Full Spice models of these types of chips are generally not available, you use behavioral models, if you can't find a model provided by the maker you'll have to make one yourself.
LTSpice was designed to support Linear Technologies full line of switch mode supplies perhaps you can find and equivalent model?
For what reason do you need full simulation of the chip?
The LTSpice people will certainly not help you with this!
I have built functional models that are pretty close to TI's parts. It will take some work.
There are functions for X^2, Compare, amplifiers, I would loose CT and RSET and replace that with a saw tooth generator. You will learn a large amount about this part and spice if you make a model.
For anyone stumbling across this thread years later looking for this component, you can try using the attached subckt. It's a 2854 instead of the 3854, but they are similar in many ways.