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BQ51050B: Coil capacitors

Part Number: BQ51050B

Hi,

I am using the BQ51050B in my design and I have problems figuring out the correct capacitor values to use with my coil.

First I tried 760 308 101 303 with the values seen in the schematic. Next I tried the 760308102207 with C1=150nF, C2=1.9nF and also C1=340nF, C2 3.5nF (capacitors have 10% tolerances). Nothing seemed to work.

I measured the signals with oscilloscope and input voltage seems ok (signal is similar to what the charger sends), but the voltage on RECT pin is only about 100mV. I have attached pictures from my measurements. Measurements were taken with 760308102207 coil and C1=340nF, C2=3.5nF. Load is a single cell lipo battery with 1200mAh.

R29 in schematic is not populated so the CHG pin is currently floating.

Ideally I would like to use the first (smaller) coil but I am aware of potential problems due to size so the second one is also an option.

     

  • Something is not right with the voltage at RECT pin. You should see a DC level at that point but your scope shots show an AC pulse.
    The schematic looks OK. Is the problem seen on a number of units? Could you have problems with assembly of the pcb?
  • I only have 1 unit (I tested 2 ICs) which is hand soldered with hot air. I inspected the board and everything seemed ok but I will recheck.
    Is the smaller coil and calculated values ok? It has quite high inductance so I beleive that should compensate for the size.
  • This does not look like a coil tuning issue -- way too much AC on the voltage at RECT. Input at AC1 and AC2 is full way rectified and filtered at RECT, it is an unregulated voltage at that point but should look a lot better than the scope shots. Suspect AC is shorted to RECT or something is not grounded.