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):
This schematic is the one proposed by Linear Technolgy in LTC4365 datasheet and it is available inside LTSPICE IV by selecting “Edit”, “Component”, “LTC4365” and “Open this macromodel’s test fixture”. The components are set to have a OV protection threshold of 18V and an UV threshold of 3.5V:
The diagram shows relationship between Vin (blue) and Vout (green) on the y axis on a milliseconds scale (x axis full scale at 200ms). They are selected by adding a voltage probe at the input and at the output of the circuit once the simulation is run. As visible the protection response is very fast (about 10ms to drop the output to zero volt). The double mosfet has the function to protect the circuit in case of reverse voltage:
The LTC4365 has a recovery delay timer which introduce a delay of 36ms that filters noise at VIN and helps prevent chatter at VOUT: after either an OV or UV fault has occurred, the input supply must return to the desired operating voltage window for at least 36ms. The recovery timer is also active while the LTC4365 is powering up: the 36ms timer starts once VIN rises above UV threshold and VIN lies within the user selectable UV/OV power good window.
A voltage divider made by three resistors applied to two comparators set the UV and OV threshold as follow:
The data sheet gives details to calculate R1, R2 and R3 in order to set the UV and OV threshold as necessary.
A possible release of such protection could be the one we made for testing purposes: