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Cheapest way to do 2 cells (alkaline) and a 5V USB jack both to 3.3V output

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alank2:
The idea would be, if plugged into USB, the power comes from USB, but then the 5V has to be regulated down to 3.3V.

Also need to support 2 alkaline cells (1.6V to 3.2V) that boosts to 3.3V.

I've been using some Shottky diodes to combine 5V USB and 3 alkaline cells (2.4V to 4.8V) so the output natually follows the highest voltage (USB if plugged in), but I want to do a dual power source USB and 2 cells that both go to 3.3V.

Should I put the 2 cells through a switcher step up to 3.3V and the USB to a linear 3.3V step down and then try to combine them with ?

I saw some step up / step down ones, but they were pricy if I was looking at something with leads on it like a soic-8...

alank2:
Current needs 200mA max.

schmitt trigger:
Linear Tech have some unique buck/boost ICs which include the power Mosfet.
I would start from there.
And feed it from either USB or battery via Oring diodes.

alank2:
I saw one of them, but as the voltage goes lower, so does its current capability.  Do you have a part number I should look at?  Any other suggestions even if it isn't a single part solution?

mariush:
See SEPIC regulators.
For example  LM2621 : http://www.ti.com/lit/ds/symlink/lm2621.pdf
Page 14 has example circuit for 1.25v to 14v -> 3.3v 500mA max and you have explanations on how to pick values if you only need 200mA (for increased efficiency and other things)

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