Need help to backup cellphone - I need output power 5V 1A from AA batteries
The folks over at adafruit has a great tutorial on how to do just this 

https://learn.adafruit.com/minty-boost

It is tried and tested on both android and iphone. 

Using a linear regulator is not practical due to power wasted. A boost regulator is what is going to give you the max efficiency from the batteries.