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LSF0102: Pull-up resistor selection

Hideki Hayashi1
Expert 7900 points
Part Number: LSF0102

Tool/software:

Hi team,

Although the LSF0102 supports frequencies up to 10 MHz, the waveform is dull even with 2.4 MHz signals, as shown below, and customer suspect the pull-up resistor is the cause.


1. The LSF0102 can be used with either push-pull or open train, but what determines this switching?
If there is a pull-up resistor, does it operate in open-drain mode, and if not, does it operate in push-pull mode?


2. I checked the data sheet for the proper value of the constant of the pull-up resistor, and the description is as follows.
Based on the statement “TThe device driving the low state at 0.175V must sink current from one or more of the pull-up resistors and maintain VOL,” I do not understand what specific resistor value should be selected.
What specific calculations are necessary?

3. Is it sufficient to determine the upper limit of the pull-up resistor based on the required current value for the IC to be connected? Are there any particular specifications for LSF0102?

Regards

Hayashi

  • 0
    TI__Mastermind 30596 points

    Hi Hayashi-san,

    To achieve faster speeds with the LSF0102, external pullups need to be sized stronger and output cap. load needs to be reduced. The inverse relationship between the two can be calculated below: 

    1.) The LSF0102 is a passive translator with open drain outputs supporting push-pull drivers/open drain drivers. It does not know what driver architecture is- for input voltages below VREFA, the switch is on, and for input voltages VREA or higher, the switch is in high-Z (and external pullups determine the output high state)

    2.) This is in equation 3 in the figure you have provided. 

    3.) LSF0102 recommended operating conditions is for 64mA of pass current through FET. 

    Customer may also reference this set of logic minute videos for better understanding of the LSF: www.ti.com/.../understanding-the-lsf-family-of-bidirectional--multi-voltage-lev.html

    Regards,

    Jack