Current VLSI (very large scale integrated circuit) chips implemented with submicron process technology have extremely small geometries and operate at low power supply voltages, such as 3 volts or less. Such VLSI chips are susceptible to electrical overstress applied to an external pad of the chip. For example, a voltage in excess of the rated voltage of transistors connected to an external pad may cause those transistors to fail. The electrical overstress can be applied to the chip at any point during its life, such as during testing or use. However, some configurations are more susceptible to electrical overstress than others. For example, chips connected to external devices or connectors are particularly susceptible to inadvertent application of an overvoltage. One specific example is a USB (universal serial bidirectional) communication port, which is in common usage on computer equipment.
Circuits are known that protect output drivers against overvoltage in the case where the power supply voltage is turned on. However, such circuits do not protect the output driver in cases where the power supply voltage is turned off, is at a low voltage, is open circuited or is connected to ground. Nonetheless, it is desirable to provide overvoltage protection under these conditions in order to prevent inadvertent damage to such circuits. The overvoltage may occur at any time and is not limited to periods when the power supply voltage is turned on. For example, some manufacturers may require the USB port to withstand an overvoltage of 5.25 volts, regardless of whether the power supply voltage is on or off.
Accordingly, there is a need for improved methods and apparatus for overvoltage protection of output drivers in integrated circuits.