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LM95235 Datasheet(PDF) 21 Page - National Semiconductor (TI) |
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LM95235 Datasheet(HTML) 21 Page - National Semiconductor (TI) |
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21 / 25 page  3.0 Applications Hints The LM95235 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 be- cause 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 LM95235’s pins. This presumes that the ambient air tem- perature 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 tempera- ture of the LM95235 die will be at an intermediate tempera- ture 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 tempera- ture much more strongly than will the air temperature. To measure temperature external to the LM95235’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 LM95235’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 LM95235’s TruTherm technology allows accurate sens- ing of integrated thermal diodes, such as those found on most processors. With TruTherm technology turned off, the LM95235 can measure a diode-connected transistor such as the MMBT3904 or the thermal diode found in an AMD pro- cessor. The LM95235 has been optimized to measure the remote thermal diode integrated in a typical Intel processor on 65 nm or 90 nm process or an MMBT3904 transistor. Using the Remote Diode Model Select register either pair of remote inputs can be assigned to be either a typical Intel processor on 65 nm or 90 nm process or an MMBT3904. 3.1 DIODE NON-IDEALITY 3.1.1 Diode Non-Ideality Factor Effect on Accuracy When a transistor is connected as a diode, the following relationship holds for variables V BE, T and IF: (1) where: • q = 1.6x10 −19 Coulombs (the electron charge), • T = Absolute Temperature in Kelvin • k = 1.38x10 −23 joules/K (Boltzmann’s constant), • η is the non-ideality factor of the process the diode is manufactured on, • I S = Saturation Current and is process dependent, • I f = Forward Current through the base-emitter junction • V BE = Base-Emitter Voltage drop In the active region, the -1 term is negligible and may be eliminated, yielding the following equation (2) In Equation (2), η and I S are dependant upon the process that was used in the fabrication of the particular diode. By forcing two currents with a very controlled ratio (I F2 /IF1) and measuring the resulting voltage difference, it is possible to eliminate the I S term. Solving for the forward voltage differ- ence yields the relationship: (3) Solving Equation (3) for temperature yields: (4) Equation (4) holds true when a diode connected transistor such as the MMBT3904 is used. When this “diode” equation is applied to an integrated diode such as a processor tran- sistor with its collector tied to GND as shown in Figure 7 it will yield a wide non-ideality spread. This wide non-ideality spread is not due to true process variation but due to the fact that Equation (4) is an approximation. TruTherm technology uses the transistor equation, Equation (5), which is a more accurate representation of the topology of the thermal diode found in an FPGA or processor. (5) www.national.com 21 |
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