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BQ51050B charging voltage problem

ErikNorth
Prodigy 50 points
Other Parts Discussed in Thread: BQ51050B, BQ25100, TIDA-00318, BQ24072, BQ24232

Hi,

We have been working on a smartwatch that uses wireless charging. At the moment our design includes BQ51050B, however we have been experiencing some problems.

One I am trying to solve at the moment is related to the charging voltage. For some reason the charging voltage never goes over 4.03V and that causes the battery to never fully charge. I checked Vrect and it is around 5.1V which seems as good value (from datasheet). Also it seems that the charging current is lower than it should be.

Have you ever experienced something similar? What could be a potential issue?

I have attached our schematic. I need to mention that 10k pulldown on TS/CTRL is missing in the schematic, however it is present on the board.

Thank you.

Regards,
/Erik


  • 0
    TI__Genius 14890 points
    Erik,

    How did you calculate the AC1 capacitor (C401)? That value must be tied to the inductance of the coil. Additionally, since that will pass all of the current, we recommend 3 capacitors in parallel.
    What is the physical size of the coil? This helps to understand the coupling.
    What is the transmitter being used?
    VRECT was a good place to check. It should follow VBAT.
    The schematic shows 190mA for charge current. What are you seeing?

    Regards,

    Dick
  • 0 in reply to Dick Stacey
    Prodigy 50 points

    Dear Dick,

    Thank you for a fast response.
    Our solution has a PCB antenna and my colleague did the inductance measurement and the calculation of AC1, however he is not here at the moment so cannot say more.
    Just to test, I disconnected our antenna and attached one from Molex (146179-4011) and measured its inductance and got Ls = 22uF and Ls' = 27uF.
    Then I calculated using equations from the datasheet:

    c1 = 1 / ((2*pi*100000)^2*(27/1000000)) = 9.3816e-08F ~= 94nF (47nF + 47nF)
    c2 = 1 / ((1000000*2*pi)^2 * (22/1000000) - (1/(94/100000000))) = 1.1528e-09F ~= 1.15nF (1nF + 150pF)

    However the result was the same, charging voltage did not rise higher than 4.02 - 4.03V.

    Both our PCB and Molex antenna are fairly small, 26mm and 25mm respectively.

    We are using off-the-shelf Qi compliant charger. I have also modified one with smaller antenna (28mm) from Wurth (760 308 101 103) but both give same result.

    The charging current is more than a half less than it should be.


    Regards,
    Erik

  • 0 in reply to ErikNorth
    Prodigy 50 points
    Dear Dick,

    I grabbed another board and it worked better than the previous, the charging voltage was 4.16V, however it never reached 4.2V. The charging current was also higher, I would say 2/3 of what it should be.

    I start to think that some of the boards have bad soldering as this is actually the part with the smallest pitch on our board. The boards are soldered in a factory but I will try to take the non-functional board and re-solder with hot-air gun and see if that really is the problem.


    In general I have concerns about BQ51050B as our battery is quite small (350mAh) and this charger might not be the best choice. I have looked at the TIDA-00318 solution and it seems to be quite good for smaller batteries. Regarding this, I want to ask if there is any other charger similar to BQ25100 but with a PowerPath? I have found BQ24072, but maybe there is something better.
    I think that charging the battery and powering MCU and other external peripherals separately will make the charging process faster, and we also might want to make MCU run a bit more while it is charging.

    Regards,
    Erik
  • 0 in reply to ErikNorth
    TI__Genius 14890 points

    Erik,

    The bq24072 looks like a good choice.

    The bq24232 is another device you might compare it with.

    Keep us posted on the results of the rework.

    Regards,

    Dick