Do we need another SPICE?
Analog circuit simulation has been inseparable from analog IC design. SPICE simulators are the only way to check circuitry prior to integration onto a chip. Further, the SPICE simulation allows measurements of currents and voltages that are virtually impossible to do any other way. The success of these analog circuit simulators has made circuit simulation spread to board level circuit design. It is easier in many cases to simulate rather than breadboard, and the ability to analyze the circuit in the simulation for performance and problems speeds the design of well-understood, robust circuits.
Given the number of commercially available SPICE simulators why should a new simulator be written? Because certain analog functions are extremely difficult to simulate with commercially available SPICE simulators. Switch-mode power supplies have fast high frequency switching square waves as well as slow overall loop response. This means simulations must run for thousands to hundreds of thousands of cycles in order to see the overall response of a switching regulator. Commercially available SPICE's simply take too long for this to be a useful simulation method. Simulation times for a switch-mode power supply must be in minutes not hours for a simulator to be useful.
There have been analog circuit simulation methods that have shown some success in speeding up switch mode power supply simulation but at a cost of making simplifying assumptions which don't allow arbitrary control logic and fully simulate the complexity of the switching waveforms. A new SPICE with integrated logic primitives that perform the switch mode control provides a better answer. It can give fast simulation times, yield detailed waveforms, and still allows the flexibility for arbitrary circuit modifications.
LTspice is a new SPICE that was developed to simulate analog circuits fast enough to make simulation of complex SMPS systems interactive. Incorporated into the new SPICE are circuit elements to model practical board level components. Capacitors and inductors can be modeled with series resistance and other parasitic aspects of their behavior without using sub-circuits or internal nodes. Also, a simulation circuit element was developed for power MOSFET's that accurately exhibits their usual gate charge behavior without using sub-circuits or internal nodes. Reducing the number of nodes the simulator needs to solve significantly reduces the computation required for a given simulation without compromising the accuracy or detail of the switching waveforms. Another benefit of these new simulation devices is that convergence problems are easier to avoid since they, like the board level component the model, have finite impedance at all frequencies.
Modern switch mode power supplies include controller logic with multiple modes of operation. For example, devices may change from pulse switch modulation to burst-mode or to cycle skipping depending on the circuit's operation. An original new mixed-mode compiler and simulator were written into LTspice that allows these products to be realistically modeled in a computationally fast manner.
But despite LTspice's close association with SMPS design, it not a SMPS-specific SPICE but simply a SPICE program fast enough to simulate a SMPS interactively.
There are currently approximately fifteen hundred Linear Technology products modeled in LTspice. The program is freely downloadable from the Linear Technology website and is a high-performance, general-purpose SPICE simulator. Included are demonstration files that allow you to watch step-load response, start-up and transient behavior on a cycle-by-cycle basis. Included with the SPICE is a full-featured schematic entry program for entering new circuits.