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TCA9509: IOL limitation from being damaged

Ann_Lien
Genius 15350 points
Part Number: TCA9509


Tool/software:

Hello expert,

My customer would like to adjust the level for the VOL and they did modify the pull-up resistor on A side. May I know if we have limitation for the IOL value from being damaged? And also, could I simply calculate the IOL based on equation = pull-up voltage / pull-up resistor? Thanks a lot!

Best regards,

Ann Lien

  • 0
    Guru 319730 points

    A pull-up resistor can only increase the VOL, which is usually not what you want. What is the actual problem you're trying to solve?

    The pull-up current is (VCC − VOL) / R, but on the TCA9509's A side, you have to add 1 mA for the internal current source. The I²C specification limits the total pull-up current to 3 mA or 6 mA, depending on circumstances.

  • 0
    TI__Guru 54948 points

    Hi Ann,

    The TCA9509 is very sensitive to VOL on A-side. It has a very low VILC requirement with VILC(min) = 110mV. Every I2C device connected to the A-side must be able to drive an output low voltage <110mV to successful transmit a low signal from A-side to B-side. Adding pull-up resistors to A-side is not recommended. If the customer needs to do so, they need to account for the internal current source 1mA on A-side, as well as the current supplied from the added pull-up resistor on A-side to ensure they can have a low enough VOL on A-side. 

    My recommendation is to remove pull-up resistors all together on A-side of the device. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    TI__Genius 15350 points

    Hello Tyler,

    Okay and I will let customer know it to see if any further help is needed. Thanks a lot!

    Best regards,

    Ann Lien

  • 0 in reply to Ann_Lien
    TI__Genius 15350 points

    Hello Tyler,

    customer's reply is shown below.If A side pull-up resistor is needed, what is the limitation for it? For this case, TCA9509 is connected to MUX then I2C device. I will try to ask the diagram for their application to you. Thanks!

    Hi, Ann

    We know pull-up impact Vol, but we do not have choice as there is mux after repeater, and pull-up need to be added after mux.

    Our question is whether this pull-up destroys repeater in the A-side or not.

    Please advise

    Thanks a lot

    Best regards,

    Ann Lien

  • 0 in reply to Ann_Lien
    TI__Guru 54948 points

    Hi Ann,

    It wouldn't necessarily destroy the repeater on A-side unless the pull-up were to be super strong and source a lot of pull-up current from the supply. 

    If a pull-up is needed on the MUX side, then I suggest making the pull-up resistor larger than usual since the TCA9509 A-side implements an internal 1 mA current source and requires very low VILC for this buffer to work. 

    THe pull-up resistor might need to be on the order of 20kohm for this to work, but this pull-up would be enough resistance to keep to a VCC in the idle state when the TCA9509 is not connected. 

    Regards,

    Tyler