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LM88CIMMX-B Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM88CIMMX-B Datasheet(HTML) 7 Page - National Semiconductor (TI) |
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7 / 9 page  2.0 Application Hints (Continued) of the LM88’s temperature. The LM88 has been optimized to measure the remote diode of a Pentium type processor as shown in Figure 3. 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 af- fected, and often dominated, by the temperature of its leads. As with any IC, the LM88 and accompanying wiring and circuits must be kept insulated and dry, to avoid leakage and corrosion. This is especially true if the circuit may operate at cold temperatures where condensation can occur. Printed-circuit coatings and varnishes such as Humiseal and epoxy paints or dips are often used to ensure that moisture cannot corrode the LM88 or its connections. Moisture may also cause leakage on the diode wiring and therefore affect the accuracy of the temperature set-points. Most silicon diodes do not lend themselves well to this application. It is recommended that a 2N3904 transistor base emitter junction be used with the collector tied to the base. A diode connected 2N3904 approximates the junction avail- able on a Pentium III microprocessor for temperature mea- surement. Therefore, the LM88 can sense the temperature of this diode effectively. 2.3 EFFECTS OF THE DIODE NON-IDEALITY FACTOR ON ACCURACY The technique used in today’s remote temperature sensors is to measure the change in V BE at two different operating points of a diode. For a bias current ratio of N:1, this differ- ence is given as: where: — η is the non-ideality factor of the process the diode is manufactured on, — q is the electron charge, — k is the Boltzmann’s constant, — N is the current ratio, — T is the absolute temperature in ˚K. The temperature sensor then measures ∆V BE and converts to IT digital data. In this equation, k and q are well defined universal constants, and N is a parameter controlled by the temperature sensor. The only other parameter is η, which depends on the diode that is used for measurement. Since ∆V BE is proportional to both η and T, the variations in η cannot be distinguished from variations in temperature. Since the non-ideality factor is not controlled by the tempera- ture sensor, it will directly add to the inaccuracy of the sensor. For the Pentium II, Intel specifies a ±1% variation in η from part to part. As an example, assume a temperature sensor has an accuracy specification of ±3 ˚C at room temperature of 25 ˚C and the process used to manufacture the diode has a non-ideality variation of ±1%. The resulting accuracy of the temperature sensor at room temperature will be: T ACC = ± 3˚C+(±1% of 298 ˚K) = ±6˚C . The additional inaccuracy in the temperature measurement caused by η can be eliminated if each temperature sensor is calibrated with the remote diode that it will be paired with. 2.4 PCB LAYOUT to MINIMIZE NOISE In a noisy environment, such as a processor motherboard, layout considerations are very critical. Noise induced on traces running between the remote temperature diode sen- sor and the LM88 can cause temperature conversion errors. The following guidelines should be followed: 1. Place a 0.1 µF power supply bypass capacitor as close as possible to the V DD pin and the recommended 2.2 nF capacitor as close as possible to the D+ and D− pins. Make sure the traces to the two 2.2nF capacitor are matched. 2. The recommended 2.2nF diode bypass capacitor actu- ally has a range of 200pF to 3.3nF. The average tem- perature accuracy will not change over that capacitance range. Increasing the capacitance will lower the corner frequency where differential noise error will start to affect the temperature reading thus producing a reading that is more stable. Conversely, lowering the capacitance will increase the corner frequency where differential noise error starts to affect the temperature reading thus pro- ducing a reading that is less stable. 3. Ideally, the LM88 should be placed within 10cm of the remote diode pins with the traces being as straight, short and identical as possible. Trace resistance of 1 Ω can cause as much as 1˚C of error. This error can be com- pensated by using the Remote Temperature Offset Reg- isters, since the value placed in these registers will automatically be subtracted or added to the remote tem- perature reading. 4. Diode traces should be surrounded by a GND guard ring to either side, above and below if possible. This GND guard should not go between the D+ and D− lines so that in the event that noise does couple to the diode lines, it would be coupled common mode and rejected- .(See Figure 4) 5. Avoid routing diode traces in close proximity to power supply switching or filtering inductors. 6. Avoid running diode traces close to or parallel to high speed digital and bus lines. Diode traces should be kept at least 2cm apart from the high speed digital traces. 7. If it is necessary to cross high speed digital traces, the diode traces and the high speed digital traces should cross at a 90 degree angle. 10132615 FIGURE 3. Pentium or 3904 Temperature vs LM88 Temperature Set-point www.national.com 7 |
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