Simplest compact USB/Vin auto switch

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ive searched around , found several different designs usb power/external power auto switcher, trying figure out 1 simplest/smallest footprint application.

im designing custom pcb using due (sam3x). im trying make pcb small possible. going omit usb power capability altogether because out of board space...but after thinking, made executive decision remove due jtag header , put usb switching circuit in place. figured miss having option power board off of usb or external lot more miss having jtag header (from reading, seems no 1 ever uses it).

i found several designs , im not sure 1 smallest, while still working properly.

i removed atmega16u2 , associated "programming port" circuitry; have due's native usb port on there now.

the main power supply, used due design lm2734 switching power supply 5v, , linear ncp1117 drop further 3.3v feed due.

some designs use op-amp , single transistor switch...others use 2 transistors , no op-amp.

some designs seem totally "self-sustaining" , automatic/separate, others seem use uotgvbof pin on due control switching?

and seems 1 design doesnt use transistors @ all, simple diode on usb +5v line prevent due power supply backfeeding pc's usb port.....?

which simpler/easier/more compact method? need use uotgvbof pin on due/sam3x?

it should worth noting not using due's native usb port peripherals or anything....i using programming due , thats it. think can ignore related usb on go, , usb "id" pin??

thanks
ben

bump....anyone have ideas? want confirm before send board off manufactured.

thanks
ben


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