There are many circuit simulators available which are based on the original SPICE (Simulation Program with Integrated Circuit Emphasis) code developed at Berkley university at the end of ’60. Linear Technology developed their own version called LTSPICE IV very useful when you need to simulate circuits which use components made by LT. LTSPICE IV can be downloaded free of charge from http://ltspice.linear.com/software/LTspiceIV.exe. Let’s see how is possible to design a very fast overvoltage, undervoltage and reverse protection using few components (the pictures have been extracted from the official datasheet available at http://cds.linear.com/docs/Datasheet/4365f.pdf and from LTSPICE IV available at http://www.linear.com/designtools/software/#LTspice):

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