JAJSE55E March 2013 – October 2017 LMT84
The LMT84 is an analog output temperature sensor. The temperature-sensing element is comprised of a simple base emitter junction that is forward biased by a current source. The temperature-sensing element is then buffered by an amplifier and provided to the OUT pin. The amplifier has a simple push-pull output stage thus providing a low impedance output source.
The output voltage of the LMT84, across the complete operating temperature range, is shown in Table 3. This table is the reference from which the LMT84 accuracy specifications (listed in the Accuracy Characteristics section) are determined. This table can be used, for example, in a host processor look-up table. A file containing this data is available for download at the LMT84 product folder under Tools and Software Models.
Although the LMT84 is very linear, the response does have a slight umbrella parabolic shape. This shape is very accurately reflected in Table 3. The transfer table can be calculated by using the parabolic equation (Equation 1).
The parabolic equation is an approximation of the transfer table and the accuracy of the equation degrades slightly at the temperature range extremes. Equation 1 can be solved for T, resulting in:
For an even less accurate linear approximation, a line can easily be calculated over the desired temperature range from the table using the two-point equation (Equation 3):
For example, if the user wanted to resolve this equation, over a temperature range of 20°C to 50°C, they would proceed as follows:
Using this method of linear approximation, the transfer function can be approximated for one or more temperature ranges of interest.
The LMT84 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 LMT84 die is directly attached to the GND pin. The temperatures of the lands and traces to the other leads of the LMT84 will also affect the temperature reading.
Alternatively, the LMT84 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 LMT84 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 LMT84 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) is the parameter used to calculate the rise of a device junction temperature due to its power dissipation. Use Equation 7 to calculate the rise in the LMT84 die temperature:
For example, in an application where TA = 30°C, VDD = 5 V, IS = 5.4 μA, VOUT = 871 mV, and IL = 2 μA, the junction temperature would be 30.015°C, showing a self-heating error of only 0.015°C. Because the junction temperature of the LMT84 device is the actual temperature being measured, take care to minimize the load current that the LMT84 is required to drive. Thermal Information shows the thermal resistance of the LMT84.
A push-pull output gives the LMT84 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 LMT84 is ideal for this and other applications which require strong source or sink current.
The LMT84 supply-noise gain (the ratio of the AC signal on VOUT to the AC signal on VDD) was measured during bench tests. The 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 LMT84.
The LMT84 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 LMT84 can drive a capacitive load less than or equal to 1100 pF as shown in Figure 11. For capacitive loads greater than 1100 pF, a series resistor may be required on the output, as shown in Figure 12.
|1.1 nF to 99 nF||3 kΩ|
|100 nF to 999 nF||1.5 kΩ|
|1 μF||800 Ω|
The LMT84 is very linear over temperature and supply voltage range. Due to the intrinsic behavior of an NMOS or PMOS rail-to-rail buffer, a slight shift in the output can occur when the supply voltage is ramped over the operating range of the device. The location of the shift is determined by the relative levels of VDD and VOUT. The shift typically occurs when VDD – VOUT = 1 V.
This slight shift (a few millivolts) takes place over a wide change (approximately 200 mV) in VDD or VOUT. Because the shift takes place over a wide temperature change of 5°C to 20°C, VOUT is always monotonic. The accuracy specifications in the Accuracy Characteristics table already include this possible shift.