SNIS170E January   2014  – October 2017 LMT87

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Tables
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Accuracy Characteristics
    6. 7.6 Electrical Characteristics
    7. 7.7 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 LMT87 Transfer Function
    4. 8.4 Device Functional Modes
      1. 8.4.1 Mounting and Thermal Conductivity
      2. 8.4.2 Output Noise Considerations
      3. 8.4.3 Capacitive Loads
      4. 8.4.4 Output Voltage Shift
  9. 9 Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Connection to ADC
        1. 9.2.1.1 Design Requirements
        2. 9.2.1.2 Detailed Design Procedure
        3. 9.2.1.3 Application Curve
      2. 9.2.2 Conserving Power Dissipation With Shutdown
        1. 9.2.2.1 Design Requirements
        2. 9.2.2.2 Detailed Design Procedure
        3. 9.2.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Receiving Notification of Documentation Updates
    2. 12.2 Community Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

8 Detailed Description

8.1 Overview

The LMT87 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.

8.2 Functional Block Diagram

Full-Range Celsius Temperature Sensor (−50°C to +150°C)
LMT87 FBD_01_SNIS170.gif

8.3 Feature Description

8.3.1 LMT87 Transfer Function

The output voltage of the LMT87, across the complete operating temperature range, is shown in Table 3. This table is the reference from which the LMT87 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 LMT87 product folder under Tools and Software Models.

Table 3. LMT87 Transfer Table

TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)
–50 3277 –10 2767 30 2231 70 1679 110 1115
–49 3266 –9 2754 31 2217 71 1665 111 1101
–48 3254 –8 2740 32 2204 72 1651 112 1087
–47 3243 –7 2727 33 2190 73 1637 113 1073
–46 3232 –6 2714 34 2176 74 1623 114 1058
–45 3221 –5 2700 35 2163 75 1609 115 1044
–44 3210 –4 2687 36 2149 76 1595 116 1030
–43 3199 –3 2674 37 2136 77 1581 117 1015
–42 3186 –2 2660 38 2122 78 1567 118 1001
–41 3173 –1 2647 39 2108 79 1553 119 987
–40 3160 0 2633 40 2095 80 1539 120 973
–39 3147 1 2620 41 2081 81 1525 121 958
–38 3134 2 2607 42 2067 82 1511 122 944
–37 3121 3 2593 43 2054 83 1497 123 929
–36 3108 4 2580 44 2040 84 1483 124 915
–35 3095 5 2567 45 2026 85 1469 125 901
–34 3082 6 2553 46 2012 86 1455 126 886
–33 3069 7 2540 47 1999 87 1441 127 872
–32 3056 8 2527 48 1985 88 1427 128 858
–31 3043 9 2513 49 1971 89 1413 129 843
–30 3030 10 2500 50 1958 90 1399 130 829
–29 3017 11 2486 51 1944 91 1385 131 814
–28 3004 12 2473 52 1930 92 1371 132 800
–27 2991 13 2459 53 1916 93 1356 133 786
–26 2978 14 2446 54 1902 94 1342 134 771
–25 2965 15 2433 55 1888 95 1328 135 757
–24 2952 16 2419 56 1875 96 1314 136 742
–23 2938 17 2406 57 1861 97 1300 137 728
–22 2925 18 2392 58 1847 98 1286 138 713
–21 2912 19 2379 59 1833 99 1272 139 699
–20 2899 20 2365 60 1819 100 1257 140 684
–19 2886 21 2352 61 1805 101 1243 141 670
–18 2873 22 2338 62 1791 102 1229 142 655
–17 2859 23 2325 63 1777 103 1215 143 640
–16 2846 24 2311 64 1763 104 1201 144 626
–15 2833 25 2298 65 1749 105 1186 145 611
–14 2820 26 2285 66 1735 106 1172 146 597
–13 2807 27 2271 67 1721 107 1158 147 582
–12 2793 28 2258 68 1707 108 1144 148 568
–11 2780 29 2244 69 1693 109 1130 149 553
150 538

Although the LMT87 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).

Equation 1. LMT87 ParaEq_G11_SNIS170.gif

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:

Equation 2. LMT87 ParEqSol_SNIS170.gif

For an even less accurate linear transfer function approximation, a line can easily be calculated over the desired temperature range from Table 3 using the two-point equation (Equation 3):

Equation 3. LMT87 equation_1_nis170.gif

where

  • V is in mV,
  • T is in °C,
  • T1 and V1 are the coordinates of the lowest temperature,
  • and T2 and V2 are the coordinates of the highest temperature.

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:

Equation 4. LMT87 equation_2_nis170.gif
Equation 5. LMT87 equation_3_nis170.gif
Equation 6. LMT87 equation_4_nis170.gif

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 LMT87 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 LMT87 die is directly attached to the GND pin. The temperatures of the lands and traces to the other leads of the LMT87 will also affect the temperature reading.

Alternatively, the LMT87 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 LMT87 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 LMT87 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 LMT87 die temperature:

Equation 7. LMT87 equation_5_nis170.gif

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 = 2231 mV, and IL = 2 μA, the junction temperature would be 30.014°C, showing a self-heating error of only 0.014°C. Because the junction temperature of the LMT87 is the actual temperature being measured, take care to minimize the load current that the LMT87 is required to drive. Thermal Information shows the thermal resistance of the LMT87.

8.4.2 Output Noise Considerations

A push-pull output gives the LMT87 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 LMT87 is ideal for this and other applications which require strong source or sink current.

The LMT87 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 LMT87.

8.4.3 Capacitive Loads

The LMT87 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 LMT87 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.

LMT87 no_decoupling_cap_loads_less_nis170.gif Figure 11. LMT87 No Decoupling Required for Capacitive Loads Less Than 1100 pF
LMT87 series_resister_cap_loads_greater_nis170.gif Figure 12. LMT87 with Series Resistor for Capacitive Loading Greater Than 1100 pF

Table 4. Recommended Series Resistor Values

CLOAD MINIMUM RS
1.1 nF to 99 nF 3 kΩ
100 nF to 999 nF 1.5 kΩ
1 μF 800 Ω

8.4.4 Output Voltage Shift

The LMT87 is very linear over temperature and supply voltage range. Due to the intrinsic behavior of an NMOS/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.