SNIS168E March   2013  – October 2017 LMT85

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 LMT85 Transfer Function
    4. 8.4 Device Functional Modes
      1. 8.4.1 Mounting and Thermal Conductivity
      2. 8.4.2 Output and 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 an 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 LMT85 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).

LMT85 FBD_SNIS168.gif

8.3 Feature Description

8.3.1 LMT85 Transfer Function

The output voltage of the LMT85, across the complete operating temperature range, is shown in Table 3. This table is the reference from which the LMT85 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 LMT85 product folder under Tools and Software Models.

Table 3. LMT85 Transfer Table

TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)
-50 1955 -10 1648 30 1324 70 991 110 651
-49 1949 -9 1639 31 1316 71 983 111 642
-48 1942 -8 1631 32 1308 72 974 112 634
-47 1935 -7 1623 33 1299 73 966 113 625
-46 1928 -6 1615 34 1291 74 957 114 617
-45 1921 -5 1607 35 1283 75 949 115 608
-44 1915 -4 1599 36 1275 76 941 116 599
-43 1908 -3 1591 37 1267 77 932 117 591
-42 1900 -2 1583 38 1258 78 924 118 582
-41 1892 -1 1575 39 1250 79 915 119 573
-40 1885 0 1567 40 1242 80 907 120 565
-39 1877 1 1559 41 1234 81 898 121 556
-38 1869 2 1551 42 1225 82 890 122 547
-37 1861 3 1543 43 1217 83 881 123 539
-36 1853 4 1535 44 1209 84 873 124 530
-35 1845 5 1527 45 1201 85 865 125 521
-34 1838 6 1519 46 1192 86 856 126 513
-33 1830 7 1511 47 1184 87 848 127 504
-32 1822 8 1502 48 1176 88 839 128 495
-31 1814 9 1494 49 1167 89 831 129 487
-30 1806 10 1486 50 1159 90 822 130 478
-29 1798 11 1478 51 1151 91 814 131 469
-28 1790 12 1470 52 1143 92 805 132 460
-27 1783 13 1462 53 1134 93 797 133 452
-26 1775 14 1454 54 1126 94 788 134 443
-25 1767 15 1446 55 1118 95 779 135 434
-24 1759 16 1438 56 1109 96 771 136 425
-23 1751 17 1430 57 1101 97 762 137 416
-22 1743 18 1421 58 1093 98 754 138 408
-21 1735 19 1413 59 1084 99 745 139 399
-20 1727 20 1405 60 1076 100 737 140 390
-19 1719 21 1397 61 1067 101 728 141 381
-18 1711 22 1389 62 1059 102 720 142 372
-17 1703 23 1381 63 1051 103 711 143 363
-16 1695 24 1373 64 1042 104 702 144 354
-15 1687 25 1365 65 1034 105 694 145 346
-14 1679 26 1356 66 1025 106 685 146 337
-13 1671 27 1348 67 1017 107 677 147 328
-12 1663 28 1340 68 1008 108 668 148 319
-11 1656 29 1332 69 1000 109 660 149 310
150 301

Although the LMT85 is very linear, its 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. LMT85 ParaEq_G01_SNIS168.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. LMT85 ParaEq_GainSolutionForT1_SNIS168.gif

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

Equation 3. LMT85 equation_1_nis168.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. LMT85 equation_2_nis168.gif
Equation 5. LMT85 equation_3_nis168.gif
Equation 6. LMT85 equation_4_nis168.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 LMT85 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 die is directly attached to the GND pin. The temperatures of the lands and traces to the other leads of the LMT85 will also affect the temperature reading.

Alternatively, the LMT85 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 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 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 LMT85 die temperature:

Equation 7. LMT85 equation_5_nis168.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 = 1324 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 LMT85 is the actual temperature being measured, take care to minimize the load current that the LMT85 is required to drive. Thermal Information shows the thermal resistance of the LMT85.

8.4.2 Output and Noise Considerations

A push-pull output gives the LMT85 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 device is ideal for this and other applications which require strong source or sink current.

The LMT85 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. 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.

8.4.3 Capacitive Loads

The LMT85 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 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.

LMT85 no_decoupling_cap_loads_less_nis168.gif Figure 11. LMT85 No Decoupling Required for Capacitive Loads Less Than 1100 pF
LMT85 series_resister_cap_loads_greater_nis168.gif Figure 12. LMT85 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 LMT85 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.