Travel adapter 220V to 110V

I'm traveling to Mongolia with a 3GS and need to know if all I need is a simple plug adapter or whether I need a plug adapter and step-down voltage converter for my iPhone. I understand the concept of a plug adapter simply allowing a different plug type to access the same voltage (220V in this case), but I worry that the charger and iPhone won't be able to handle the 220V when its built for 110V power here in the US.

Any thoughts?