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'[EE]About multiple power supplies to a PIC'
2006\09\22@130912 by Rolf

face picon face
Hi all.

I am investigating the possibility of using a rechargeable Li-Ion cell
in a device. I currently have a working (breadboard) prototype which
uses a Boost converter (LT1111-5) to generate 5V for PIC18F4620, LCD
w/Backlight, and other components (Imax would be about 100mA worst case)
from 2 x NiMH AA cells.

The 18650 LiIon cell would give me more capacity (larger V and mAh), and
would be (much) easier to fit in the target enclosure to be used in the
final product.

With either the 2 x AA or 1 x LiIon solution I would need to undo
multiple screws to replace the discharged cells with recharged ones. I
intend to solve this by including a charging circuit in the device
(probably the MCP73863). I will be making 2 devices in total, one for
me, one for my friend. We both have Nokia phones which have a wall-wart
power-supply that would be good enough for supplying the charging
circuit. The charging chip *can* accept up to 12V input though.

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.
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!

In regards to 1, it may be useful to see the "Typical Application"
diagrams on page2 of the MCP73863 (charging IC) datasheet here:  
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


In regards to 2, a Linear Tech. charge controller (LTC4062) circuit has
a charging chip with a "micropower comparator" which is described here
(perhaps a good basis for a shut-off circuit):
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1037,C1078,C1088,P9551,D6878

If there are any pages I can go to that will better educate me in this
process I would appreciate some links. My "Gargoyling" returns far too
many irrelevant links when I search for "power supply", "dual power",
etc. I need some more directed pointers.

The piclist archive seems barren in this subject too.

Thanks in advance

Rolf

2006\09\22@141011 by John Chung

picon face

>
> 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.

{Quote hidden}

 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:  
>
ww1.microchip.com/downloads/en/DeviceDoc/21893c.pdf
> The LT1111-5datasheet is here:
>
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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