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USB Charger


Moderator: MisterX

USB Charger

Postby Coughinhk » June 21st, 2012, 8:09 pm


I"m using this US$5 LM2577 boost to create a 9V regulated supply, suitable for charging from a double-headed USB cable.


The ad says it is 90% efficient but the data sheet for the LM2577 suggests 80%. A dual usb cable (two identical plugs at the same end) can draw 5V @ 1A = 5W. 80% output is 4W. 4W @ 9V is 444mA, which is lower than the original board requirement. So changing R1 to 2K lowers the maximum charge rate to 445mA (according to the formula on page 17 of the MCP73864 data sheet).

It works ! And the LM2577 only gets warm, not hot, during charging. It takes a little longer to charge than before, and it's one more gadget to carry around, but I can live with that as a trade off against the ability to charge from my notebook, and it is lighter and smaller than a charger.

The board is an awkward size for a case though, so I'm still hunting for something that it will fit in well.

Just thought I'd share this on the off chance anyone else is looking for something like this.


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