| Electronics > Projects, Designs, and Technical Stuff |
| 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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