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TCA9555: I2C Bus address setting

Alan Chang
Intellectual 1960 points
Part Number: TCA9555

Hi team,

     I have a question about TCA9555 bus address setting. 1.8V is applied for Vcc pin and A0~A2 pin. 10 kohm is used on pull-up to 1.8V and  pull- down to GND.

However, the pull-up voltage is 0.35V only. Do I need resistor for pull-up /down? or tie VCC and GND directly? 

1.Want to know is there internal pull high or pull low resistor in TCA9555PWR pin:21?
2.Is this any problem to have 10K ohm pull high resistor connect to pin:21
3.Do you have any suggestion for this pull high resistor value R163?

  

  • 0
    TI__Mastermind 49040 points

    Hey Alan,

    "However, the pull-up voltage is 0.35V only."

    That shouldn't be right. The address pins of our device are inputs only (CMOS gates) so our device shouldn't be pulling the line down to 0.35V. Can you double check to see if  there might be a large pull down resistor on the pin populated by accident? Also use a scopeprobe to look at this (not a Digital MultiMeter)

    "Do I need resistor for pull-up /down? or tie VCC and GND directly? "

    Since it is a CMOS gate input only, you can get away with using direct connect instead of resistors. Internally there is a dampening resistor before the FET's gate to lower inrush currents.

    1.Want to know is there internal pull high or pull low resistor in TCA9555PWR pin:21?

    No, there is not. Something seems wrong from that 0.35V measurement.


    2.Is this any problem to have 10K ohm pull high resistor connect to pin:21

    No, this is expected.


    3.Do you have any suggestion for this pull high resistor value R163?

    Any value will work. Higher gives you a weaker pull up though while lower gives you a stronger pull up. As long as the low voltage is below 30% of Vcc and the high voltage is above 70% of Vcc like our recommended operating conditions specify.

     

    -Bobby

  • 0 in reply to BOBBY
    TI__Intellectual 1960 points

    Hi Bobby,

       Thanks for your answer, another question below:

       "Internally there is a dampening resistor before the FET's gate to lower inrush currents."

    How can I calculate the external resistance? 

  • 0 in reply to Alan Chang
    TI__Intellectual 1960 points

    Hi team,

       can you help to answer below question?

       "Internally there is a dampening resistor before the FET's gate to lower inrush currents."

    How can I calculate the external resistance? 

      

  • 0 in reply to Alan Chang
    TI__Mastermind 49040 points

    Hey Alan,

    You can choose pull up/pull down resistor you want considering the leakage current that is acceptable for you. A smaller value gives you a stronger pullup/pulldown (but more leakage) while a larger one would may more susceptible to noise. You can even just bypass the resistor and just hook it up straight to GND or Vcc.

    -Bobby