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TPS22916: Question about the ON pin smart power down function

Jiayu Ye
Intellectual 1790 points
Part Number: TPS22916

Dear,

I can find the below description in TPS22916 datasheet. However, in our design, when the GPIO of MCU is Hi-Z state before power up, the connected ON pin of TPS22916 is actually floating, can we also regard this as Von<VIL? Thanks.

  • 0
    TI__Mastermind 32650 points

    Hi Jiayu,

    A floating node does not guarantee that the voltage on this pin will be pulled below V_IL. The way the smart pull down works is that the voltage on this pin has to be brought to V_IL or lower after which, the pin is pulled to GND internally. Until this condition is met, the smart pull down will not be connected.

    In your system, unless the node floats below the V_IL threshold, the smart pull down will not be used and as such, I think it is necessary to use some sort of external pull down.

  • 0 in reply to Shreyas Dmello
    TI__Intellectual 1790 points

    Hi Shreyas Dmello,

    According to your description, a floating node does not guarantee that the voltage on this pin will be pulled below V_IL and we have to add an external pull down resister. Then what's the meaning of the smart power down function? If we can ensure the state of the connected GPIO for this EN pin is definite. Then actually there is no need for this internal smart pull down function. Thanks. 

  • 0 in reply to Jiayu Ye
    TI__Mastermind 32650 points

    Hi Jiayu,

    The smart pull down is an effort to avoid any undefined states when the ON pin voltage is disconnected. 

    This avoids any situation where coupling from external sources could inadvertently pull the ON voltage to these states causing unwanted turn on.

    As mentioned earlier and in Table 8-3, the voltage on the pin has to be equal to or less than V_Il for it to be connected internally. 

    Thanks,

  • 0 in reply to Shreyas Dmello
    TI__Intellectual 1790 points

    Hi Shreyas Dmello，

    Thanks for reply. Understand. But in my case, the state of GPIO connected to 22916‘s ON pin is Hi-Z when the MCU is not powered on. I think this is equal to what you described as "The smart pull down is an effort to avoid any undefined states when the ON pin voltage is disconnected " What is your opinion?

  • 0 in reply to Jiayu Ye
    TI__Mastermind 32650 points

    Hi Jiayu,

    The smart ON pull down will not trigger until the voltage on this pin leaks down to below V_il. Once this is triggered, the pin will be held down and no external coupling will be able to cause a rise in voltage on this pin which could result in an unwanted/unsupported state of operation of the device.

    These cases are not one and the same. There is no guarantee that once the GPIO input goes to high-Z that the voltage on this pin WILL fall below V_il especially if there are any capacitors on this node.

    If in your system, this is seen then the smart pull down is sufficient to turn off the device. If not, an external pull down has to be used.

  • 0 in reply to Shreyas Dmello
    TI__Intellectual 1790 points

    Hi Shreyas Dmello，

    Thanks for reply.

    My confusion is "The smart ON pull down will not trigger until the voltage on this pin leaks down to below V_il. "

    If the voltage on ON pin is below V_il, it means the input is LOW, then why I need this internal pull down connection.

    If the voltage on ON pin is above V_ih caused by external coupling, I though this internal pull down is disconnected, then how to avoid an unwanted output of TPS22916?

    BTW, If in your system, this is seen then the smart pull down is sufficient to turn off the device. If not, an external pull down has to be used.

    How to judge it is sufficient or not?

    Thanks.

  • 0 in reply to Jiayu Ye
    TI__Mastermind 32650 points

    Hi Jiayu,

    The easiest way to to understand the smart pull down is:

    1. If the voltage on the ON pin is brought below V_IL, the device will hold the ON pin down with the smart ON pull down. The voltage on ON will not go higher that V_ih by external coupling as there is a 750kOhm path to GND.

    2. If an external source is used to pull ON high, the smart ON will be disconnected at V_IH.

    3. You will have to test your system and check if the ON voltage drops below V_IL for the smart pull down to be the only pull down used.