That's what my circuit does, using the same button for powering-up and
for input.
The switch feeds the bases of two transistors with emitters to ground,
one collector goes to the power circuit and the other collector goes to
a MCU pin. A third transistor is operated by a GPIO of the MCU and its
collector is tied in parallel with the other transistor that commands
the power circuit.

I use also an RC circuit coming from the external power input that
generates a pulse and turns on the circuit when external power is first
applied but allows the circuit to power itself off afterwards.

For the buck-boost, Linear Technology has some nice battery management
chips (with what they call "PowerPath") with battery charger and several
boost and buck-boost circuits. I used one of them, don't remember right
now exactly which one.

Check the LTC3566.


Cheers,

Isaac



On 11/03/2015 22:52, Lee Mulvogue wrote:
> Just been going back through Isaac's thread from 23/Feb as I'm looking at
> something similar
> http://www.piclist.com/techref/postbot.asp?by=3Dthread&id=3D%5BEE%5D+Char=
ger+and+power+circuit+for+mobile+equipmen&w=3Dbody&tgt=3Dpost&at=3D20150123=
104616a#104616a
>
> and have a couple of related questions;
>
> - if using a momentary button, switching to ground, to latch the power
> supply (MCU holding it there), is it possible to re-use that button for
> input purposes?  I currently have two buttons switching to ground used as
> inputs, but no latched power system.  I'd like to disable the buck-boost
> on complete shutdown, but don't have room for additional switches, and
> double-pole switches of the style I'm using/need don't seem to exist.
>
> - what type of diodes were D1, D2, D3?
>
> - if I always need both 3.3V and 5V from a li-ion when in operation, with
> the ability to boost from a min of about 3.0V, is there much benefit to
> using two buck-boosts, over one buck-boost to 5V and one LDO for the 3.3V=
?
>
> Thanks
> Lee
>



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