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'[EE]About multiple power supplies to a PIC'
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
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.
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:
The LT1111-5datasheet is here:
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):
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
> 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.
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
> In regards to 1, it may be useful to see the
> "Typical Application"
> diagrams on page2 of the MCP73863 (charging IC)
> datasheet here:
> The LT1111-5datasheet is here:
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.
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
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