Compact Thermal Sensors for Dense CPU Thermal Monitoring and Regulation: A Review

Nearly all integrated circuits contain power and thermal management circuits. These two functions are intimately connected and help improve the power performance of the system. In recent generation computers, the functionality and complexity of the thermal sensing and management circuits have accelerated in order to improve the performance. Integrated thermal sensors are utilized to monitor the peak temperatures of the CPU's to ensure that they do not exceed the reliability limits. There are many hot-spots on the CPU which have to be monitored, and these locations are not necessarily obvious during the design stage. As a result, it is required that there be multiple sensors on the CPU to enable accurate coverage. Thermal sensors are needed for cold-spot monitoring as well, in order to determine the required V
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level during low voltage, low power modes. The sensors are also used for fan regulation and power estimation. There can be many tens of sensors in a product, which require them to be small and low power. A variety of mechanisms have been utilized to form the sensing element. These include the parasitic PNP transistor, MOS transistors, resistors, as well as the thermal diffusivity of Si. In the past ten years, a trend towards miniaturization of thermal sensors has occurred. This review paper surveys the recent miniaturized integrated sensors which have been reported in the literature. The design tradeoffs for the CPU application are discussed, as well as the relative advantages of the different sensing mechanisms and their performances.