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BQ25504: Improving start up time when using supercap as storage device

marcus marcus
Prodigy 50 points
Part Number: BQ25504
Other Parts Discussed in Thread: TPS62743, BQ25570

Hi experts. I'm looking at the bq25504 for an application and have some questions.

The target application will be using a very low power PV as input (MPPV ~1.5V, ~45uW), a supercap connected to VBAT for storage (5V, ~0.22F), and a load that requires an average of ~2uA@2.0V.

Looking at the datasheet, it seems that any positive voltage differential between VSTOR and VBAT (above the NFET body diode drop) will always flow to VBAT. As such, to get out of cold start mode, not only does CSTOR need to be charged above VSTOR_CHGEN, but the supercap on VBAT does also.

Our application is portable and it's likely that the supercap will become fully discharged somewhat regularly.

Given the much better efficiency of the main boost charger (plus MPPT tracking rather than the PV cell being clamped to VIN_CS) vs cold start mode, it would seem to be far better for the ultracap to remain disconnected until CSTOR reaches VSTOR_CHGEN plus some margin, and only then be connected (and disconnected once CSTOR falls to VSTOR_CHGEN). This would allow the supercap to be charged from 0V to VSTOR_CHGEN in main boost mode instead of cold start mode, which is likely to be tens of times quicker.

Are my assumptions here valid? If so, could this be achieved by settings the VBAT_OK_PROG to say 2.2V (i.e. VSTOR_CHGEN + 0.2-0.4V) and using the VBAT_OK output to close a separate FET that connects the supercap to VBAT? In this way:

- With the supercap and CSTORE at 0V and the PV cell in sunlight, the device enters cold start mode and charges CSTORE (supercap disconnected)
- V/CSTORE hits ~1.8V and the device switches to main boost mode with MPPT
1 - V/CSTORE hits 2.2V + VBAT_OK_HYST, VBAT_OK goes high and the supercap is connected to VBAT
2 - CSTORE discharges into the supercap
3 - V/CSTORE falls to VBAT_UV, VBAT_OK goes low, the supercap is disconnected and CSTORE starts charging again (with the device remaining in main boost mode) and back to 1.

The load would be driven directly from the supercap via a suitable buck converter like the TPS62743.

Does this design make sense?

  • 0
    TI__Guru**** 166626 points
    Marcus,

    Your logic is sound and I tried almost the exact same type of ratchet-like charger but found it a bit cumbersome. I added a series resistor to the supercap and then slowly shorted the resistor with a parallel PFET driven by a resistor divider from VSTOR. Alternatively, an even simpler solution is using the bq25570 with integrated buck converter and placing the supercap at the buck converter output powered from VSTOR. The buck converter has integrated soft start and input undervoltage protection with current limit that prevents VSTOR from collapsing while slowly charging the supercap.
  • 0 in reply to Jeff F
    Prodigy 50 points
    Thanks for the quick and helpful reply Jeff.

    Subsequent to my post I came across an article detailing using the buck output of bq25570 in this way. Will definitely run the numbers on it.

    I also noted that several other vendors' offerings (e.g. ADP5091) isolate the VBAT output during cold start for this reason and so might be more suitable.