> 
> My questions....
> 1. how do I build a circuit that allows the device
> to use power from the 
> wall-wart when available, but from the LiIon
> otherwise. Specifically, 
> the device should *not* use any power that is
> recharging the LiIon cell. 
  
  There are sample circuits on the net. Basically it
use a diode for the battery to only allow current to
flow in absence of the walmart adapter.

> Additionally, the wall-wart supply will also have to
> be regulated 
> *after* feeding the charger down to 5V from 5.7
> *nominal*. (I was 
> thinking a resistor/zener/schottky combination -
> 50Ohm, 5.1V Zener, and 
> then a schottky, but this wold not be as effective
> if the input voltage 
> was at the charger's max of 12V...).
> 2. How do I build a protection circuit so that the
> device will be shut 
> down if the LiIon voltage drops below 2.5V. This
> must not draw more than 
> a few uA from the cell!

  Okay, you can use the ADC module from the MCU to
determine the voltage that you want to *shutdown* and
just wait till the power satifies your voltage
requirement.

John



> 
> In regards to 1, it may be useful to see the
> "Typical Application" 
> diagrams on page2 of the MCP73863 (charging IC)
> datasheet here:  
>
http://ww1.microchip.com/downloads/en/DeviceDoc/21893c.pdf
> The LT1111-5datasheet is here: 
>
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1033,P1459,D2092
> 
> 
> 

  You can check on of the latest articles from circuit
cellar for more information on the charging circuit. I
can't remember which article off hand. 


John

PS: If you want a really cheap comparator and charging
solution the PIC already have a comparator built in.
Just charge the battey until the reference voltage is
reached.



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