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I've been whipping out quick-and-dirty USB-to-3.3 V supplies for XBee experiments. All it powers is the XBee (ser 1) and a couple of low intensity LEDs on the DIO lines, with a maximum total drawn current of 50 mA. A simple 2950 LDO, a 1N4148 diode for sanity, and a couple of tantalum capacitors. In most cases I can squeeze everything onto the back of the breakout/mounting PCB of the XBees.

Would this simplistic solution be OK for the simplistic 3.3 V requirement?

(Note: the regulator is a 2950 LDO, not 7833 as Fritzing insists on inserting into the schematic.) Additionally, I also have a 22 pF ceramic capacitor off the VCC of the XBee to ground (but it's not a part of this power circuit).

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  • \$\begingroup\$ I see nothing wrong with this; as long as you get a nice steady voltage at the right level and you don't draw too much current. I can't find your exact LDO with the information provided to check. Edit: Somehow I missed the link you provided, as long as you don't draw more than 100mA. \$\endgroup\$ – Nick Williams Jun 21 '13 at 18:59
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    \$\begingroup\$ "a 1N4148 diode for sanity". What's the point of the diode? If it's being used from a USB plug, it won't be able to be reversed. I assume the USB connection is soldered directly to the pcb. \$\endgroup\$ – horta Jun 9 '14 at 14:47
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Yes. Though, depending on how stable the USB 5 V source is, and its capacitor/filter on the source side, you may not even need the input capacitor.

And some 3.3 V regulators do not need an output capacitor either under most situations, but the LP2950 does (minimum 2.2 µF, with an ESR of 5 Ω).

Additionally, for a truly minimal 5 V to 3.3 V circuit, you could go with an SMD IC.

For example, the LDCL015XX33 is a capacitor-less SMD (SOT23-5) 3.3 V regulator with a 150 mA output current.

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  • \$\begingroup\$ These xbees are mostly hanging off laptops (literally, in many cases) so the 5V source is pretty stable, good point about not needing the source cap. I'll look into the IC as well - these 2950's are cheaper than candy and their long legs make it easier to finagle a fit into tight spaces and solder directly to suitable PCB traces. I used tantalum caps which are compact and robust enough to jam into place. \$\endgroup\$ – Ron J. Jun 22 '13 at 1:03

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