PIC32 Microstick II Basic Question - Backward current

[ez24]

Hi
I want to plug in my Microstick II but :

Quotes from this site: http://wiki69.pst69.homeip.net/index.php/PIC32_Tutorial

It is a good idea for more advanced experiments to have a 3.3 V source at your hands. If you are interested in using the USB communications as shown in the experiments later on, then a combined (USB & Power) breakout board is an ideal companion. But remember not to power the Microstick from this source as the onboard regulator is not certified for backward current (there is indeed no documentation available).

How can backward current occur?  Other than something stupid like adding 5 volts to the PIC ?   I do not understand this paragraph?

thanks

[Bruce Abbott]

Backward current can occur if the regulator's output voltage is higher than its input voltage. This situation exists when the regulator is not powered through USB and you supply an external voltage to the PIC.

However this is only a problem if something draws high current from the regulator's input to ground. The Microstick II has a diode which protects the regulator from shorts or reverse polarity on the USB connector. The only other components on the input are a 200k resistor and 4.7uF capacitor. If you apply external power then a short surge of reverse current will pass through the regulator as the capacitor charges, but this probably won't hurt it.