Hi, I really loce those small boards, but one question is arising. Is it possible to power the board through the 3.3v pin? That would mean backfeeding the LDO. Is that a problem? There is no usb connected and no power on the 5v pin
Thanks, markus
If nothing is connected to the USB connector, battery, or 5V pin (no input voltage supplied to the LDO), it would be possible to supply 3.3V to the 3V3 pin. However, the 3.3V must be constant voltage.
I thought the 5v is connected to the usb 5v source… i found this out by shorting or overcurrenting it… or was that in a dream…
I thought the 5v is connected to the usb 5v source…
The following link provides a summary of the power supply circuit. For your reference!
not to argue, but doesnt it show the usb and 5v pins are directly connected?
In the schematic on the Wiki, VUSB is output on the 5V pin.
Hi, thank you for the answer. I asked in a German forum as well and more or less the same answer, with a lot of additional discussion why:
https://www.mikrocontroller.net/topic/568055?goto=7673395#7673395
Yes a very good forum, You see the caveat in the end"if your pants are full" loose translation on that if you do Hook it up this way,
Here is the Main Jists and I take NO credit for the findings.
Jens G. wrote:
> The manufacturers usually do not provide any details because this
mode of operation is not usually the norm and therefore the manufacturers
save themselves the effort of
specifying this in detail with all possible circumstances.
This is available on request if you buy enough of the components.
Basically, the operating case is uncritical if you consider a few points
noted:
* for bipolar voltage regulators (eg 7812) the output voltage
not be more than 6 V above the input voltage. Otherwise the
Output transistor. In the data sheets, a diode is usually
Output to input recommended, which limits the difference to 0.7 V.
This only applies to regulators that have more than 6 V at the output
With 5 V or 3.3 V this is not an issue, a 7805 does not need
Diode.
* For LDOs with MOSFETs, no significant current may flow through the
Backgate diode. Every MOSFET acts backwards as a diode, so does
the internal one. That means that nothing should be connected to the input that
draws current permanently. An electrolytic capacitor is OK, a complete circuit on the
next higher voltage level.
* The reset generator often looks at the input. That is, if the input is open
and output is externally powered, the reset pin is often activated. This disturbs
when flashing.
Of course, this only applies to LDOs with a reset pin or watchdog.
GL 
PJ
