Is there a diode that fits on the USB power jumper pads?

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jwatte

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For development, I want to power the Teensy with the USB cable I use to program it.
When installed in the system, though, it is to be powered by external 5V.
And, in some cases, I'd like to both keep the USB plugged in, and power it externally, for diagnostics in-system.
And, sometimes, but not during the entire power cycle, there will be an on-board Raspberry Pi that communicates to the Teensy using USB. This is actually the main configuration -- Teensy is always-on, RPi is sometimes-on, RPI -> Teensy uses USB for communication.

My brilliant idea is to find a low-drop diode (Schottky most likely) that fits on the USB-VCC jumper pads, and solder it in there after cutting the jumper.
This would still power the Teensy (with a 0.25V drop on the 5V rail) from USB, but would not back-power USB from Teensy when the RPi is off.

Two questions:
1) Is there something wrong with this idea?
2) What's a good SMD diode that fits on those pads? It doesn't need to support more than whatever the Teensy draws (100 mA is fine?); I will never USB power the Teensy when anything else is drawing from the 5V rail.

I'm most importantly looking for the right footprint! Some diodes I find are 0.6x0.3mm (which is really small!) and 1.0x0.5mm (which seems like a better fit) -- then they suddenly jump to 4mm, which seems too big.
Isn't there something that's 1.6mmx0.8mm? That seems like it'd fit just right!

Also, wouldn't it be cool if the Teensy added such a diode in parallel with the jumper already, so that when the jumper is cut, it already works? :)
 
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SOD-323 (2.6 x 1.35mm)
SOD-523 (1.8 x 0.9mm)
SOD-723 (1.45 x 0.65mm)

are smaller diode packages
 
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Checked (on a T3.2) with some calipers and can confirm that 1.6mmx0.8mm looks like it would fit fine. Try an 0603 package (.06" x .03", darn close to 1.6mm x 0.8mm). 0603 are hard, but definitely possible to solder by hand (if you have steady hands and good eyes).
 
I looked for 0603 imperial, but DigiKey had nothing. It's smaller than that for signal (0402 and down) and it's 4mm type stuff for power (because it needs a higher surface area for cooling, presumably.)

IN the picture, it looks like it's tacked on between the USB pad and the other end of the jumper pads, which seems creative. I was looking for something that mounts to the two jumper pads directly, because I think conventionally!

I think I'm going to try the 0402 using the "plate first, use tweezers and heat into pad" method and see how it goes :)
http://www.comchiptech.com/cms/UserFiles/CDBQC0130L-HF RevB681116.pdf

Edit: Turns out Mouser has it in 0603! http://www.mouser.com/ds/2/80/CDBURT0530LL-HF RevB850212-464702.pdf
 
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HWGuy's suggestion of SOD-523 is a good one too. Something like this would be much easier to solder than anything with pads only under the chip. But your idea ought to work for soldering one of those 0603s on if that's what you ordered.

There's too many diode packages out there for this guy ;)
 
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