BQ24650 output query

Hi Adam,

Ultimately, you should design your controller for the current you intend to use (A "hand-wavy" answer, I know...). By controller, do you mean the BQ24650?  If that is the case, you set your charging current by changing the sense resistor designated "Rsr" in the typical application circuit given in the datasheet. Should you want to increase the charging current, the only real "design" change would be at that resistor, as long as the rest of your components can handle the increased current. If that is the case, you would use Equation 3 in the datasheet:

So if you would like to double your charging current, you should use half as much sense resistance.

Typically, single Li-Ion cells are charged up to 4.2 V. You can adjust the resistors in the voltage divider at the Vfb pin to do this. Since all output power flows through the buck converter, you will have to regulate down to 3.3 V for your Particle Core, as you said. That being said, you would start seeing issues pop up once your battery voltage starts to drop below 3.3V.

Alternatively, you could use a buck-boost converter to output 3.3 V. That should be able to handle lower battery voltages at the expense of being slightly more complicated than an LDO. If you plan on pulling a significant amount of current with your system, a switching regulator will usually be more efficient, as well.