USB supply/charger - tips needed

[kaning]

Hi all,

Im building a portable project where i use internal Li-Po battery as power supply. This internal battery is charged via mini usb/usb whatever.

And here is where i need some guidance, how to plan the schematic of this particular part? when battery is charged, it feeds all circuit; when battery is empty and i plug USB it feeds circuit and charges battery.

General purpose question is: what kind of circuit, IC should be used when you have some project fed by internal battery and you want to charge it via USB while On, so it charges battery and supply circuit??(as we see in pretty much portable device(mp3,phone,laptop etc? many thanks in advance

 

[erikl]

No matter if USB is supplying power or not, simply always have USB connected loading the Li-Po - via IC or simple current & voltage limitation. The Li-Po always feeds all circuitry.

 

[kaning]

Im using MCP73831 as Li-Po charger and actually it has "Integrated System Load Sharing and Battery Charge Management". And the way you are saying a parallel configuration would be enough, something like this below?

it seems a litle bit inneficient since usb already has the 5v i need for the rest of the circuit so its a waste of energy regulating it to 4,2v and then step it up to 5v again. :/
Im looking for a solution that would make circuit work something like this on the presence of USB port connection.

Or im thinking wrong? what am i missing? many thanks

 

[erikl]

You're of course right in terms of efficiency. Your 2^nd version should work, provided your STEP-UP controller doesn't allow for feedback from output to input. Then this solution would be more efficient.