Wide measurement range temperature sensitive IC

Digital "remotely placed" temperature sensors, using an external bipolar transistor as the sensing element, have been relied upon for many years to detect the temperature of high-speed, high-performance chips such as microprocessors, graphics processors, and FPGAs . Accurate temperature monitoring is critical to ensuring optimal performance and preventing catastrophic failures. Temperature monitoring devices assist the system in controlling fans and implementing clock throttling functions to keep the operating temperature of high-performance ICs within the necessary range. At higher temperatures, it can be used to shut down the system to prevent failure. As performance and power levels increase, remote temperature monitoring functions become more and more important and more and more difficult to implement.

Almost all traditional digital temperature sensor ICs have an upper limit of less than 128 °C, and many sensors have a limit of less than 100 °C. In many cases, the limits of traditional sensors are sufficient, but sometimes it is necessary to measure temperatures up to 150 °C. In these cases, a temperature sensor with a wide measurement range is required.

A typical digital temperature sensor IC uses a sign bit and 8 size bits to represent temperature, with the lowest bit representing 1 °C and the highest bit representing 64 °C. The representation method with the highest bit being 64 °C limits the maximum measurable temperature to less than 128 °C.

A wide-range temperature sensor should be able to measure values ​​far beyond the 128 °C limit—often up to 150 °C. The most convenient way to achieve this is to use the highest bit with a weight of 128 °C, which extends the temperature range to 255 °C, far beyond the useful range. Due to the characteristics of the semiconductor junction that measures the temperature, the measurement accuracy decreases rapidly above about 150 °C .

For temperature monitoring of circuit boards, temperatures above 127 °C are unlikely to be encountered. However, temperature monitoring of high-power chips such as microprocessors, graphics processors or FPGAs requires a wider measurement range.

These high-power ICs operate at temperatures above the normal temperature range, so their monitoring requires sensors with a wider temperature range. The maximum operating temperature is a function of clock speed, process, device packaging, and various design factors. Signal integrity often degrades as temperature increases, until the circuit no longer guarantees its performance specifications. In many CPUs and graphics processors, this problem occurs at about 100 °C, but in some high-performance circuits, the normal operating temperature range extends to 145 °C. As the maximum absolute temperature of the chip approaches the upper limit of the operating range, it becomes more critical to monitor the temperature and shut down the system when necessary to avoid failure ( Figure 1) .

On some high-performance processors, the physical characteristics of the temperature measuring diode will cause the measured temperature value to deviate, in other words, the measured temperature will be much higher than the actual temperature. In this case, the temperature sensor needs to measure the apparent temperature far higher than the normal operating temperature range.

Remote temperature sensing principle

The most common way to measure temperature with a "remote diode" is to pass two different currents through the diode, usually in a ratio of about 10:1 . ( The diode is not a two-terminal device like a 1N4001 , it is actually a bipolar transistor connected in diode form, and the ideality factor of a two-terminal diode does not apply to remote diode temperature sensors . ) The voltage across the diode for each current level is measured, and the temperature is calculated according to the following equation:

In this formula, IH is the upper diode bias current, IL is the lower diode bias current, VH is the diode voltage generated by IH, VL is the diode voltage generated by IL , n is the ideality factor of the diode, k is the Boltzmann constant 1.38x10-23J/o(K) , T is the temperature expressed in oK , and q is the charge of an electron (1.60x10-19C) . If IH/IL=10 , the above formula can be simplified to VH- VL = 1.986 × 10-4 × nT .

The " n " term in the above equation is called the ideality factor, which is closely related to the process and is close to 1.0 for most transistors. A remote diode temperature sensor generates a precisely proportional current and measures the corresponding voltage, then scales and shifts the voltage measurement to produce temperature data. The internal analog-to-digital converter must be able to accurately measure small voltage signals as well as large common-mode values; a 1 °C temperature change is accompanied by a signal of about 200mV .

Although wide-range remote temperature sensors are new to the market, the need for them is unmistakable. As new system requirements emerge, system designers will see more ICs with wider temperature measurement ranges .