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TMP75: SDA High Question

Kyle Mehringer
Intellectual 1520 points
Part Number: TMP75

Tool/software:

Hello,

We have a quick question on the TMP75. It's been seen that when TMP75 is installed into a system, but not powered, the SDA line rises up to 3.3V for the host. While this is expected and favorable, it is somewhat unexpected based on the datasheet. There is no provision for “Partial Power Down”, but the device behaves like it is.

Can you help to explain why we may see this?

Thank you,

Kyle

  • 0
    TI__Genius 9720 points

    Hello Kyle,

    I would not recommend using our device in this way. Please follow the recommend voltage range to power up the device. If you are looking to save power, you can put the device in Shutdown mode. When the device is in shutdown mode, writing 1 to the OS bit starts a single temperature conversion. The device returns to the shutdown state at the completion of the single conversion. This is mentioned in section 7.4.1 and 7.4.2 in the datasheet. As you see below, you can save more power with shutdown current.

     

    Best Regards,

    Meredith McKean

  • 0 in reply to Meredith McKean
    TI__Intellectual 1520 points

    Hello Meredith,

    Thank you for the response. So this application has been in production for a long time and they haven't seen any issues. We're just trying to understand why might the device behave like this? Are there certain conditions that could be explained?

    Thank you,

    Kyle

  • 0 in reply to Kyle Mehringer
    TI__Mastermind 43785 points

    Hi Kyle,

    Where is the power coming from? What is powered on when the TMP75 is not powered? 

    In the scenario where the I2C pull-ups are powered and TMP75 is not, it is completely normal for the SCL/SDA to have the pull-up voltage despite the TMP75 being unpowered. The TMP75 will not interfere with the bus.

    In the scenario where the I2C pull-ups are not powered and the TMP75 is, the TMP75 will not provide power to the SCL/SDA pins. 

    Both of these scenarios are described by the TMP75's Abs Max specification, which states the SCL/SDA pins can be up to 6.5V completely independent of the V+ pin. 

    The A2 pin, on the other hand, is not allowed to exceed V+. If the A2 pin is pulled to a voltage higher than V+ pin, it will start to conduct from A2 to V+ through its ESD protection structure. Even if this were true for the SDA pin, it would not explain your situation. The ESD structure would never conduct in the other direction.

    If you have removed your TMP75 and verified that the voltage issue away, then you have a damaged TMP75.

    thanks,

    ren