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BQ25504 Problem when connecting battery

Robbertvs
Prodigy 60 points
Other Parts Discussed in Thread: BQ25504

Hello,


I am in the process of designing a board with a BQ25504 harvester. I have made a prototype which isn't working as expected.

I pretty much used the typical application for solar cells with a 7mAh Li-coincell (3,1V) from the datasheet, circuit is attached. I am using two small 35x14mm solar cells which are capable of supplying around 60uA at 2,7V combined.


6330.schematic.pdf


On the EVB it takes about 2 minutes for the boost converter to fully start and output a voltage of 3,1V. When I hotplug a battery (V=2,9V) it starts charging the battery with around 50uA. VIN_DC regulates to 0,78*2,7=2,1V.

On my prototype it takes about 4 minutes for the boost converter to start and output a voltage of 3,1V. When I hotplug the same battery, it looks like the boost converter stops and can't start up again (although it tries to every 16 seconds). The solar cells are heavily loaded and practically short-circuited (V=~0V, I=78uA)

In both cases, no load was attached.

When the prototype or EVB are running without a battery attached, everything looks OK (also with a small load).

Can anyone point me in the right direction as to why the EVB is working fine and the prototype isn't?

Thanks in advance!

Robbert

-edit-

I noticed a strange behaviour when measuring the quiescent current of the prototype. I'm measuring a current of 335nA, but every 80 seconds the current rises to about 550nA and then slowly returns to 335nA. The EVB doesn't have this behaviour.

  • 0
    Prodigy 60 points

    I have done some more measurements and have found a possible cause why the input voltage drops. The VREF_SAMP capacitor apparently discharges. Because I can't measure on this capacitor as this will discharge it, I have captured the input voltage which should follow the voltage on the VREF_SAMP capacitor.


    It discharges with 0,44nA (i=10*10^-9 * (0,7/15,68)=4,46*10^-10).

    The datasheet says the leakage current of this cap should be less than 2nA @ 3V. If I calculate this, the cap will discharge 3,2V at a rate of 2nA in 16 seconds (16*(2/10)=3,2V).

    I'm using the following capacitor: CC0603KRX7R9BB103. I also soldered this capacitor on the EVB where it functions without problems.

    Could someone help me explain what's happening?

  • 0 in reply to Robbertvs
    Prodigy 130 points

    Hi Robbertvs,

    Have you got a any solution for this? I am also facing the same problem!

    Priyesh

  • 0 in reply to Priyesh P P
    TI__Guru**** 166116 points
    You need to select a low leakage capacitor. Typically, the higher voltage rating on the capacitor, the lower the leakage. Also, solder flux residue can create parasitic resistors in the 100k-1M range across components.
  • 0 in reply to Priyesh P P
    Prodigy 60 points

    It turned out to be a problem with the cleaning of the PCB. We didn't clean the PCB thoroughly enough which created a leakage current (which Jeff F also mentions).