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Where the sensitive circuit is not in any case subjected to excessively high supply voltages, a means of disconnecting the supply shall be provided. One way to do this is to blow the fuse with a thyristor. A less destructive alternative is to use a MOSFET to disconnect the power supply. The surge protection IC, LTC1696 of Linear Technology (www.Linen-tech.com), became available recently and is suitable for activating and controlling such a device. It operates from 2.7V to 27V mains and can be connected to the unregulated input of a voltage regulator. The two voltages can be monitored using feedback pins FB1 and FB2, appropriately divided using potential dividers.
The trigger threshold for FB1 and FB2 is +0.88 V. The value of the upper resistor in the potential divider can be calculated using the following formula: R1 = 33 kÎ©× [(VLIMIT – 0.88V)/ 0.88V] The value of the capacitor connected to the TIMER/RESET pin will set the delay before the protection interrupts. The charge current of this capacitor depends non-linearly on how much the voltage exceeds the threshold value. The larger the overvoltage, the faster the integrated circuit activates. Once triggered, the IC remains in this state until either the input voltage is removed or the internal latch is cleared using a MOSFET connected to the TIMER/RESET input.