Top-down design in MATLAB® and Simulink® is about speed of design and speed of simulation. Using circuit simulation, you can iterate on design alternatives to look for the best solution, but these rapid change-and-evaluate cycles are time-consuming when simulated in low-level SPICE models. Top-down design in Simulink uses behavioral modeling to achieve significant speed compared to SPICE modeling.
Loop simulation in SPICE tools can be slow. By contrast, Simulink's built-in solvers are designed and proven to simulate feedback loops efficiently and accurately, resulting in faster simulation times for mixed-signal devices such as ADCs, PLLs, and SERDES. Top-down design in Simulink not only speeds simulation, its speed makes it possible for the user to evaluate design alternatives for complex systems.
Top-down design in Simulink includes behavioral circuit simulation using SimPowerSystems™. SimPowerSystems lets you model RLC elements, op-amps, and switches. Electronic circuit models created with SimPowerSystems simulate faster than lower-level SPICE models.
The ability to combine both design abstractions and circuit-level elements in the same Simulink model enables you to reuse behavioral models as test harnesses for SPICE models or for prototype devices.