Overvoltage Protection

When a sensitive circuit
must under no circumstances have too high a supply voltage applied,
then some means of disconnecting the supply must be provided. One way to
achieve this is to trigger a thyristor to blow a fuse. A less
destructive alternative possibility is to use a MOSFET to disconnect the supply. An over-voltage protection IC, the LTC1696 from Linear Technology (http://www.linear-tech.com),
has recently become available, which is suitable for triggering and
driving such a device. It operates from a power supply in the range 2.7 V
to 27 V and can be connected to the unregulated input of a voltage
regulator. Two voltages can be monitored using feedback pins FB1 and
FB2, suitably divided down using potential dividers.

The trigger threshold for both 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 kΩ× [(VLIMIT – 0.88 V)/0.88 V] The value of the capacitor connected to the TIMER/RESET
pin sets the delay before the protection is triggered. The charging
current for this capacitor depends non-linearly on the amount by which
the voltage exceeds the threshold value. The greater the over-voltage,
the faster the IC triggers. Once triggered the IC remains in that state
until either the input voltage is removed or the internal latch is
cleared using the MOSFET connected to the TIMER/RESET input.

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