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LSF0102-Q1: Circuit review and pull-up resistor inquiries

MJ Jeon
Genius 5240 points
Part Number: LSF0102-Q1
Other Parts Discussed in Thread: TCA9517-Q1, TCA9517

Hi, TI Support Team

Inquiries were received during the design of the LSF0102-Q1 from a customer.

Please review the attached circuit.

The recommended value for the pull-up resistor of HOST (Modem) connected to LSF0102-Q1 is 2.2kohm.

Are there any possible problems when applying the corresponding resistor?

Or is there a value you can recommend?

Additionally, what is the difference between the LSF0102-Q1 and the TCA9517-Q1?

Thanks.

Regards,

MJ

  • +1
    TI__Mastermind 45725 points

    Hey MJ,

    I don't support the TCA9517 device so I can't comment on its functionality. As for the LSF device, those pull-ups are no problem for it. The size of the pull-up will determine the rise time and thus the overall data rate. I2C speeds are typically slow so 2.2 Kohm will likely work just fine for that.

  • +1
    Guru 316690 points

    The LSF is a passive switch. When a line is pulled low, there is a direct connection between the two sides, and both pull-up resistors work in parallel. Check that all devices are able to drive the resulting current.

    The TCA9517 is a buffer, so both sides are isolated.

  • 0 in reply to Clemens Ladisch
    Genius 5240 points

    Hi, Clemens

    Thank you for your answer.

    Do you have any comments on the overall schematic?

    If you have any opinions on the circuit diagram in the first inquiry, please respond.

    Thanks.

    Regards,

    MJ

  • +1 in reply to MJ Jeon
    Guru 316690 points

    My comments do apply to the schematic. 1 kΩ || 2.2 kΩ ≈ 688 Ω, so you might want to use larger resistors, for example, 4.7 kΩ on both sides.

    With the TCA9517, you could use 2.2 kΩ on both sides.

    Why were 1 kΩ resistors chosen for the 3.3 V side?

  • 0 in reply to Clemens Ladisch
    Genius 5240 points

    Hi, Clemens

    Pull-up resistor values are applied based on the table below.

    Based on the above table, it is a question to ask if it is possible to apply 2.2kohm based on modem for 1.8V line while applying pull-up resistor.

    Are you okay?

    Is there no problem for the whole circuit?

    Thanks.

    Regards,

    MJ

  • +1 in reply to MJ Jeon
    TI__Mastermind 45725 points

    Hey MJ,

    There are no issues with the circuit. I recommend watching these training videos over the LSF family. If the interfacing devices aren't able to sink the current from the pull-ups on both sides of the device, then you will need to increase the size. Video 6 in the training series will help you calculate resistor sizes for your system.

  • +1 in reply to MJ Jeon
    Guru 316690 points

    The smallest current in that table is 3 mA, but this does not mean that 3 mA would be the smallest current that you can use.

    You need high currents only for high-frequency signals. I²C is very slow, and typically works with 1 mA or less.