GPIO 5V disables USB

[browntastic]

I'm running RetroPie 2.6.0 on a Raspberry Pi B and am having a problem with the GPIO and USB.

Everything was working fine until I attached a 7" TFT screen http://www.ebay.ca/itm/VGA-AV-Lcd-contr ... 53fce7c97e and powered it through the GPIO. I'm using a 12V step up converter https://www.creatroninc.com/product/12v ... -u3v50f12/ connected to 5V and Ground (I have tried using different 5V and Ground pins).

When starting up the Pi, I have used a connected keyboard to type in the command line until the Emulation Station screen comes up. After this, anything connected (keyboard, USB controller) is unresponsive. When the screen is powered by another source the keyboard or controller works fine.

Since the keyboard works for a little while I don't think it's a power issue. Could it be that a config file is being ignored for some reason?

Thanks for any help you can give to solve this problem. I was very happy to have finally figured out how to power the screen, and this is a real bummer.

[klricks]

It's most definitely a power issue. A model B will only allow 750mA total. If you attempt to draw more than that then the RPi and especially the USB devices will become unstable. You need to use a dual outlet 5V power supply OR 'Y' cable.
Other option is to get a dual 12V and 5V supply and lose the step up converter. http://www.ebay.com/itm/Hard-Disk-Exter ... 48797f761e
OR
Use a single 12V supply and add a 5V switching 'buck' regulator for the RPi. It's much easier to step down DC voltage than it is to step up.

[browntastic]

I guess it was just wishful thinking that it wasn't a power issue. Two questions for you:

Would the B+ supply more current?

What do you mean when you say it's easier to buck rather than boost?