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LM95231 Datasheet(PDF) 19 Page - Texas Instruments |
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LM95231 Datasheet(HTML) 19 Page - Texas Instruments |
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19 / 30 page  IF = IS e Vbe KVt Vt = k T q IF = IS x e -1 VBE K x Vt LM95231 www.ti.com SNIS139E – FEBRUARY 2005 – REVISED MARCH 2013 DIE REVISION CODE REGISTER (Read Address FFh) The default value is A1h. This register will increment by 1 every time there is a revision to the die by Texas Instruments. Applications Hints The LM95231 can be applied easily in the same way as other integrated-circuit temperature sensors, and its remote diode sensing capability allows it to be used in new ways as well. It can be soldered to a printed circuit board, and because the path of best thermal conductivity is between the die and the pins, its temperature will effectively be that of the printed circuit board lands and traces soldered to the LM95231's pins. This presumes that the ambient air temperature is almost the same as the surface temperature of the printed circuit board; if the air temperature is much higher or lower than the surface temperature, the actual temperature of the LM95231 die will be at an intermediate temperature between the surface and air temperatures. Again, the primary thermal conduction path is through the leads, so the circuit board temperature will contribute to the die temperature much more strongly than will the air temperature. To measure temperature external to the LM95231's die, use a remote diode. This diode can be located on the die of a target IC, allowing measurement of the IC's temperature, independent of the LM95231's temperature. A discrete diode can also be used to sense the temperature of external objects or ambient air. Remember that a discrete diode's temperature will be affected, and often dominated, by the temperature of its leads. Most silicon diodes do not lend themselves well to this application. It is recommended that an MMBT3904 transistor base emitter junction be used with the collector tied to the base. The LM95231's TruTherm technology allows accurate sensing of integrated thermal diodes, such as those found on processors. With TruTherm technology turned off, the LM95231 can measure a diode connected transistor such as the MMBT3904. The LM95231 has been optimized to measure the remote thermal diode integrated in a Pentium 4 processor on 90nm process or an MMBT3904 transistor. Using the Remote Diode Model Select register either pair of remote inputs can be assigned to be either a Pentium 4 processor on 90nm process or an MMBT3904. DIODE NON-IDEALITY Diode Non-Ideality Factor Effect on Accuracy When a transistor is connected as a diode, the following relationship holds for variables VBE, T and IF: where • • q = 1.6×10 −19 Coulombs (the electron charge), • T = Absolute Temperature in Kelvin • k = 1.38×10 −23joules/K (Boltzmann's constant), • η is the non-ideality factor of the process the diode is manufactured on, • IS = Saturation Current and is process dependent, • If= Forward Current through the base emitter junction • VBE = Base Emitter Voltage drop (1) In the active region, the -1 term is negligible and may be eliminated, yielding the following equation (2) In Equation 2, η and IS are dependant upon the process that was used in the fabrication of the particular diode. By forcing two currents with a very controlled ratio(IF2/IF1) and measuring the resulting voltage difference, it is possible to eliminate the IS term. Solving for the forward voltage difference yields the relationship: Copyright © 2005–2013, Texas Instruments Incorporated Submit Documentation Feedback 19 Product Folder Links: LM95231 |
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