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LSF0102-Q1: Do the pull-up resistors on either side of the device needs to draw equal current

Francis Pettitt
Prodigy 10 points
Part Number: LSF0102-Q1
Other Parts Discussed in Thread: TCA9517-Q1, , PCA9306-Q1

Hi, I'm using this device for I2C and SPI level shifting. I can only sink 3mA on one side (at 3V3) due to limited processor current sink capability but the other side (shifted to 5V) could potentially sink 10mA or more - it's used in a noisy environment so I may want to do this. Do the pull-up resistors on either side of the device need to source equal current or can there be an imbalance in current (on A vs B pins)?

Thanks!

  • 0
    TI__Guru* 97046 points

    Hi Francis,

    I would highly recommend watching this short video series on the LSF family of voltage level translators: https://training.ti.com/node/1139264?context=1139264


    The short answer is that your total current from the pull-up resistors must be less than 3mA for your setup to work.

  • 0
    TI__Guru* 97046 points

    Hi Francis,

    I see that you marked this answer did not resolve your issue.

    Can you be more specific in how I can better help you? I both gave you a resource to learn more and listed a quick and easy direct answer to your question.

  • 0
    Guru 316690 points

    The LSF itself does not care.

    But when the switch is on, the driving device must be able to sink the total current through both pull-up resistors. So if one of your devices cannot handle more than 3 mA, you have to size the pull-up resistors so that the sum of the two currents does not exceed 3 mA.

    If you want to have more current on the B side, you have to avoid passive switches like the LSF0102-Q1 or PCA9306-Q1, and use an I²C buffer, i.e., the TCA9517-Q1.