the drive strength of your other devices; if the resistors are too small, their VOL becomes too high.)

    The datasheet shows some example values, assuming that you want to achieve a certain pull-up current. If you want another current value, you can use smaller or larger resistors.

    It is hard to predict the capacitances. It is usually easier to measure the waveforms with an oscilloscope, and adjust the resistors until you get good signals.

    The LSF is cheap, but it might not be a good choice for high-speed signals. A possible alternative would be the TXB0304 (which requires short traces, and does not work with strong pull-up/-down resistors).

  • 0
    TI__Mastermind 44691 points

    Hey Hattori-San,

    Adding on to Clemens' response, are the signals that require translation auto-bidirectional signals per channel? If not, there may be more suitable translators such as the direction controlled/ fixed translators that will allow for higher data rates and better driving capabilities.

    Regards,

    Jack 

  • 0 in reply to Jack Guan
    TI__Genius 11605 points

    Jack-san,

    Thank you for your comments.  This signal is bi-directional.  I have already told it to my customer.

    Thanks and best regards,
    M.HATTORI.