Control failure by sensing the heat

Controlling the amount of heat produced or dissipated by today's more powerful electronic equipment becomes critical as the real estate shrinks enabling heat related problems to occur more easily. It can be a tricky challenge to resolve but several companies are offering possible solutions.

In the PC, for example, the temperature sensor is being used to monitor the central processing unit (CPU), the graphical processing unit (GPU), hard disk drive (HDD), memory chips, DVD circuits, and ambient temperature within the system. “The high temperatures can adversely impact performance and reliability, observed Carl Liepold, Chief Technology Officer at Andigilog. “Typically, the CPU will have an onboard diode that may be connected as a remote sensor to the system temperature monitoring IC. This is becoming more critical as CPU and graphics chips continue to increase clock speeds,” added Mr. Liepold.

Andigilog offers the aTS75, a 2-wire digital temperature with a thermal alarm. This interfaces to the PC's serial System Management Bus (SMB). Using a standard protocol, the IC may be programmed to report when the sensor temperature has gone above or below a set point. The temperature is reported via the SMB with 9- to 12-bit resolution and a single-pin output will provide the alarm CPU interrupt or may be used to turn on a fan by using an external MOSFET driver. The alarm can operate in two modes — interrupt mode and comparator mode, which allows flexibility for many types of applications. The aTS75 is typically accurate to +/-2C over the full temperature range of -40C to 125C and to +/-1C over the range of 0C to 100C.

If you need a sensor for a space constrained application then the combined features of accuracy, small size, and low power, make the ADT7301 from Analog Devices ideal for you. It meets the needs for medical equipment, automotive, cell phones, hard disk drives, electronic test equipment, thermostat controls, portable battery management and even process control. “Given that there were over 150 million PCs shipped this year the applications for thermal management controllers are very broad,” said Paul Errico, Product Marketing Manager for Temperature and System Management at Analog Devices.

However, before you go out and buy a sensor understand that there are several factors that affect the use of your sensor design. “The measurement method, interface standard, number of zones, fan drive technique, layout requirements and platform are all variables that must be solved with application specific solutions to address the performance and cost,” said Mr. Errico. “All PC segments need temperature sensors to monitor zone temperature for the CPU, graphics processor, ASICs, voltage regulation module, memory zone, hard drive zone, PCMCIA zone, ambient zone, fan blades, power supply zones (both silver box and regulation) and fan tray zones,” he said. ADI offers several sensors that support these applications including the ADT7460, ADT7463, ADT7467, ADT7468, ADT7466, ADM1034, ADM1026, ADT7461, and the TMP05.

For example, the ADT7461 is a digital temperature sensor with capabilities important for next-generation processors. The default temperature measurement range is 0 to 127 degrees C. It cancels thermal measurement inaccuracies caused by resistance in series with the thermal diode sensor by automatically switching current sources, comparing the resulting output voltages, and calculating the corrected temperature. Additional on-chip signal conditioning eliminates the effects of noise in the sensor circuits. The ADT7461 is well suited for a range of applications including desktop and notebook computers, industrial controls, automotive, instrumentation, and embedded systems.

ADI's ADT7460 and ADT7463 have a local temperature sensor channel and two remote temperature channels that may be connected to an on-chip diode-connected transistor on a CPU. These three temperature channels may be used as the basis for automatic fan speed control to drive fans using pulse width modulation (PWM). In general, the greater the number of fans in a system, the better the cooling, but this is to the detriment of system acoustics. Automatic fan speed control reduces acoustic noise by optimizing fan speed according to measured temperature. Reducing fan speed can also decrease system current consumption. The thermal validation of the system is one of the most important steps of the design process, so these values should be carefully selected. The ADT7460 and ADT7463 allow the speed of four fans to be monitored. Each temperature channel has a thermal calibration block. This allows the designer to individually configure the thermal characteristics of each temperature channel. For example, you may decide to run the CPU fan when CPU temperature increases above 60 degrees C, and a chassis fan when the local temperature increases above 40 degrees C. Note that the designer has individual control over parameters such as minimum PWM duty cycle, fan speed failure thresholds, and even ramp control of the PWM outputs. This ultimately allows graceful fan speed changes that are less perceptible to the system user. After the system hardware configuration is determined, the fans can be assigned to particular temperature channels. Not only can fans be assigned to individual channels, but the behavior of fans is also configurable.

Take a look at the National Semiconductor LM93 hardware monitor; it has a two wire digital interface compatible with SMBus 2.0. Using an 8-bit Sigma Delta A/D converter, the LM93 measures the temperature of two remote diode connected transistors as well as its own die, and 16 power supply voltages. To set fan speed, the LM93 has two PWM outputs that are each controlled by up to four temperature zones. The fan-control algorithm is lookup table based. The LM93 includes a digital filter that can be invoked to smooth temperature readings for better control of fan speed. The LM93 has four tachometer inputs to measure fan speed. Limit and status registers for all measured values are included. “For processor monitoring, most of our customer are requesting +/-1?C accuracy,” said Zaryab Hamavand, Technical Marketing Manager for the Data Conversion Systems Group at National Semiconductor. “For the GPU monitoring, it varies depending on the vendors, some requesting +/-1?C accuracy and some +/-3?C. For the ambient monitoring they request about +/-2?C accuracy,” added Mr. Hamavand.

