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Maxim > Design Support > Technical Documents > Application Notes > Temperature Sensors and Thermal Management > APP 3178
Keywords: extended-range, temperature-sensing, ics, temp sensors, applications
APPLICATION NOTE 3178
Extended-Range Temperature-Sensing ICs
Apr 27, 2004
Abstract: Using remote temperature sensors to monitor die temperature of high-performance microprocessors
and graphics processors is a common technique for managing power and thermal operating characteristics. As
clock speeds, circuit densities, and power levels increase, die operating temperatures can exceed the limits of
conventional temperature sensors. New temperature sensors from Maxim can measure temperatures
previously beyond the range of such devices. This article discusses extended-range temperature sensors,
their theory, and applications.
Digital, remote temperature sensors that use an external bipolar transistor as the sensing element are widely
relied on for monitoring die temperature on high-speed, high-performance ICs like microprocessors, graphics
processors, and FPGAs. Monitoring temperature accurately is critically important for ensuring best
performance and protecting against catastrophic failure. A temperature monitor lets the system perform fan
control and clock-throttling functions to keep high-performance ICs within the necessary operating temperature
ranges. At higher temperatures, it can be used to shut the system down to prevent failure. As performance
and power levels increase, the remote temperature-monitoring function becomes even more important, yet is
more difficult to perform.
Virtually all conventional digital temperature-sensor ICs have upper measurement limits of less than 128°C;
many are limited to 100°C. Frequently, a conventional temperature range is sufficient. However, there are
times when it is important to measure temperatures as high as 150°C. In such cases, extended-range
temperature sensors are necessary.
Sensing Extended Temperatures
The typical digital temperature-sensor IC represents temperature data using one sign bit and seven magnitude
bits with an LSB of 1°C and an MSB of 64°C. Although some digital sensors have a few additional bits to
express temperature with more resolution, the 64°C MSB limits the highest measured temperature to less than
128°C. An extended-range temperature sensor can measure values well above this 128°C limit—often as high
as 150°C. The most convenient way of performing this task is by providing an MSB with a weight of 128°C. In
this case, the temperature data range extends to 255°C, which is well beyond the useful range as it is unlikely
that temperatures over 127°C will be encountered. Accuracy degrades rapidly at temperatures above
approximately 150°C because of the limitations in semiconductor junctions used to measure temperature.
The maximum operating temperature for some highpower ICs depends on clock speed, process, device
package, and various design factors. Often, signal integrity degrades with increasing temperature until the
circuit no longer meets its specifications. In many CPUs and graphics processors, this happens around 100°C;
but in some high-performance circuits, normal operation can extend to 145°C. If the device can function
properly at an extended temperature, accurately measuring the temperature is important for keeping it within
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