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Original question:

BQ24725A: Hot Plugging a SMBus Battery to a BQ24725 Charger System Causes AC Power Path to Latch OFF

BQ24725A: Battery Charger does not reliably select AC Adapter Power Path with Battery installed in system

Randy Holmberg
Prodigy 40 points
Part Number: BQ24725A


This is a follow up to an earlier thread on the same issue. I have been able to change bit one of the Charge Options register (0x12) from '1' to '0' and have observed the same issue previously mentioned. On some boards, the AC Power Path is turned OFF when I do power mode transitions (turn AC Power ON/OFF or plug/unplug the battery) and apparently it is not caused by an AC Over current issue. I would like help to figure out what else could cause the battery charger to do this.

  • 0
    TI__Mastermind 34461 points

    Hello Randy,

    Can you verify that it is an IC issue and not a board issue. Will you swap the IC from a working board with the IC from a nonworking board and see if you can recreate this issue on both boards?

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 40 points

    That is a good thing to try in concept, but difficult to move IC's from one board to the next without damaging parts.

  • 0 in reply to Randy Holmberg
    TI__Mastermind 34461 points

    Hello Randy,

    Can you replace the IC on the nonworking board with a new IC to see if the problem still occurs? We need to determine if it an IC issue or an issue with the board. Another option would be to order the BQ24725A EVM board and modify it to replicate your board and see if you can replicate the issue.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 40 points

    Changing out the part is a reasonable suggestion. One concern is that if I replace the part and the symptoms go away, then it may be difficult to determine what is needed in my design to prevent this from happening. I will plan to order the evaluation board as you recommend.

  • 0 in reply to Randy Holmberg
    TI__Mastermind 34461 points

    Hello Randy,

    Please let me know what happens when you replace the IC.

    Best Regards, 

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 40 points

    Hi Christian,

    I changed Charge Options Register bits and seem to have fixed the symptoms on two boards that had been problematic. Bit 8 was changed from default value '1' to '0' and bit 7 was changed from default value '0' to '1'. Bit 8 disabled high side MOSFET short circuit protection and Bit 7 would have increased the comparator threshold for the low side Mosfet. I will test more variants to see which one (or both) of these changes were needed to keep the AC Power path from switching off on us. I would like help understanding why I would need to change one or both of these register settings from the defaults to allow the charger to operate reliably for our application. Assuming this behavior is not typical for other customers, what did I do in my design that could have caused the IFAULT_HI Comparator or IFAULT_LOW Comparator tests to fail intermittently on some of my boards. I am happy I can make register changes that seem to fix the symptoms but really want to understand the root cause better.

    Thank you,

    Randy Holmberg

  • 0 in reply to Randy Holmberg
    TI__Mastermind 34461 points

    Hello Randy,

    Please refer to section 8.4.18 Inductor Short, MOSFET Short Protection of the datasheet:

    I recommed checking to see if the MOSFETs that you are using are not shorted.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 40 points

    Christian,

    When you ask to check to see if the MOSFETS are shorted, I would say no. With the register bits changed, the battery operation and charging functions work as expected. It appears that one or both of the checks are intermittently failing with the default settings. 

    Randy

  • 0 in reply to Randy Holmberg
    Prodigy 40 points

    Maybe a better question to ask is if the Mosfet threshold tests are relying on the RDS ON to be in some specific range. My design uses the BUK7D25-40E Mosfet for the charger. The BUK7D25-40E is similar to the SiS412DN shown on the example schematics.

  • 0 in reply to Randy Holmberg
    TI__Mastermind 34461 points

    Hi Randy, 

    You are correct. The RDS(ON) could also be causing this problem.

    Randy Holmberg said:
    the Mosfet threshold tests are relying on the RDS ON to be in some specific range

    Please refer to section 9.2.1.2.9 BQ24725A Design Guideline

    The equation above can be used to determine the voltage drop sensed by the IC. The total voltage drop must be below the the high side short circuit protection threshold to prevent charger shut down.( 750mV).