A technique is proposed for increasing the computational efficiency of a mixed-mode FDTD ciruit simulator. The FDTD algorithm is coupled to a set of equations which describes both linear and non linear lumped elements. The simulation time step is adaptively adjusted by monitorng the convergence of the Newton-Raphson procedure adopted for the non-linear solution. This considerably speeds up the simulation, with respect to the adoption of a worst-case, fixed time step. The propagation of a high speed digital signal along a transmission lines is presented as an example. Simulation results are compared with SPICE outputs, showing excellent agreement.