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BQ25886: Charge current increases over time

Kauko Putki
Prodigy 25 points
Part Number: BQ25886


Hello,

 

I’m having problems with the charge current on BQ25886 increasing over time. It starts out with the correct value but increases about 1.5 mA - 2 mA / minute, until it hits the CV stage or input current limit.

In the schematic the CE pin is pulled high when the battery is not inserted, and a resistor in the battery pack pulls it low. I have also tied CE low permanently, but it has no effect. I have also tried lower charge currents, but the behavior is the same.

Sparying cold spray on the IC does temporarily bring the charging current down, but it does increase back again shortly. The IC does not get overly hot, the charging efficiency is 90+%, but cooling it down does seem to have an effect.

I do have the EVK, and I have experimented with it as well. It also seems to increase the charge current over time, but much more slowly, at about 0.25 mA / minute.

 

I have attached images of the schematic and layout.

Thanks!

  • 0
    TI__Mastermind 18285 points

    Hi Kauko Putki,

    It appears to be temperature related. I will be conducting bench testing to identify the possible sources causing this issue. I am suspecting that as the temperature of the board changes, the resistivity of these resistors can possibly be altered, thus resulting in a changing charging current. Can you confirm the resistor value of R24 and R22 during the execution of your test, and confirm that it remains the same?

    Best Regards,

    Juan Ospina

  • 0
    Prodigy 25 points

    Hi,

    I did test the effect of temperature on R24 by raising it off the board on top of long leads to isolate if from the heat from the board, but it didn't have an effect. I also measured both R24 and R22 on the board while testing and they did not seem to change value due to heating.

    -Kauko

  • 0 in reply to Kauko Putki
    TI__Mastermind 18285 points

    Hi Kauko,

    I have a few questions to further get insight on your question.

    - Can you please let me know what your ICHGSET Resistor value is and any information in regards to that resistor. Additionally, what is your VSET resistor value.

    - Do you have a log of the change in ICHG over time?

    - Can you please provide a reading of the temperature at the start of charging and at the end?

    - Does the charge current continually increase or does it reach a point of stability?

    I hope to get a better Idea of the issue if you can provide answers to these questions.

    Best Regards,

    Juan Ospina

  • 0 in reply to Juan Ospina
    Prodigy 25 points

    Hi,

    "- Can you please let me know what your ICHGSET Resistor value is and any information in regards to that resistor. Additionally, what is your VSET resistor value."

    It is 5k6 1% +/-100ppm, part number MCR01MZPF5601. VSET is 150k.

    "- Do you have a log of the change in ICHG over time?"

    I have attached a log of IC temperature + charge current with both 5k6 and 5k resistors.

    "- Does the charge current continually increase or does it reach a point of stability?"

    It seems to continually increase, until it hits the ILIM current, or when charging enters CV mode.

    Looking at the data a bit closer, it seems that the charging current starts at a lower value than was set, and then goes slightly over it, until hitting CV. It doesn't seem to go very much over the set current, but curiously starts at a lower value. Here it seems to start at 50-60mA lower current, but in some of my tests it has been 100mA lower than the set current.

    -Kauko

  • 0 in reply to Kauko Putki
    TI__Mastermind 18285 points

    Hello Kauko,

    Thank you for providing the information I requested. Based on the measurements you provided, it appears as though charge current remains within the specified charge current for your current settings. As specified by the datasheet, Ichg has an accuracy of +/-7.5% of the configured charge current. Based on the ICHG resistor values, the charge current seems to remain within those parameters.

    Can you please detail what your concern may be with this behavior?

    Best Regards,

    Juan Ospina