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different behaviors with same test

Giacomo Mezzetti
Prodigy 10 points
Part Number: BQ25570

Hi,

we build a small system with two BQ25570 for piezoelectric and solar harvesting source as shown in the figures:

Vbat = 3.3 V to 4 V depending on the battery charge

What we expect is that Vout will come out with 3.3V even when the harvesting sources are not physically attached to Vin_dc, and Vstore set to battery voltage level (Vbat).
What we experienced is that it depends. Sometimes we have 3.3V (with a system load attached to Vout), sometimes we measure low level voltage, like 1.4 V or 0 V, with Vstor not charging.
If the harvesting sources are attached to Vin_dc, Vstore starts to increase until it reach ~2 V and then got o battery voltage level.
Shouldn't Vout level always be on 3.3V (set with Vout_set pin) if a charged battery is connect to Vbat?
Thanks.
  • 0
    TI__Guru**** 167146 points

    HI Giacomo,

    The internal PFET between VSTOR and VBAT, used to provide battery undervoltage protection, has a body diode that allows current flow from VSTOR to VBAT.  If VBAT <VBAT_UV=2.1V typical, the PFET stays off to prevent the load on VSTOR (including the buck converter) from over-discharging the battery.  This device cannot handle battery hotplug, i.e. attaching a battery when VIN_DC is present or VSTOR or VBAT>100mV.  If the charger is in this state, attaching a source to VIN_DC turns on the PFET.  If the battery has its own undervoltage protection, I recommend shorting VSTOR=VBAT.

    Regards,
    Jeff