Scroll Bar - Simple but need help

I have dynamic text coming into a window. I have a scrollBar attached to it. Just like yahoo chat, or msn chat, or any messenger chat, the scrollBar goes DOWN with the text, so you don’t have to scroll down, it automatically scrolls down as someone types. With default scrollBar in AS3, everytime new text comes in, the scrollBar moves up, so in order to see the text you would need to grab the bar and pull it down. Anyone know a simple fix?