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LMT85 Datasheet(PDF) 11 Page - Texas Instruments |
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LMT85 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 26 page J A JA DD S DD OUT L T = T + (V I ) + (V - V ) I T ª º ¬ ¼ LMT85, LMT85-Q1 www.ti.com SNIS168C – MARCH 2013 – REVISED OCTOBER 2015 Using this method of linear approximation, the transfer function can be approximated for one or more temperature ranges of interest. 8.4 Device Functional Modes 8.4.1 Mounting and Thermal Conductivity The LMT85/LMT85-Q1 can be applied easily in the same way as other integrated-circuit temperature sensors. It can be glued or cemented to a surface. To ensure good thermal conductivity, the backside of the LMT85/LMT85-Q1 die is directly attached to the GND pin (Pin 2 for the SOT/SC70/DCK package). The temperatures of the lands and traces to the other leads of the LMT85/LMT85-Q1 will also affect the temperature reading. Alternatively, the LMT85/LMT85-Q1 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 LMT85/LMT85-Q1 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. If moisture creates a short circuit from the output to ground or VDD, the output from the LMT85/LMT85-Q1 will not be correct. Printed-circuit coatings are often used to ensure that moisture cannot corrode the leads or circuit traces. The thermal resistance junction to ambient (RθJA or θJA) parameter used to calculate the rise of a device junction temperature due to its power dissipation. The equation used to calculate the rise in the LMT85/LMT85-Q1 die temperature is: (7) where TA is the ambient temperature, IS is the supply current, ILis the load current on the output, and VO is the output voltage. For example, in an application where TA = 30°C, VDD = 5 V, IS = 5.4 μA, VOUT = 1324 mV, and IL = 2 μA, the junction temperature would be 30.014°C, showing a self-heating error of only 0.014°C. Since the LMT85/LMT85-Q1's junction temperature is the actual temperature being measured, care should be taken to minimize the load current that the LMT85/LMT85-Q1 is required to drive. For the thermal resistance of the LMT85/LMT85Q1 in different packages see sectionThermal Information(1). 8.4.2 Output and Noise Considerations A push-pull output gives the LMT85/LMT85-Q1 the ability to sink and source significant current. This is beneficial when, for example, driving dynamic loads like an input stage on an analog-to-digital converter (ADC). In these applications the source current is required to quickly charge the input capacitor of the ADC. The LMT85/LMT85- Q1 are ideal for this and other applications which require strong source or sink current. The LMT85/LMT85-Q1's supply-noise gain (the ratio of the AC signal on VOUT to the AC signal on VDD) was measured during bench tests. Its typical attenuation is shown in Figure 8 found in the Typical Characteristics section. A load capacitor on the output can help to filter noise. For operation in very noisy environments, some bypass capacitance should be present on the supply within approximately 5 centimeters of the LMT85/LMT85-Q1. 8.4.3 Capacitive Loads The LMT85/LMT85-Q1 handles capacitive loading well. In an extremely noisy environment, or when driving a switched sampling input on an ADC, it may be necessary to add some filtering to minimize noise coupling. Without any precautions, the LMT85/LMT85-Q1 can drive a capacitive load less than or equal to 1100 pF as shown in Figure 10. For capacitive loads greater than 1100 pF, a series resistor may be required on the output, as shown in Figure 11. (1) For information on self-heating and thermal response time see section Mounting and Thermal Conductivity. Copyright © 2013–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 |
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