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BQ25886: PMID not outputting to the inductor

Eric Metzler
Prodigy 50 points
Part Number: BQ25886

I have a BQ25886EVM evaluation board and my own board with a BQ25886EVM that I am comparing to each other. The Evaluation boards works as expected but my board is not.

In my troubleshooting I have noticed that on my board I am not getting VBUS voltage on PMID pin to power the boost circuit. On a scope a ~4Hz sawtooth waveform that goes up to VBUS voltage is on both the PMID and SW pins. While on the evaluation board my VBUS voltage (+5V) is on PMID and a ~1.6MHz square wave is on SW.

What would be causing this issue?

  • 0
    TI__Genius 10385 points

    Eric,

    Take a look at this schematic checklist and verify that your circuit is configured properly. As for why this is happening, I have a few questions: Is VBUS falling to 0V? During what operation does this occur (charging, standby, OTG, not charging, all of them or some of them)?

    This information will help me determine a root cause.

    Thanks,

    Ricardo

    3302.BQ2588x_SchematicChecklist_AddPCBcomment.pdf

  • 0 in reply to Ricardo
    Prodigy 50 points

    Ricardo,

    Thanks for the schematic checklist, that is a really helpful document. I have all the the required and most of the recommended and optional components.

    VBUS is not falling, it is at a steady 5V. This happens on all the states mentioned. The batteries output fine but that doesn't mean much because of the diode from BAT to SYS. When I plug the 5V charging power neither the /PG or STAT LEDs light up. I have ILIM tied to GND for max input current. OTG is disabled, that pin is tied to GND.

  • 0 in reply to Eric Metzler
    TI__Genius 10385 points

    Eric,

    Looking at section 8.3.8 of the datasheet could it be possible that something in your board design is violating the conditions in 8.3.8.1? If any of the conditions in 8.3.8.1 are violated, /PG goes high (no light) and the STAT LED would not be active as charging would be disabled. When you measure the voltage at VBUS are you measuring it at the power supply or at the IC pin?

  • 0 in reply to Ricardo
    Prodigy 50 points

    Thanks Ricardo,

    I haven't gotten back to this project since my last post. I am waiting on parts and a new board layout to arrive, I am quite confident that somewhere along the way the BQ25886 chip was damaged.

  • 0 in reply to Eric Metzler
    TI__Genius 10385 points

    Eric,

    Would it be possible for you to swap the "known bad" IC on your board with the "known good" IC on the EVM?

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 50 points

    Ricardo,

    My new boards and components came in and 4 boards have been assembled. 3 of them seem to be working, or close to working. The LEDs light up when plugged in to charge and the batteries are getting 8.4V which is what I have VSET set to.

    What doesn't seem to be right is that the board makes a high pitch squealing noise and when a scope is attached to the SW pin the square wave in intermittent. Also the 8.4V to the battery is a ripple to it when viewed on a scope.

    The fourth board doesn't seem to be working at all, same symptoms as before.

    Thanks,

    Eric

  • 0 in reply to Eric Metzler
    TI__Genius 10385 points

    Eric,

    In regards to the squeal, this is most likely attributed to the capacitors or inductor used. In electrolytic capacitors the squeal is due to the components leaking and failing. I feel this would be immediately apparent to you so this most likely is not the cause. In ceramic capacitors, this squeal is caused by the capacitor expanding when a voltage is applied. This can be resolved by selecting ceramic capacitors with a higher DC voltage rating. Similarly, the inductors can produce a coil whine caused by vibrations. This can be resolved by choosing an inductor with a higher saturation current rating or by securing the inductor to the board to reduce vibration. We recommend that you follow the BOM provided in the EVM user guide for optimal performance.

    Would it be possible for you to swap the IC on the fourth board that doesn't work with the IC on the EVM?

    Thanks,

    Ricardo