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BQ25504 occasionally jumps back into cold start mode - causes glitches on VBAT_OK

Neel Karkhanis
Prodigy 80 points
Other Parts Discussed in Thread: BQ25504

Hi TI community,


I have a design containing a BQ25504 device, which takes input from a single-cell solar cell (IXYS KXOB22-12X1F) and charges two supercaps connected in series (Tecate TPL-3.3/10X20F).  The BQ successfully charges the caps, but when they are fully charged, it often behaves like it's lapsing back into cold start mode, pulling VBAT_OK down and shutting off my downstream system.

The scope shot below shows this behavior when it happens.  Note that channel 2 - VSTOR - has a 4.7V offset; the shot below shows its behavior around a 4.7V base.

  • VBAT_OV set to 4.696V (ROV1 = 4.42M, ROV2 = 6.65M)
  • VBAT_UV set to 2.199V (RUV1 = 6.19M, RUV2 = 4.70M)
  • VBAT_OK set to 2.210V, VBAT_OK_HYST set to 2.804V (ROK1 = 4.53M, ROK2 = 3.48M, ROK3=2.15M)

I tried cleaning flux off the board and adding a small (15pF) cap across the lower resistors of the resistor dividers; does not change the problem or its severity at all.

How should I proceed?  What would cause the chip to re-enter cold start mode (thereby dropping VBAT_OK and turning off the VBAT - VSTOR PFET) when VSTOR is up at 4.7V?

Note: there are no transients on VSTOR that pull it down to the VBAT_UV (or, worse yet, VSTOR_CHGEN) levels prior to the reset.  I have verified this dozens of times.

  • TI__Guru**** 166626 points

    Try adding a 0.01uF capacitor as close as possible between the IC VSTOR and VSS pins.  This cap should filter any noise into VSTOR that is causing the IC to lapse back into cold start.

  • in reply to Jeff F
    Prodigy 80 points

    This worked.  I added 2x 10nF caps from VSTOR to VSS, one on the front of the board and one on the back, with short wires.  Total length from VSTOR pin through the front side cap to VSS pin is about 5mm.

    With the caps, the system stayed stable for more than 12 hrs in a row.  When I subsequently removed them, the system resumed its behavior of rebooting (from VBAT_OK going down) every 5 - 10 mins.


    My existing board has a 100nF cap from VSTOR to VSS, but it is not as close as possible to the pins.  Total path length was about 10 - 15 mm.


    These 10nF caps may have solved the problem.

  • in reply to Neel Karkhanis
    Prodigy 80 points
    Question about layout:

    The BQ25504 datasheet provides a recommended layout on page 26. However, it's not clear where and how to connect together the AVSS and VSS pins.

    Should I connect them together far from the switching power components with a ferrite bead? Leave them unconnected?
  • in reply to Neel Karkhanis
    TI__Guru**** 166626 points
    Hello Neel,

    I recommend connecting them together with a short trace from the AVSS pin to the thermal pad.