LTspice/SwitcherCAD III is a complete and fully functional SPICE program (electronic circuit simulator) that is available free of charge from the Linear Technology Corporation (LTC). Because of its superior performance, excellent community support and ease of file sharing, it is rapidly replacing all other SPICE programs (regardless of price) as the simulator of choice for hobbyists, students and professionals alike.
The purpose of this topic is to explore and explain some of the many useful or quirky features that have never appeared in the standard documentation whether due to simple oversight, the feature being considered not important enough, not polished enough or functionally obsolete – or even due to the feature being considered proprietary to another brand of SPICE or to LTspice itself. LTC considers some of these undocumented features as fair game for open discussion in public forums such as the LTspice Yahoo users group, whereas for others, it considers any such open discussions as a violation of its License Agreement.
"Fair game" is any feature that is or has ever been part of the normal distribution, i.e., appears or ever has appeared in the Help file, as plain text in any of the included sample or example files, in any program menu available during normal use of the program, or in any of the materials, presentation files or handouts from any LTspice seminar presentation. Such items are all considered as having been officially "documented" and are specifically allowed as discussion topics in public forums such as the LTspice Yahoo users group. However, be advised that any items that have been dropped from the documentation, even if still functional, should generally be considered obsolete and in risk of being purged from the program code at any time (fortunately, such items are quite rare).
As to the classification of anything not covered above, you must make your own common sense judgment or ask the advice of the users group moderator or the program author via private email. Clearly any standard, generic SPICE feature that works in LTspice would be okay for general use and discussion regardless of its state of documentation in LTspice. A lot of the standard devices have undocumented parameters (e.g., tempcos) or syntax (e.g., Pspice specific compatibility) that would fall into this category. Just as clearly, any undocumented A-device that is specific to LTC’s encrypted, high performance SMPS IC models would likely be considered proprietary knowledge to be protected with due diligence from release to the public domain, lest LTC’s competitors gain the de facto permission to freely copy them in their own circuit simulator offerings (however, it is difficult to see how LTC could legitimately prevent private individuals from making use of such undocumented features in their own simulations or discussing them via private communications). For these reasons, this last category of undocumented features will not be directly discussed here.