Converting a 5v usb battery to 3v3

[kolonelkadat]

Is it possible to convert one of those cheapo 5v usb battery back ups to output 3v3 instead? I figured I could just change a cap or something but it looks like there are a couple of micros in the circuit that im afraid will override any changes to the analog portion of the circuit.

eta: or is there a 3v3 battery/usb charging module commercially available that I could just buy?

[Zero999]

The easiet option is to just add a 3.3V regulator on the output. If a long battery life is required then a buck converter would be best.

[cowana]

The issue you'll have is most powerbanks have a single cell lipo (or paralleled lipos), giving somewhere between 3v (empty) and 4.2v (full). To get to 5v, they have a boost switch mode converter, increasing the voltage. Changing the voltage it outputs is usually as simple as twekaing a resistor value or two (within limits).

However, for a 3.3V output, this requires a significant change in topology - you're no longer boosting the voltage at all times. If the battery is 4.2v, it needs to be reduced (with a buck converter) - and if the battery voltage is 3v, it does need boosting as before.

Your simplest option is probably modifying the feedback resistors to reduce the boosted voltage to around 4.4v, then adding a linear 3.3v regulator on the output.