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I want to power a power LED from a lithium-ion battery and simultaneously from a 5V power supply if the device is plugged into the main voltage. My LED driver demands 3.3 V and up to 800 mA. The lithium-ion battery is just a single cell so the voltage will be something in between 2 V and 4.5 V during the whole discharge cycle.

I can ramp up the battery voltage from a DC/DC converter and get the voltage down with a LDO or build something myself like a discrete LDO from passive components, transistors and OpAmps (I don't have a lot of other IC's at hand). What would be the better option regarding power losses and what would be good topologies to build myself?

EDIT: The design should work for every single cell lithium chemistry and the battery should discharge at least 80 % of its battery capacity.

I use the MCP73831/2 CMU.

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  • \$\begingroup\$ You should never discharge a Li-ion cell down to 2V; it will probably damage it, and certainly decrease its cycle life. At 3.3V, the cell will be almost empty, so there's little point in stepping up the voltage when the battery goes below that; there will be almost no capacity left. \$\endgroup\$
    – ocrdu
    Jan 1, 2021 at 16:57
  • \$\begingroup\$ and Li-on never get well over 4.2V :) it tends to shorten the life cycles or even worse overcharge and making it hot + explode :) \$\endgroup\$ Jan 1, 2021 at 17:05
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    \$\begingroup\$ Have a look at my answer here: electronics.stackexchange.com/questions/539946/… and the question it answers. Basically, I would use a load-sharing charger IC followed by a low drop-out regulator to get 3.3V. The battery will be close to 3.6V over most of its useful discharge curve; once it goes a few tenths of a Volt below that, it's time to recharge anyway. \$\endgroup\$
    – ocrdu
    Jan 1, 2021 at 17:19
  • \$\begingroup\$ @ocrdu I don't know which battery will be used. My design constraint is that it should work for every singe cell lithium chemistry and discharge at least 80 % of its available capacity. And since temperature will change the discharge curve I just assumed the lowest and highest possible voltages. Thanks for your answer that probably solves most of my answer, since I'm using the MCP73831/2 CMU \$\endgroup\$
    – Maxim
    Jan 1, 2021 at 17:30
  • \$\begingroup\$ @Maxim Your requirement to work with every lithium chemistry, which is still evolving today, requires you to specify and then research the details for all of them, and their differences and similarities. If you modify your question with your work product, it would go a long way in improving your question. \$\endgroup\$
    – jonk
    Jan 2, 2021 at 12:02

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