An overvoltage protection circuit (10, 140) comprising: a reference voltage generator for providing a reference voltage that is constant during an overvoltage event; wherein the refence voltage generator includes a first filter circuit for filtering transients in a voltage signal on the first power supply terminal; a trigger circuit (44, 142, 144) coupled to receive the reference voltage and a first power supply voltage, the trigger circuit for comparing the reference voltage to the first power supply voltage, and in response to detecting that the first power supply voltage is above the reference voltage, the trigger circuit providing a trigger signal; a clamping device (32, 42) coupled between a first power supply terminal and a second power supply terminal; and a timer circuit coupled to the first filter circuit for maintaining the reference voltage for a predetermined time period, wherein the timer circuit comprises a resistive element having a first terminal coupled to the first power supply terminal, and a second terminal; and a capacitive element having a first plate electrode coupled to the second terminal of the resistive element, and a second plate electrode coupled to the second power supply terminal, characterized in that a transistor having a first current electrode is coupled to the first plate electrode of the capacitive element, a control electrode, and a second current electrode coupled to the second power supply terminal; and an edge detector having an input is coupled to receive a signal indicative of the overvoltage event, and an output coupled to the control electrode of the transistor, the edge detector for generating a pulse for starting the predetermined time period; and in that the trigger signal has a voltage proportional to a difference between the first power supply voltage and the reference voltage, and in that the clamping device is turned on in proportion to the voltage of the trigger signal to shunt current between the first and second power supply terminals. a reference voltage generator for providing a reference voltage that is constant during an overvoltage event; wherein the refence voltage generator includes a first filter circuit for filtering transients in a voltage signal on the first power supply terminal; a trigger circuit (44, 142, 144) coupled to receive the reference voltage and a first power supply voltage, the trigger circuit for comparing the reference voltage to the first power supply voltage, and in response to detecting that the first power supply voltage is above the reference voltage, the trigger circuit providing a trigger signal; a clamping device (32, 42) coupled between a first power supply terminal and a second power supply terminal; and a timer circuit coupled to the first filter circuit for maintaining the reference voltage for a predetermined time period, wherein the timer circuit comprises a resistive element having a first terminal coupled to the first power supply terminal, and a second terminal; and a capacitive element having a first plate electrode coupled to the second terminal of the resistive element, and a second plate electrode coupled to the second power supply terminal, characterized in that a transistor having a first current electrode is coupled to the first plate electrode of the capacitive element, a control electrode, and a second current electrode coupled to the second power supply terminal; and an edge detector having an input is coupled to receive a signal indicative of the overvoltage event, and an output coupled to the control electrode of the transistor, the edge detector for generating a pulse for starting the predetermined time period; and in that the trigger signal has a voltage proportional to a difference between the first power supply voltage and the reference voltage, and in that the clamping device is turned on in proportion to the voltage of the trigger signal to shunt current between the first and second power supply terminals.