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Temperature sensor applications in notebook computers

Published on Nov 04 2010 // Temperature Sensor Circuits


Temperature sensor applications in notebook computers1

 Temperature sensor applications in notebook computers

Temperature sensor applications in notebook computers3

 Temperature sensor applications in notebook computers2

Performance of continuous innovation as the computer, more and more functions are integrated into the computer. Therefore, increasing the capacity of the computer, the data includes multimedia data and 3D animation data. To meet the needs of a large number of data processing, more and more into the host chipset is the same time, CPU and chipset working frequency is also rising. More chipset and faster clock frequency means more heat generated.
For notebook computer users in addition to require a system with better performance, but in appearance, also called light, thin, small, this is another design challenge faced by staff. In the limited space, how to dissipate the heat generated by the system is a difficult problem. How to balance system performance, the system of comfort (including laptop computer case temperature, fan rotation noise), and system uptime, laptop computer design is an important issue.
Notebook computer components that need to monitor temperature
Figure 1 shows a typical system block diagram laptop, CPU heat source for the largest system, the current widespread use of laptop Intel Dothan processor power consumption is about its maximum moment of 37W, AMD Athlon processor, the moment of maximum power consumption of about 35W to 40W, Intel next-generation Merom processors will be up to the moment of maximum power consumption of 50W. CPU temperature detection is the computer important goal. Currently, both the Intel or AMD CPU, CPU internally to provide remote temperature detection with a diode to provide a temperature sensor to directly detect the temperature of CPU internal core, and its precise temperature control.

Graphics chip (GPU) is in addition to CPU, the system is another important source of heat. As higher-resolution LCD monitor, the graphics processing chip greatly increases the amount of data processing, graphics processing chip to make reliable, the current widespread use of the graphics processing chip, and CPU are the same, are included to provide remote temperature sensing diode, graphics processing chip for direct detection of the internal die temperature, and its temperature control. Laptop computers, another may need to temperature detection and control components also include DDR memory, hard drives and optical drives. Temperature measurement is to head to the embedded micro-controller laptop notebook computer can make the appropriate power management and thermal management. Accurate and reliable temperature sensing applications in the notebook computer has the following advantages:
One. Precise temperature detection allows the system to maximize performance: When the components of the actual temperature does not reach the critical point of system down-time, because the temperature sensor error, the system may be premature down-moves, which makes the system unable to maximize performance.
II. Accurate temperature detection can reduce system noise and extended computer battery: If the temperature sensor detection temperature is higher than the actual temperature of the system, will result in an early fan operation, or fan speed higher than the actual demand, which will cause the system to unnecessary fan noise and power consumption.
III. Accurate temperature detection can improve the system stability, increase the competitiveness of products: If the temperature sensors detect the temperature is lower than the actual temperature, actual temperature may be in the system has reached the critical point down the system frequency remained higher operating frequency, resulting in paralysis or even damage the system. In addition, accurate temperature sensing allows the system to use the minimum cooling modules, cooling modules that can reduce costs and increase product competitiveness.
The temperature sensor used laptop
Thermistor and integrated temperature sensor is commonly used in notebook computers of two temperature sensors, we will explore the following two temperature sensors and the use of works.
Thermistor
Thermistor resistance characteristics by changes in temperature can be divided into positive temperature coefficient thermistors, negative temperature coefficient thermistor and the critical temperature coefficient thermistor. Positive temperature coefficient thermistor and temperature coefficient of the critical characteristics of the thermistor resistance will change drastically in a given temperature, suitable for temperature measurement or limit set in a smaller temperature range. Negative temperature coefficient thermistor is mainly manganese oxide, cobalt oxide, nickel oxide, copper and aluminum composite metal oxides such as sintering, the metal oxide materials have semiconductor properties, when the temperature is low, within the semiconductor electron – hole small number of children, so resistance is higher. When the temperature increases, the thermal resistance of the electron – hole increase in the number of children, the conductivity increased, resistance decreased. Figure 2 shows a typical negative temperature coefficient thermistor characteristic curve, the relationship between resistance and temperature, type the following:

