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TMP411-Q1 Datasheet(PDF) 29 Page - Texas Instruments |
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TMP411-Q1 Datasheet(HTML) 29 Page - Texas Instruments |
29 / 40 page ![]() 1.004 1.008 ( ) (273.15 100 ) 1.008 1.48 ERR ERR T C T C - = ´ + ° = - ° 1.008 ( ) (273.15 ( )) 1.008 ERR n T T C - = ´ + ° TMP411-Q1 www.ti.com SBOS527F – DECEMBER 2010 – REVISED NOVEMBER 2013 REMOTE SENSING 4. Tight control of VBE characteristics indicated by small variations in hFE (that is, 50 to 150). The TMP411-Q1 is designed to be used with either discrete transistors or substrate transistors built into Based on these criteria, two recommended small- processor chips and ASICs. Either NPN or PNP signal transistors are the 2N3904 (NPN) or 2N3906 transistors can be used, as long as the base-emitter (PNP). junction is used as the remote temperature sense. Either a transistor or diode connection can also be MEASUREMENT ACCURACY AND THERMAL used; see Figure 11. CONSIDERATIONS Errors in remote temperature-sensor readings are the The temperature measurement accuracy of the consequence of the ideality factor and current TMP411-Q1 depends on the remote and/or local excitation used by the TMP411-Q1 versus the temperature sensor being at the same temperature manufacturer-specified operating current for a given as the system point being monitored. Clearly, if the transistor. Some manufacturers specify a high-level temperature sensor is not in good thermal contact and low-level current for the temperature-sensing with the part of the system being monitored, then substrate transistors. The TMP411-Q1 uses 6 μA for there is a delay in the response of the sensor to a ILOW and 120 μA for IHIGH. The TMP411-Q1 allows for temperature change in the system. For remote different n-factor values; see the N-Factor Correction temperature sensing applications using a substrate Register section. transistor (or a small, SOT23 transistor) placed close to the device being monitored, this delay is usually The ideality factor (n) is a measured characteristic of not a concern. a remote temperature sensor diode as compared to an ideal diode. The ideality factor for the TMP411-Q1 The local temperature sensor inside the TMP411-Q1 is trimmed to be 1.008. For transistors whose ideality monitors the ambient air around the device. The factor does not match the TMP411-Q1, Equation 4 thermal time constant for the TMP411-Q1 is can be used to calculate the temperature error. Note approximately two seconds. This constant implies that for the equation to be used correctly, actual that if the ambient air changes quickly by 100°C, it temperature (°C) must be converted to Kelvin (K). would take the TMP411-Q1 about 10 seconds (that is, five thermal time constants) to settle to within 1°C of the final value. In most applications, the TMP411- (4) Q1 package is in electrical and therefore thermal contact with the printed circuit board (PCB), as well where: as subjected to forced airflow. The accuracy of the n = Ideality factor of remote temperature sensor measured temperature directly depends on how T(°C) = actual temperature accurately the PCB and forced airflow temperatures represent the temperature that the TMP411-Q1 is TERR = Error in TMP411-Q1 reading due to n ≠ measuring. Additionally, the internal power dissipation 1.008 of the TMP411-Q1 can cause the temperature to rise Degree delta is the same for °C and °K above the ambient or PCB temperature. The internal For n = 1.004 and T(°C) = 100°C: power dissipated as a result of exciting the remote temperature sensor is negligible because of the small currents used. For a 5.5-V supply and maximum conversion rate of eight conversions per second, the TMP411-Q1 dissipates 1.82 mW (PDIQ = 5.5 V × 330 (5) μA). If both the ALERT/THERM2 and THERM pins If a discrete transistor is used as the remote are each sinking 1 mA, an additional power of 0.8 temperature sensor with the TMP411-Q1, the best mW is dissipated (PDOUT = 1 mA × 0.4 V + 1 mA × accuracy can be achieved by selecting the transistor 0.4 V = 0.8 mW). Total power dissipation is then 2.62 according to the following criteria: mW (PDIQ + PDOUT) and, with a θJA of 150°C/W, 1. Base-emitter voltage > 0.25 V at 6 μA, at the causes the junction temperature to rise approximately highest sensed temperature. 0.393°C above the ambient. 2. Base-emitter voltage < 0.95 V at 120 μA, at the lowest sensed temperature. 3. Base resistance < 100 Ω. Copyright © 2010–2013, Texas Instruments Incorporated Submit Documentation Feedback 29 Product Folder Links: TMP411-Q1 |
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