Buck, buck-boost / SEPIC or LDO?

Buck-boost / SEPIC

When looking at discharge curves from a Li Ion battery, it can be seen that at a battery voltage of 3V3, the battery holds only 15% of its original capacity.  There's little use in draining it further.

LDO

An LDO that can deliver 1A at 3V3 will be bigger, more expensive and will have a higher quiescent current than buck converter.

I plan to use it when the 5V is connected for a longer period of time.  The Li Ion would only be a backup power supply.  In that case, an LDO is really a no-go because of its low efficiency at 5V input.

Buck converter

If currents >1A are desired,  this will be a more efficient solution.  Both energy and cost-wise.

• AP3429A

Input surge/reverse voltage protection

This will consist of a fuse and a TVS-diode.

An SMD-fuse holder, holding a standard 2410-size fuse will be used.  Littelfuse sells these as there OMNI-BLOK 154 series.  Chinese knock-offs can be found on AliExpress.

The TVS-diode should be able to dissipate reverse current or over voltage until the fuse opens.  An SMC (DO214AB) package can hold 1500W or 3000W types.  So there's room for experiment.  The TVS-diode will keep the input voltage below 9.2V, so we must be sure that the battery charger and the buck converter can also handle this.  The AP3429 only accepts max. 6V, so we'll have to add extra safety measures.