The answer depends on how much power you are using off each of the rails that go to the expansion port. Here is a list of the available rails, how much they can supply, and how much power you can probably expect from the rail:
5V_MAIN This rail is limited by the 3A fuse. It will normally source ~ 1.0A minus whatever power is supplied to attached USB devices. The remaining power is used by the PMIC/regulator. If you need more power, you can install a higher current fuse, and use a higher rated power supply. I would not exceed 5A to the board.
LCD_3V2 This rail is supplied by a 1A regulator. The regulator supplies many things including Audio, Ethernet, SD3 card slot. The major load is Ethernet. You can probably get 200 - 500 mA out of this rail, but if you are looking for a consistent load > 200 mA, you may want to generate it yourself from 5V
VIOHI_2V775 This rail supplies 150 mA of current. ~100 mA is availabe for use on the Expansion Port.
VLDO8_1V8 This rail supplies 200 mA of current. ~ 150 mA is available for use on the Expansion Port.
VLDO9_1V5 This rail supplies 100 mA of current, and all of it is available on for use on the Expansion Port. In addition, this rail can have it's voltge adjusted between 1.25V - 3.6V since the rail is not used anywhere else on the Quick Start board.
VLCD_BLT This is a LED backlight rail only and will supply ~ 20 mA
There are no guarantees that you will get the above listed power out of each rail. If you need power close to the upper limits discussed above, you are best off to default to an LDO powered by 5V_MAIN.
