I understand this is a long and complex datasheet. This device 1) serves your purpose and 2) is available. Read the ds slowly, and buy a board off ebay to play with it
I have read the datasheet again, and I am pretty sure I am right. As a hobbyist and beginner, maybe I am wrong but :
On
datasheet:
In section 7.1 "Overview", page 13, end of page :
"In addition to the boost charging front end, the bq25570 provides the system with an externally programmable
regulated supply
via the buck converter."
In section 7.3.7 "Step Down (Buck) Converter", page 20 :
"The buck regulator input power is internally connected to VSTOR and steps the VSTOR voltage down to a lower
regulated voltage at the OUT pin."
As VSTOR is the main output of the energy harvesting input and not battery, if battery is under 3.3V, it is not possible to generate 3.3V.
I precise here that my battery will not reach 3.3V.
On e2e.ti.com, on post "
BQ25570: is it possible to get 3.3v output from BQ25570 Vout? even battery voltage is Less than 3v?" :
"The BQ25570 VOUT output is from a buck converter and would require a slightly higher voltage than 3.3V to provide 3.3V output. Can I recommend BQ25505 followed by BQ63024 integrated solution or TPS61170 configured as a SEPIC converter (d/s figure 19 page 21)?"
A buck/boost (or here SEPIC) is needed to generate 3.3V with a battery voltage lower than 3.3V. The BQ25570 seem to be a buck boost charger, with a buck converter on top of the battery as output power supply.
Moreover, BQ25570 is not available in little quantity (as a hobbyist, I need under 10 pieces for now). The only stock I found is in Rochester Electronics website, but MOQ is 49pcs