Electronic devices are being used today to perform a wide variety of tasks. Many different areas of business, industry, government, education, entertainment, and most recently, the home, are tapping into the enormous and rapidly growing list of applications developed for today's increasingly powerful electronic devices. Such devices include, for example, handheld electronic devices such as mobile instrument equipment, portable computers, portable medical electronics devices, and fixed electronic devices such as machine tool controllers, large server computer systems, and robotic servo mechanisms.
As electronic devices and machines controlled by electronics become increasingly ubiquitous and widespread in their use, there is increasing interest in improving the performance and the functionality of the electronics. For example, increasing the performance and software execution speed of computer system devices is of great interest.
There are many methods used by designers to increase the functionality of electronic devices. For example, with digital computer systems, software execution speed is increased by increasing the processor “clock speed.” Another method used by designers, with both digital electronics and analog electronics, is to increase the density of the electrical components within integrated circuit dies. For example, many high-performance integrated circuit processors include tens of millions of transistors integrated into a single die (e.g., 60 million transistors or more). As density increases, the operating speeds possible within a given design also increase, for example, as circuit traces are packed ever more closely together. Another method for increasing performance is to increase the efficiency of heat removal from a high-density high-performance integrated circuit. As component density increases, the thermal energy that must be dissipated per unit area of silicon also tends to increase. To maintain high performance, stable operating temperature must maintained. Accordingly, the use of carefully designed heat dissipation devices (e.g., heat sink fans, liquid cooling, heat spreaders, etc.) with high-performance processors has become relatively standardized.
Thus, performance enhancing techniques such as increased component density, increased clock speed, and increased heat dissipation are carefully balanced in order to obtain an optimum performance level. Over heating leads to erratic functional behavior of the device, such as, for example, computational errors, unpredictable behavior, or even physical destruction of the device. As more and more functions are integrated into smaller and smaller semiconductor dies, the operating speeds can be increased, however, the resulting increased switching activity leads to greater heat generation. Additionally, circuits having a high degree of integration are generally much more sensitive to thermal overloads and are more easily damaged by excessive heat. Such circuits tend to the specifically designed to function with very small operating currents, thus, current spikes related to thermal transients can easily damage them.
To protect such sensitive circuits from damage, is desirable to provide thermal protection which functions to shut down, or otherwise limit, current to a given electronic device when the device operates outside of safe thermal limits. Such thermal protection circuitry needs to be tailored to the operating conditions of the overall incorporating electronic device. In the case of a low power devices, the thermal protection circuitry needs to function properly while consuming low power. For example, in some electronic systems, like low operating current regulators, there is a need for a thermal shutdown circuit that will operate at about 1 microamp.
Generally, a thermal shutdown circuit for an integrated circuit device acts like a thermometer in that it senses a die temperature and shuts down the normal function of the device when the temperature exceeds a given threshold, or shutdown temperature (e.g., approximately 180 deg C.). Since the operating current of the integrated circuit device is much reduced during thermal shutdown, there is no chance that internal dissipation will significantly raise the operating temperature of the integrated circuit device during the shutdown. Thus, in the case of a regulator integrated circuit device, effective thermal protection circuitry makes the device practically “blowout” proof.
There exists problems with prior art thermal shutdown circuits. Prior art thermal shutdown circuits often employ large resistors in their design. The large resistors lead to correspondingly large back gates, or “tubs”, which in turn produce a corresponding back gate leakage current, or tub leakage current. Since the tub leakage current doubles with every 10 degrees centigrade, the size of the resistor needs to be quite large (e.g., 9M) in order to compensate for the change in resistance caused by the tub leakage current. Additionally, the changing tub leakage current causes the shutdown voltage to change as temperature changes, making the actuation of the shut down less accurate. Additionally, prior art thermal shutdown circuits often required circuit elements, such as zener diodes, that limit the voltages the device can process.
Thus, what is required is a thermal shutdown circuit for integrated circuit devices that is dependent on device temperature. What is require is a circuit that is independent of the tub/substrate diode leakage currents. Additionally, the required solution should not required excessively large resistors and should not require a large operating current. The present invention provides a novel solution to these requirements.