I'm building a PCB that will have a circuit that recharges a Li-ion battery from the USB power. The board should also be able to be powered both when it is draining power from the battery and when it is being recharged from the USB port. I found a IC from Linear Technologies called LTC4055 which seems to do all that I need, except that the top output supply current is of only 500mA (I need at least 800mA). Someone has an idea of how to build such circuit?

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    \$\begingroup\$ The 900 uA figures is incorrect The LTC4055 will supply up to 500 mA to the peripheral and battery combined. What is your battery type and capacity. If you think you can use up o 800 m then having 500 mA available will not make a lot of difference. Because of the way that LiIon batteries charge, IF your n=battery is of 800 mAh capacity then it will only take about 20 minutes longer to charge with 500 mA max supply (Say 3h-20 instead of 3h). \$\endgroup\$ – Russell McMahon Mar 28 '12 at 10:54
  • \$\begingroup\$ My battery is a 2000mAh with a typical charging current of 400mA and maximum of 1A (this one, btw). I noticed that it can recharge the battery at up to 500mA, and what I wanted was to be able to drain the power from the USB port up to 500mA and the remaining would come from the battery, if the circuit is under heavy load, since our circuit has a power consumption that varies a lot with the usage (it is a robot mainboard). \$\endgroup\$ – Gabriel Diego Mar 28 '12 at 12:44
  • \$\begingroup\$ Th open source Maple single board computer from Leaflabs implements LiPo battery supply + charging. Since it is open source and reasonably well documented you could use it as a reference design. \$\endgroup\$ – JonnyBoats Mar 28 '12 at 17:32

"I wanted was to be able to drain the power from the USB port up to 500mA and the remaining would come from the battery."

An analogy: Your car has a battery. It is charged from the alternator. All power used is coming from the battery, BUT if the battery is somewhat discharged (say you just started it in sub-zero weather,) you will get power from the alternator but it comes through the battery. (Of course if the battery is completely discharged you will need to run the engine for 30-60 minutes so the alternator can recharge the battery with enough energy to start the engine the next time.)

If I'm reading the datasheet correctly the LTC4055 is very much like an car's alternator. Have your PCB connected directly to the battery. The LTC4055 can be attached between the USB and the battery.

The question then becomes what happens if the battery is completely discharged?

Can the LTC4055 supply sufficient current to recharge the battery while letting the USB port power the PCB? Again referring to the datasheet the LTC4055 goes into a low current trickle charge mode if your battery is completely discharged and the rest of the USB current goes to load. Sounds like it is the right approach, or at least worth trying.


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