Switching power source on plugin for battery charger

[Zebov]

I'm designing my first battery charging circuit for a device and need
some assistance. As a preface, I am charging a 3.7V 850mAh Polymer
LiIon battery from a 5V wall-wart through a MAX1555.

I have a battery charge circuit and I have the rest of my circuit
which runs off of 3.3V (3.7V down-converted to 3.3V via a MAX710 step-
down) pulling about 100mA. My problem comes when I merge the two
parts.

I don't want to overcharge my battery and would like the circuit to be
running while the battery is charging. The problem is, if I just
connect the battery + to the +rail of the circuit, it will draw 100mA
+battery charging draw. Will this confuse the MAX1555 and make it not
turn off when it's supposed to?

My first thought is to somehow switch power sources from the battery
to the 5v wall (through a step-down to get 3.3V) whenever the 5V
supply is plugged in. I'm trying to think of the best way to do
this. I was thinking of just putting steering diodes between the
battery and wall rail but don't think this will work well because the
battery will be switches on once it is charged and I'll run into the
same problem as if I was just running off the battery charging rail by
itself (as mentioned previously). Any suggestions? Thanks ahead of
time for the help.

- Wes

 

[Martin]

My first thought is to somehow switch power sources from the battery
to the 5v wall (through a step-down to get 3.3V) whenever the 5V
supply is plugged in. I'm trying to think of the best way to do
this. I was thinking of just putting steering diodes between the
battery and wall rail but don't think this will work well because the
battery will be switches on once it is charged and I'll run into the
same problem as if I was just running off the battery charging rail by
itself (as mentioned previously). Any suggestions? Thanks ahead of
time for the help.

- Wes

If the power supply is plugged in via a plug and jack, consider using
headphone type plugs and jacks. Some (not all, not even most) of them
contain a terminal that is disconnected when the plug is inserted.
They are meant to be used with devices with speakers, where the
speaker is disconnected when the headphone plug is inserted. I don't
offhand remember what this feature is called.

Please note that if you do this your battery will be totally
disconnected (i.e. neither charging nor discharging) while the device
is on the suppply

Martin