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BQ24295 PMID pin

Natallia Holubeva1
Expert 6550 points

Hello e2e,

My customer is wondering:

If the boost converter is turned off, do you know if PMID is isolated by turned-off transistors?  If current leaks from by battery on VDEV through the charge chip into the PMID pin, will current flow into the pin? 

Thank you!

  • 0
    TI__Genius 13700 points

    Hello Natallia,

    Are you asking if RBFET(Q1) & HSFET)Q2) will be opened when the boost converter is disabled?

    The RBFET is used to isolate the INPUT and PMID to prevent reverse conduction when the input voltage drops to low as discussed on Pg. 22 of the datasheet. So if the input is not present then RBFET will be open.

    The operation of HSFET is focused on bucking the voltage from the input down when charging is possible. If there is no potential on the PMID pin and no voltage on the input the HSFET has the potential to leak current through its body diode if there is a potential on the SYS/SYS pins.

    Could you clarify what you mean by VDEV?

  • 0 in reply to Ryan May
    TI__Expert 6550 points

    Hello Ryan,

    From the customer:

    The thing I take away regarding the PMID pin is it has three modes:

    1. When VBUS is present, it’s connected by Q1 to PMID.  Even if Q1 were off, current would flow from VBUS to PMID through the body diode.
    2. In boost mode, the 5V output is on this pin.
    3. When BUS is not present and the boost mode is not enabled, PMID is connected to SYS through Q2’s body diode.  I’m supposed to connect stuff that can operate at the battery voltage, e.g. the LDO that runs the digital part of my board, to SYS.  Except for in shipping mode or lockout, BAT is connected to SYS by Q4, and this SYS voltage will appear on PMID b/c of Q2’s body diode.

    If I’m not using Shipping Mode and I don’t bring QON out of my product’s enclosure, am I still right to connect my system to SYS instead of BAT?  What can I do to keep a software error from initiating Shipping Mode and bricking the product?

    Thank you

  • 0 in reply to Natallia Holubeva1
    TI__Genius 13700 points

    Hello Natallia,

    Those 3 points are correct and very well stated.

    To keep the device out of shipping mode the customer is recommend to first be careful with their design both in hardware noise and proper coding since the only way to enter Shipping Mode is not something that statistically should happen by accident. However if that were to happen the solution to renable to BATFET is simple.

    Even if the device does enter shipping mode by some accident they could power their host controller through the SYS pin by connecting an adapter to initiate a process to check to make sure the BATFET is closed.

    Let me know if there are any other questions.

  • 0 in reply to Ryan May
    TI__Expert 6550 points

    Ryan,

    Does providing 5V at VBUS guarantee power is available (through the inductor) at the SYS pin? 

    Thank you

  • 0 in reply to Natallia Holubeva1
    TI__Genius 13700 points

    Hello Natallia,

    By placing 5V on VBUS you will first guarantee that there is voltage present on the PMID pin.

    It will run through tests to ensure that the input is good.

    The only case where the would not be voltage expected at the SYS pin when a proper voltage is present on the IN pin is if the EN_HIZ is written with 1.