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BQ24650: Two-layer PCB unstable

Chris Tanner
Prodigy 10 points
Part Number: BQ24650

Tool/software:

Hi, I have had this two-layer PCB layout running fine for a year or so, but the most recent batch of PCBs has a problem where it goes into an unstable mode:

- the STAT1 LED flashes briefly once every two seconds
- BTST net is as shown on the trace during the charging phase (which cycles on and off)

Charging li-ion at a target of 16.8v, using a 21V 10W solar panel, at 1A max current. While the battery is low voltage and the charge current is high, everything seems OK, until some point (which varies) during charge when it becomes unstable.

I am OK to make modifications to the PCB layout if needed, but would need to know the source of the problem so I can change the appropriate section. Or even another component I can retrofit to existing PCBs to ensure stability.

Gerber attached.

Unstable on BTST, approx 16v at peak.

GerberWM-V12.zip

  • 0
    TI__Mastermind 31761 points

    Hello Chris,

    Can you provide your schematic?

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 10 points

    Yes, here you go.

    The 2.5mR resistor is for current sensing elsewhere.

    LEDs are connected from VREF to the STAT lines with a 200R resistor.

  • 0 in reply to Chris Tanner
    TI__Mastermind 31761 points

    Hello Chris,

    Chris Tanner said:
    Hi, I have had this two-layer PCB layout running fine for a year or so, but the most recent batch of PCBs has a problem where it goes into an unstable mode:

    Have you taken the IC off a working board and placed in on a non-working board, to confirm issue follows IC or board? Is there any difference in this latest batch of boards?

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 10 points

    Hi Christian

    I don't have the facilities to remove and refit this IC unfortunately. I have ascertained some more information:

    - Placing a finger on the PCB in *just* the right place, somewhere over the IC and nearby components, restores stability, and I get the "charge complete" LED coming on. I can't repeat this predictably, and don't think it would be useful to spend the time doing so! It's a capacitive fix; there are no loose joints.

    - I had one previous batch PCB which was not showing problems, but now is ... which led me to think that perhaps there was a voltage spike from a power supply in place of a solar panel which has damaged the IC. But I've tried one of the new batch from the new PCB stack, which has never had a power supply attached, and it exhibits the same problem.

    So in summary it appears that all of the new boards so far have the problem, and it's possible to get old boards to fall into the same problem.

    The new boards are the same schematic apart from I have replaced a coin cell CR2032 (for an RTC) with a LP2985-33DBVR regulator, so we can use the primary battery for maintaining RTC. The instability remains even when I isolate this section of the circuit, so I'm pretty sure it's not that.

    The PCB layout for the charging components (and everything nearby) is the same as per the previous version.