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ADT7408 Datasheet(PDF) 21 Page - Analog Devices |
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ADT7408 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 24 page  ADT7408 Rev. 0 | Page 21 of 24 APPLICATION INFORMATION THERMAL RESPONSE TIME The time required for a temperature sensor to settle to a specified accuracy is a function of the thermal mass of the sensor and the thermal conductivity between the sensor and the object being sensed. Thermal mass is often considered equivalent to capaci- tance. Thermal conductivity is commonly specified using the symbol Q and can be thought of as thermal resistance. It is commonly specified in units of degrees per watt of power transferred across the thermal joint. Thus, the time required for the ADT7408 to settle to the desired accuracy is dependent on the package selected, the thermal contact established in that particular application, and the equivalent power of the heat source. In most applications, the settling time is best determined empirically. SELF-HEATING EFFECTS The temperature measurement accuracy of the ADT7408 might be degraded in some applications due to self-heating. Errors can be introduced from the quiescent dissipation and power dissipated when converting. The magnitude of these temperature errors is dependent on the thermal conductivity of the ADT7408 package, the mounting technique, and the effects of airflow. At 25°C, static dissipation in the ADT7408 is typically 778 μW operating at 3.3 V. In the 8-lead LFCSP_VD package mounted in free air, this accounts for a temperature increase due to self-heating of ΔT = PDISS × θJA = 778 μW × 85°C/W = 0.066°C Current dissipated through the device should be kept to a minimum by applying shutdown when the device can be put in the idle state, because it has a proportional effect on the temperature error. SUPPLY DECOUPLING The ADT7408 should be decoupled with a 0.1 μF ceramic capacitor between VDD and GND. This is particularly important when the ADT7408 is mounted remotely from the power supply. Precision analog products, such as the ADT7408, require a well- filtered power source. Because the ADT7408 operates from a single supply, it might seem convenient to tap into the digital logic power supply. Unfortunately, the logic supply is often a switch-mode design, which generates noise in the 20 kHz to 1 MHz range. In addition, fast logic gates can generate glitches hundreds of mV in amplitude due to wiring resistance and inductance. If possible, the ADT7408 should be powered directly from the system power supply. This arrangement, shown in Figure 18, isolates the analog section from the logic switching transients. Even if a separate power supply trace is not available, however, generous supply bypassing reduces supply-line-induced errors. Local supply bypassing consisting of a 0.1 μF ceramic capacitor is critical for the temperature accuracy specifications to be achieved. This decoupling capacitor must be placed as close as possible to the ADT7408 VDD pin. TTL/CMOS LOGIC CIRCUITS POWER SUPPLY ADT7408 0.1µF Figure 18. Using Separate Traces to Reduce Power Supply Noise TEMPERATURE MONITORING The ADT7408 is ideal for monitoring the thermal environment within electronic equipment. For example, the surface-mounted package accurately reflects the exact thermal conditions that affect nearby integrated circuits. The ADT7408 measures and converts the temperature at the surface of its own semiconductor chip. When the ADT7408 is used to measure the temperature of a nearby heat source, the thermal impedance between the heat source and the ADT7408 must be considered. Often, a thermocouple or other temperature sensor is used to measure the temperature of the source, while the temperature is monitored by reading back from the ADT7408 temperature value register. Once the thermal impedance is determined, the heat source temperature can be inferred from the ADT7408 output. As much as 60% of the heat transferred from the heat source to the thermal sensor on the ADT7408 die is discharged via the copper tracks, the package pins, and the bond pads. Of the pins on the ADT7408, the GND pin (VSS pin) transfers most of the heat. Therefore, when the temperature of a heat source is being measured, thermal resistance between the ADT7408 VSS pin and the heat source should be reduced as much as possible. An example of the ADT7408’s unique properties is shown in monitoring a high power dissipation DIMM module. Ideally, the ADT7408 device should be mounted in the middle between the two memory chips’ major heat sources (see Figure 19). The ADT7408 produces a linear temperature output, while needing only two I/O pins and requiring no external characterization. LEFT BOTTOM MIDDLE RIGHT SO-DIMM THERMAL SENSOR LOCATIONS TOP Figure 19. Locations of ADT7408 on DIMM Module |
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