Additionally, National's hardware monitoring product manages voltages and fans for uses other than the temperature of the CPU and the motherboard. The company has a series of digital temperature sensors that monitors ambient temperature for different devices. Recently National Semi introduced a series of new devices from the SensorPath family, targeted for PC market. The LM95010 is a digital output temperature sensor that has single-wire interface compatible with the SensorPath interface. It uses an analog temperature sensing technique that generates a differential voltage that is proportional to temperature. This voltage is digitized using a Sigma-Delta analog-to-digital converter. The LM95010 is part of a hardware monitor system, comprised of two parts: the PC system health controller, such as a Super I/O unit, and up to seven slaves of which four can be LM95010 sensors. Using SensorPath, the LM95010 is controlled by the master and reports its own die temperature to the master. SensorPath data also is pulse width encoded, thereby allowing the LM95010 to be easily connected to many general purpose micro-controllers.

Maxim Integrated Products also offers sensor products for your designs. It provides the MAX6642, a two-channel, digital temperature sensor that accurately measures the temperature of a remote PN junction within +/-1C and its own die within +/-2C. It features a single over-temperature alarm and operates through an SMBus-compatible, two-wire interface. It comes in eight factory-preset versions, occupying different addresses on the SMBus to prevent conflicts in multiprocessor-based systems. The device operates over a temperature range from -55C to +150C with performance optimized from +60C to +100C.

The company's MAX6646 and MAX6647, two-channel digital temperature sensors, also measure the temperature of a remote thermal diode within +/-1C and its own die to within +/-2C. Both temperature sensors include two programmable alarms and report temperature in digital form over the SMBus interface. Similar to the MAX6642, these devices also occupy different addresses on the SMBus and when used in conjunction with the MAX6649, up to three local and three remote temperatures can be monitored on a single SMBus line. These devices operate over a temperature range of -55C to +145C with performance optimized from +100C to +145C. The 6646 and 6647 are ideal for multiprocessor systems, high-performance CPUs, and graphic processors.

The Microchip Technology TC77 and the TC72 thermal sensors are able to read temperatures without the need for any external components and communicate thermal data via 3- and 4-wire industry standard interfaces, respectively. The TC72, offered in 8-pin 3 x 3-mm packages, and the TC77, offered in the 5-pin SOT-23 and the 8-pin SOIC package, enable designers to save board space. “These new devices offer power savings while occupying the smallest possible board space,” said George Paparrizos, Product Marketing Manager for the Analog and Interface Product Division at Microchip. “In addition, they measure temperature with a maximum temperature error of 1 degree Celsius over a certain temperature range, and communicate with a variety of microcontrollers and other digital ICs.”

Another temperature monitor is the Philips NE1617. It's a two-channel temperature monitor that measures the temperature of itself and the temperature of a remote sensor. The remote sensor is a diode connected transistor. This can be in the form of either a discrete NPN/PNP transistor, such as the 2N3904/2N3906, or a diode connected PNP built into another die. The temperature of both the remote and local sensors is stored in a register that can be read via a two-wire SMBus. The temperatures are updated at a rate that is programmable via the SMBus.

Texas Instruments (TI) Incorporated also introduced a digital-output temperature sensor to the market. The company's TMP122 features an SPI-compatible interface and an SOT23 package type. The device is ideal for thermal measurements in a variety of communications, computer, consumer, industrial and instrumentation applications.

The TMP122 is capable of measuring temperatures within 0.5 degrees Celsius accuracy (1.5C max) over a temperature range of -25C to +85C, with operation from -55C to +150C. Low supply current (50uA), a shutdown feature (0.1uA) and a supply range from 2.7V to 5.5V make the device well suited for low-power applications. The TMP122 provides programmable resolution (9 to 12 bits) and programmable set points for the alert pin.

“The TMP122 is the latest addition to TI's growing portfolio of temperature sensors which offers a superior combination of accuracy, resolution, versatility and low power. Due to its programmable features, small packaging and wide temperature range, the TMP122 will find its place in a very diverse set of applications,” said Tadija Janjic, Strategic Marketing Engineer for TI's high-performance linear products.

The TMP122 is optimized for space-sensitive, low-power systems such as computer peripheral thermal protection, notebook computers, cell phones, thermostat controls, battery management and environmental monitoring.

As you can see, the growing market for sensors is certainly being supported by a significant number of companies developing temperature sensors. Now all you have to do is decide which product provides the right combination of design, foot print to control heat-related problems in your design.

Company Contacts:

Analog Devices, Inc.
Tel: 800- AnalogD (262-5643)

Andigilog, Inc.
Tel: 480-940-6200

Maxim Integrated Products
Tel: 408-737-7600

Microchip Technology, Inc.
Tel: 480-792-7668

National Semiconductor Corp.
Tel: 800-272-9959

Philips Semiconductor
Tel: 800-234-7381

Texas Instruments, Inc.
Tel: 800-477-8924

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