• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 62 subscribers
  • Views 188 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS549D22: PMB_DATA and PMB_CLK

Darren (FAE)
Expert 5483 points
Part Number: TPS549D22
Other Parts Discussed in Thread: TPS546D24A,

I have a customer asking about the drive capability of the device for I2C.

They want to pullup PMB_DATA and PMB_CLK lines to the supply (3.3V) via a 1kΩ resistor. This means the pins have to be able to sink 3.3mA.

Is this possible?

  • 0
    TI__Expert 5483 points

    Hoping for an answer soon...

    I found the TPS546D24A DS (section 7.3.24):
    The TPS546D24A supports a minimum of 20mA of sink current on PMB_CLK, PMB_DATA, and SMB_ALRT.

    Then for the TPS549D22 DS: PMB_CLK and SMB_ALRT OUTPUT PULLDOWN
    This states a max of 0.4V at the pin when sinking 20mA...so I am assuming the internal open drain? MOSFET has Rds(on) of 20Ω? But I don't understand the "Pulldown" requirement. Is this a pulldown resistor, or when the open-drain MOSFET pulls the line LOW?

    It looks like the pin should be able to sink up to 20mA and maintain an output voltage of under 0.4V.
    So a 1kΩ pullup to 3.3V (3.3mA) will be within spec...yes?

    I don't really understand this. It looks like 

  • 0 in reply to Darren (FAE)
    TI__Genius 14455 points

    Hi Darren

         With 3.3V pullup voltage and 1k resistor, the maximum current that you will have is 3.3mA. As the Vol is 0.4V at 20mA, you are within spec using 1k resistor with 3.3V.

    Imagine now that you use a 100 ohm resistor, this means that max current will be 33mA. This 33mA will flow through the MOSFET and will cause the Vol to increase to beyond 0.4V which could be say 0.66V. This could be misinterpreated as this Vol is not sufficiently low. While TPS549D22 can recognize a Vil of 0.8V, there could be other devices on the bus that could misbehave because of this high Vol.

    Hope this clarifies.

    Regards,

    Gerold

  • 0 in reply to Gerold Dhanabalan - Online Design Tools
    TI__Genius 14455 points

    Also, the downside to using a high pull-up resistor say 1Meg, is that the time constant from the 1Meg and Bus capacitance can cause the signals to rise so slowly that will limit your operation.

    FYI.