LTspice IV, from Linear Technology, is a high performance Spice III simulator, schematic capture and waveform viewer with enhancements and models for easing the simulation of switching regulators. Enhancements to Spice have made simulating switching regulators extremely fast compared to normal Spice simulators, allowing the user to view waveforms for most switching regulators in just a few minutes. LTspice IV features Macro Models for 80% of Linear Technology’s switching regulators, over 200 op amp models, as well as resistors, transistors and MOSFET models.
LTspice IV features multi-threaded solvers designed to better utilize current multicore processors. Also included are new SPARSE matrix solvers that deploy assembly language in order to approach the theoretical flop limit of current FPUs (floating point units). LTspice IV speeds up the simulation speed of medium- to large-sized circuits by a factor of three on a quad core. According to Linear Technology, LTspice IV outperforms other commercially available SPICE programs.
LTspice IV implements proprietary methods that efficiently parallelize tasks that would require as little as 5us to run single-threaded. LTspice IV also includes integrated schematic capture and waveform viewing. While it has been associated with switch-mode power design (it ships with over 1000 models of switch-mode regulators and controllers), LTspice IV is not an SMPS-specific SPICE program but rather a general-purpose SPICE that includes new spice components so it is fast enough to interactively simulate SMPSs. LTspice IV is not artificially limited by component or node count.
More info: Linear Technology LTspice IV