USB, LiPo and External power

Question

I'm creating a new project that requires a LiPo to be charged from USB, the circuit can then be powered from either the LiPo, USB or an external Voltage (don't know which will be available at any given time). Charging should however only be done from USB. My setup at the moment uses Schottky diodes to switch between these sources leading to the voltage regulator. The system uses fairly little current but the diodes will still have a voltage drop associated which I would like to minimize. (Zeners on the schematic should be schottky diodes, Kicad doesn't seem to have the symbol)

I'd appreciate some critique and suggestions about this configuration and any ideas would be appreciated.

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MCP1802 LDO Data sheet

MCP73832 - LiIon/LiPo charger - selector guide - page 16

Answer 1

Looks OKish.
Note Andy's FET circuit for about zero fwd voltage drop.

Chosen LDO has 200 mV max dropout at 100 mA load and 25 uA quiescent.
With a LiPo cell and 3V3 output you want as little voltage drop as possible and 200 mV is significant. If you use a Schottky it's even worse -
3V3 + 200 mV LDO + 300 mV say Schottky = 3V8 !!!
Your LiPo will have much capacity remaining when the regulator drops out.

Even with the FET switch you have 3V3 + 200 mV = 3V5 minimum battery voltage. The MCP1802 datasheet fig 2-13 suggests typical dropout of about 1.66 mV/mA at 25C - worse if hotter.

An LDO with lower dropout may be in order if you want to use more of your battery capacity. and something like the MOSFET switch is going to be essential.

Answer 2

If you want to get rid of the volt drop when feeding a battery this circuit might prove useful: -

Incoming power is from the left. Normally the parasitic diode in the p channel mosfet would drop 0.7V but because the gate is tied to ground the fet turns on when power is fed in because the source becomes more positive than the gate. Should there be no power fed in from the left or the input is shorted, Q1 does not turn on and the internal diode is reverse biased preventing discharge of the battery.

Do not run this on incoming supplies greater than the gate-source voltage of the fet (~15V) or you'll damage it. Check the data sheet also for reverse leakage current through the parasitic diode to see that it is low enough not to discharge the battery over the time period you expect it to stay charged for.

Picture taken from here