Using Piecewise functions for LTSpice behavioural voltages
You can use this page as a reference for the behavioral sources: http://ltwiki.org/?title=B_sources_%28complete_reference%29
Anyway, you can set up your equation like this:
V=(EQ1)*(V(test)&gt;X) + (EQ2)*(V(test)&lt;=X)
Where:
EQ1 - First equation
EQ2 - Second equation
test - Node from which you determine the testing range
X - the value you want to split the equations at
The '&gt;' and '&lt;=' parts of the function will return 1 if true, and 0 if false.