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LM26CIM5-VPA Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM26CIM5-VPA Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 9 page Applications Hints (Continued) TABLE 1. Resistive compensation for capacitive loading of V TEMP C LOAD R( Ω) ≤100pF 0 1nF 8200 10nF 3000 100nF 1000 ≥1µF 430 NOISE CONSIDERATIONS The LM26 has excellent power supply noise rejection. Listed below is a variety of signals used to test the LM26 power supply rejection. False triggering of the output was not ob- served when these signals where coupled into the V+ pin of the LM26. • square wave 400kHz, 1Vp-p • square wave 2kHz, 200mVp-p • sine wave 100Hz to 1MHz, 200mVp-p Testing was done while maintaining the temperature of the LM26 one degree centigrade way from the trip point with the output not activated. MOUNTING CONSIDERATIONS The LM26 can be applied easily in the same way as other integrated-circuit temperature sensors. It can be glued or cemented to a surface. The temperature that the LM26 is sensing will be within about +0.06˚C of the surface tempera- ture to which the LM26’s leads are attached to. This presumes that the ambient air temperature is almost the same as the surface temperature; if the air temperature were much higher or lower than the surface temperature, the actual temperature measured would be at an intermediate temperature between the surface temperature and the air temperature. To ensure good thermal conductivity, the backside of the LM26 die is directly attached to the GND pin (pin 2). The temperatures of the lands and traces to the other leads of the LM26 will also affect the temperature that is being sensed. Alternatively, the LM26 can be mounted inside a sealed-end metal tube, and can then be dipped into a bath or screwed into a threaded hole in a tank. As with any IC, the LM26 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 conden- sation 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 LM26 or its connec- tions. The junction to ambient thermal resistance ( θ JA) is the pa- rameter used to calculate the rise of a part’s junction tem- perature due to its power dissipation. For the LM26 the equation used to calculate the rise in the die junction tem- perature is as follows: (3) where T A is the ambient temperature, V + is the power supply voltage, I Q is the quiescent current, IL_TEMP is the load current on the V TEMP output, VDO is the voltage on the digital output, and I DO is the load current on the digital output. Since the LM26’s junction temperature is the actual temperature being measured, care should be taken to minimize the load current that the LM26 is required to drive. The tables shown in Figure 3 summarize the thermal resis- tance for different conditions and the rise in die temperature of the LM26 without any loading on V TEMP and a 10k pull-up resistor on an open-drain digital output with a 5.5V power supply. 10132317 a) R in series with capacitor 10132318 b) R in series with signal path FIGURE 2. Resistor placement for capacitive loading compensation of V TEMP SOT23-5 no heat sink SOT23-5 small heat sink θ JA (˚C/W) T J−TA (˚C) θ JA (˚C/W) T J−TA (˚C) Still Air 250 0.11 TBD TBD Moving Air TBD TBD TBD TBD FIGURE 3. Thermal resistance ( θ JA) and temperature rise due to self heating (T J−TA) www.national.com 6 |
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