Using SMSTP(x, r) for Smoother and Faster Simulations in Allegro PSpice

Hello everyone,

If you're using traditional step functions (like STP or Heaviside) in PSpice, you’ve probably seen issues such as sharp discontinuities, tiny timesteps, slow simulations, and convergence problems.

To overcome these limitations, Allegro PSpice introduces the SMSTP(x, r) function, which is defined as:

SMSTP(x,r)=0.5+0.5⋅tanh(r⋅x)

This gives a smooth transition from 0 → 1, and the parameter r lets you control the steepness.
Higher r = sharper but still smooth transition.

Advantages of SMSTP in Allegro PSpice:

• Simulator-Friendly: It reduces the need for tiny timesteps, thus speeding up simulations.
• Improved Convergence: Smooth transitions help solvers reach solutions more reliably.
• Tunable Behavior: The r parameter controls the steepness and domain of the transition, as shown below:
  • r = 2 transition over domain > [-1, 1]
  • r = 4 domain > [-0.5, 0.5]
  • Higher r values > sharper transitions within narrower domains

Hope this helps anyone working on switching or behavioral models! If needed, I can share example plots or circuit snippets.

Happy Learning!!