The screen is a 5" car reversing camera screen which takes the composite video feed from the Rpi It's meant to be powered by the 12V from a car battery, but actually has a switching regulator which converts this down to 3.3V. Unfortunately this built-in regulator didn't work too well with a 5V input voltage, so I had to bypass it and use another 5v to 3.3v regulator.

Originally I used an inefficient linear regulator to do this, but then found it got very hot due to the power it was dissipating (even with a heatsink), and replaced it with a better switching regulator.