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TMP411 Datasheet(PDF) 21 Page - Burr-Brown (TI) |
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TMP411 Datasheet(HTML) 21 Page - Burr-Brown (TI) |
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21 / 25 page  TMP411 SBOS383A − FEBRUARY 2007 www.ti.com 21 REMOTE SENSING The TMP411 is designed to be used with either discrete transistors or substrate transistors built into processor chips and ASICs. Either NPN or PNP transistors can be used, as long as the base-emitter junction is used as the remote temperature sense. Either a transistor or diode connection can also be used; see Figure 11. Errors in remote temperature sensor readings will be the consequence of the ideality factor and current excitation used by the TMP411 versus the manufacturer-specified operating current for a given transistor. Some manufacturers specify a high-level and low-level current for the temperature-sensing substrate transistors. The TMP411 uses 6 µA for ILOW and 120µA for IHIGH. The TMP411 allows for different n-factor values; see the N-Factor Correction Register section. The ideality factor (n) is a measured characteristic of a remote temperature sensor diode as compared to an ideal diode. The ideality factor for the TMP411 is trimmed to be 1.008. For transistors whose ideality factor does not match the TMP411, Equation 4 can be used to calculate the temperature error. Note that for the equation to be used correctly, actual temperature ( °C) must be converted to Kelvin ( °K). T ERR + n * 1.008 1.008 273.15 ) T °C Where: n = Ideality factor of remote temperature sensor T( °C) = actual temperature TERR = Error in TMP411 reading due to n ≠ 1.008 Degree delta is the same for °C and °K For n = 1.004 and T( °C) = 100°C: T ERR + 1.004 * 1.008 1.008 273.15 ) 100°C T ERR +* 1.48°C If a discrete transistor is used as the remote temperature sensor with the TMP411, the best accuracy can be achieved by selecting the transistor according to the following criteria: 1. Base-emitter voltage > 0.25V at 6 µA, at the highest sensed temperature. 2. Base-emitter voltage < 0.95V at 120 µA, at the lowest sensed temperature. 3. Base resistance < 100 Ω. 4. Tight control of VBE characteristics indicated by small variations in hFE (that is, 50 to 150). Based on these criteria, two recommended small-signal transistors are the 2N3904 (NPN) or 2N3906 (PNP). MEASUREMENT ACCURACY AND THERMAL CONSIDERATIONS The temperature measurement accuracy of the TMP411 depends on the remote and/or local temperature sensor being at the same temperature as the system point being monitored. Clearly, if the temperature sensor is not in good thermal contact with the part of the system being monitored, then there will be a delay in the response of the sensor to a temperature change in the system. For remote temperature sensing applications using a substrate transistor (or a small, SOT23 transistor) placed close to the device being monitored, this delay is usually not a concern. The local temperature sensor inside the TMP411 monitors the ambient air around the device. The thermal time constant for the TMP411 is approximately two seconds. This constant implies that if the ambient air changes quickly by 100 °C, it would take the TMP411 about 10 seconds (that is, five thermal time constants) to settle to within 1 °C of the final value. In most applications, the TMP411 package is in electrical and therefore thermal contact with the printed circuit board (PCB), as well as subjected to forced airflow. The accuracy of the measured temperature directly depends on how accurately the PCB and forced airflow temperatures represent the temperature that the TMP411 is measuring. Additionally, the internal power dissipation of the TMP411 can cause the temperature to rise above the ambient or PCB temperature. The internal power dissipated as a result of exciting the remote temperature sensor is negligible because of the small currents used. For a 5.5V supply and maximum conversion rate of eight conversions per second, the TMP411 dissipates 1.82mW (PDIQ = 5.5V × 330 µA). If both the ALERT/THERM2 and THERM pins are each sinking 1mA, an additional power of 0.8mW is dissipated (PDOUT = 1mA × 0.4V + 1mA × 0.4V = 0.8mW). Total power dissipation is then 2.62mW (PDIQ + PDOUT) and, with an qJA of 150°C/W, causes the junction temperature to rise approximately 0.393 °C above the ambient. (4) (5) |
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Similar Description - TMP411 |
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