TB067

TB067 Temperature Sensor Strategies Keep System Thermal Problems in Check Author: Microchip Technology Inc. OVER-TEMPERATURE SHUTDOWN Heat sinking the high power dissipation devices in a system is the simplest thermal management strategy. In this case, only the system must be monitored for catastrophic over-temperature conditions due to a malfunction or excessive external ambient temperature. Temperature is sensed within the system enclosure (internal system ambient) or at the heat sink itself. When measured temperature exceeds a preset limit, a fault is indicated and the system shuts down. Figure 1 illustrates a simple power supply over-temperature shutdown circuit using Microchip’s TC622 temperature switch. This device integrates a solid-state temperature sensor and user-programmable limit detector in a single TO-220 package, allowing direct attachment to the heat sink surface. The device operates from power supply voltages between 4.5V and 18.0V, and has both high true and low true logic outputs (OUT, OUT). These outputs are driven active when measured temperature equals (or exceeds) the temperature setting determined by the value of external programming resistor, RSET. In the example of Figure 1, the TC622 outputs are driven active when the heatsink temperature equals the trip point temperature set by RSET. When this occurs, the crowbar circuit is activated, causing the supply output to fold back to zero. The TC622 outputs remain active until the heatsink temperature falls a minimum of 2°C (built-in hysteresis) below the trip point temperature, at which time the device again allows normal supply operation. INTRODUCTION Various concerns must be addressed when dealing with temperature sensing applications, which fall into two broad categories: precision temperature measurement and system thermal management. Precision temperature measurement applications (such as those encountered in the process control industry) require highly accurate temperature sensors. Applications of this type frequently use a thermocouple (or higher precision sensor) in concert with low offset/low drift analog interface circuits to acquire and condition the temperature signal with minimum error. Sensor absolute accuracy and linearity are primary considerations; sensor cost is a secondary issue. System thermal management schemes, on the other hand, regulate the creation (and/or disposal) of heat within the system. Examples of system thermal management applications are CPU over-temperature detectors in personal computers and thermal shutdown circuits in power systems. The temperature sensor must be accurate enough to detect when temperature is approaching the upper boundry of the “safe” operating limit. Typically, full scale sensor measurement errors of 2°C to 5°C are adequate for such applications. The primary concerns in sensor selection are sensor cost, size, and ease of interface to the processor. VCC Heat Sink Output Device VOUT Output Device TC622 RSET SET TC622 VDD OVERTEMP OUT GND OUT Power Good Signal Crowbar Circuit Circuit Board TC622 Heat Sink Mounting Heat Sink Surface FIGURE 1: TC622 Power Supply Over Temperature Shutdown  2003 Microchip Technology Inc. DS91067A-page 1 TB067 FAN ON/OFF CONTROL Although fans commonly are used to remove heat from a system, running a fan continuously not only creates objectionable acoustic noise, but also reduces the service life of the fan motor. It is more efficient to design the system thermal characteristics so the fan operates intermittently during high system load conditions. Figure 2 demonstrates an ambient temperature regulator using the TC07 thermostat chip. Housed in a tiny MSOP package (consuming only onehalf the board area of an 8-pin SOIC package), the TC07 consists of a temperature sensor and user-programmable dual limit detection circuit. As shown in Figure 2, the output of the TC07 controls the fan through an N-Channel logic-level MOSFET. The value of the RSET resistor determines the temperature at which the fan turns on. Once started, the fan runs continuously until temperature drops below the set point determined by RHYST. The fan cycles on and off (as required) to maintain system temperature within the range programmed by RSET and RHYST. INTELLIGENT FAN MANAGEMENT Temperature-proportional fan speed control (operating the fan only as fast as required to maintain a safe system temperature) maximizes fan life and minimizes power consumption and acoustic noise. The TC646 (see Figure 3) combines this fan management methodology with fan off/on control and fault detection for the ultimate in fan management. The TC646 uses efficient pulse-width modulation drive to control fan speed. A low cost NTC thermistor connected to VIN increases the positive duty cycle of VOUT (and, therefore, fan speed) as temperature rises. Should temperature fall low enough to cause the voltage on VIN to be less than the voltage on the autoshutdown threshold input (VAS), the TC646 automatically suspends fan operation. The fan is automatically restarted and temperature-proportional speed control resumes when increasing temperature causes VIN to exceed VAS. When the BDC fan is operating normally, brief commutation pulses are present on the SENSE input as each pole of the fan is energized. The TC646 uses the presence (or absence) of these pulses to detect a stuck, disconnected, or otherwise inoperative fan. If such a condition is detected, fan operation is suspended and a FAULT interrupt is generated. The FAULT output also is asserted (but fan operation is uninterrupted) if the voltage on VIN is greater than required for full speed, indicating a bad fan bearing, blocked air intake, or system thermal runaway. +5V System Enclosure HSET RSET TC07 SET HYST VDD OUT Q1 +12V GND OUT – Heat + Excess Heat Removed from System Brushless DC Fan Heat Source (Power Device Mounted to Heat Sink) FIGURE 2: DS91067A-page 2 Ambient Temperature Sensor  2003 Microchip Technology Inc. TB067 +5V + 1 µF R1 75k NTC 10k @ 25˚C 8 Shutdown R2 1k +5V R3 33k 3V AS CB 0.01µF 2 CF CF 1 µF GND 4 5 SENSE CSENSE 0.1 µF 1 CB 0.01 µF VIN VDD 6 FAULT Thermal Shutdown RBASE 1.5k VOUT 7 +12V Fan Q1 TC646 R4 18k RSENSE 2.2Ω FIGURE 3: Intelligent Fan Management SUMMARY Thermal management is essential to safeguard systems from failure caused by sustained over-temperature conditions. Many thermal management strategies have proven effective in controlling heat generation and removal. Microchip’s thermal management products, such as the devices discussed here, help designers implement these strategies effectively.  2003 Microchip Technology Inc. DS91067A-page 3 TB067 NOTES: DS91067A-page 4  2003 Microchip Technology Inc. Note the following details of the code protection feature on Microchip devices: • • Microchip products meet the specification contained in their particular Microchip Data Sheet. Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. Microchip is willing to work with the customer who is concerned about the integrity of their code. Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” • • • Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, KEELOQ, MPLAB, PIC, PICmicro, PICSTART, PRO MATE and PowerSmart are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. FilterLab, microID, MXDEV, MXLAB, PICMASTER, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. Accuron, dsPIC, dsPICDEM.net, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP, ICEPIC, microPort, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, PICC, PICkit, PICDEM, PICDEM.net, PowerCal, PowerInfo, PowerTool, rfPIC, Select Mode, SmartSensor, SmartShunt, SmartTel and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. Serialized Quick Turn Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2003, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999 and Mountain View, California in March 2002. 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B505A, Fullhope Plaza, No. 12 Hong Kong Central Rd. Qingdao 266071, China Tel: 86-532-5027355 Fax: 86-532-5027205 Italy Microchip Technology SRL Via Quasimodo, 12 20025 Legnano (MI) Milan, Italy Tel: 39-0331-742611 Fax: 39-0331-466781 India Microchip Technology Inc. India Liaison Office Marketing Support Division Divyasree Chambers 1 Floor, Wing A (A3/A4) No. 11, O’Shaugnessey Road Bangalore, 560 025, India Tel: 91-80-2290061 Fax: 91-80-2290062 United Kingdom Microchip Ltd. 505 Eskdale Road Winnersh Triangle Wokingham Berkshire, England RG41 5TU Tel: 44 118 921 5869 Fax: 44-118 921-5820 02/12/03 DS91067A-page 6  2003 Microchip Technology Inc.