R0, R are the ambient temperature T0, T (K) the absolute temperature of the resistance. B is the thermistor constant, B a constant usually between 2500K to 5000K range.
Figure 3 shows a typical negative temperature coefficient thermistor of the application circuit. Use of laptop computers embedded microcontroller ADC (ADC) values calculated by the voltage read out the resistance of NTC, which calculate the ambient temperature. Use of negative temperature coefficient thermistor temperature measurement error of large sources of error, including NTC’s own errors, to enhance resistance of the error, bias power supply (VCC) of the error, ADC errors and errors caused by measurement noise. From cost considerations, if only to consider the negative temperature coefficient thermistor their prices, this is a cheap solution. However, if the ADC bias circuit and additional costs be taken into account, the cost may increase.

Integrated Temperature Sensor
Integrated temperature sensor is currently widely used laptop temperature sensor, with high accuracy, fast response, small size, low power consumption and easy control software interface. Figure 4 shows a typical block diagram of an integrated temperature sensor. The main mechanism of temperature sensing integrated temperature sensor inside the current source and ADC, integrated temperature sensor working principle is the use of semiconductor PN junction forward voltage drop at different temperatures have different voltage drop characteristics of the temperature measurement. By the semiconductor PN junction – voltage curve:

ICH, IDL units of mA, RP units of ohms. In this case, the stray resistance of 1 ohm will result 0.45oK temperature measurement error. If the IDH = 200uA, IDL = 20uA, the 1 ohm resistor will cause 0.9oK spurious temperature measurement error. RP’s size and remote-sensing diodes and wiring on printed circuit boards, printed circuit board layout to minimize the printed circuit board copper foil produced by the parasitic resistance. Typically, the remote diode measurement circuits may be caused by the parasitic resistance of up to 3 to 4 ohms. (IDH – IDL) in size and integrated temperature sensor is related to the different integrated temperature sensor has a different (IDH – IDL), the choice of the integrated temperature sensor, select a small (IDH – IDL) helps reduce the parasitic resistance caused by the temperature measurement error.
In addition, the results from the previous analysis that, 1mV voltage changes approximately equivalent to 5oK temperature changes, so the wiring on the printed circuit board temperature measurement accuracy greatly. General temperature sensor IC has a power input of the RC low-pass filter, to prevent high-frequency noise. Placed in the printed circuit board part time, RC filter should be placed as close to the temperature sensor near the IC power input pins. In addition, the temperature sensor IC should be placed near the location of temperature sensing diodes. For the differential (DXP, DXN) cabling to connect remote temperature diode parallel lines must be used, while the two parallel wiring to be close to each other, and as far as possible away from the magnetic components, high voltage signals, high-speed signals to avoid interference. Improper wiring may cause the printed circuit board temperature measurement error of more than 30oK.
Application
Figure 5 is a commonly used in laptop temperature control scheme. Through the SMBus interface, temperature sensor IC connected to the laptop’s embedded micro-controller, the temperature sensor IC with embedded micro-controller for the digital interface, so the temperature sensor IC in position away from the embedded microcontroller can not be noise interference problem. MAX6649 also built a local temperature sensor and the differential used to connect remote diode interfaces. MAX6649’s IDH = 100uA, IDL = 10uA, high precision, low current due to stray current source can reduce the resistance of the resulting measurement error. Differential input helps to reduce the noise. Figure 5 shows the circuit, temperature sensor IC is responsible only for temperature sensing, fan speed control by embedded micro-controller to complete, by the software. To avoid the problem of software control of the crash, MAX6649 also integrates protection, high temperature when the temperature reaches the first critical point, MAX6649 ALERT may issue an interrupt request to the embedded micro-controller, the appropriate treatment, such as lowering the processor frequency; If these measures still can not effectively control the temperature rise in temperature when the temperature reaches the second critical point, MAX6649 OVERT can be used to control the second fan or a forced shutdown of the system. Figure 5, low cost, high accuracy, the use of elasticity, etc., but the design of the software takes more effort.