Thermodynamics - internal energy

What would the curve that describes the change of internal energy ($U$) with the volume ($V$) be like if the change in the temperature is negative?

My try:

For a ideal monoatomic gas, we have $U=(3/2)NkT$, where $N$=number of particles and $k$=boltzmann constant. So $$U=(3/2)pV$$

M question is: If the pressure changes, how is it possible to describe $U$ as a function of $V$?