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This question is very similar to many other threads, but with some additional constraints.

I require a 2.7v to 3.6v supply derived from an automotive supply (typically 7 to 33v) at maximum of 1 amp. When the external supply is connected it must also correctly charge a single Li-Po cell. When the external power is disconnected, the Li-Po must automatically / transparently take over supplying the load.

The Microchip application note AN1149 for the MCP73837 shows as similar solution, but the output voltage to the load is too high, and the maximum input to the 73837 is only 6v. See http://ww1.microchip.com/downloads/en/appnotes/01149c.pdf

I have a solution which uses an LM2595S-ADJ to pre-regulate the input down to 4.5v, which is fed to the 73837. The output from the 73837 is further regulated down to 3.6v with an LDO regulator. Diodes are used to route the supply current.

Although my existing circuit works ok, it seems crazy to need three ICs, several diodes and a handful of passive components. Any suggestions as to how to simplify this would be very welcome Thanks.

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There are battery managers with Buck converters to give you what you want. You'll need to find one that fits well with your system, but here's an example: http://www.ti.com/product/BQ25703A

Integrating control with the level converter simplifies the power system. You'll want to be sure to shut down if the battery goes under voltage like the reference design schematic in the link shows.

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