SNIS187A March   2015  – July 2015 LMT70 , LMT70A

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Wide-Range Precision Active RTD or NTC Replacement (−55°C to 150°C)
  5. Revision History
  6. Device Comparison Table
  7. Pin Configuration and Functions
  8. Specifications
    1. 8.1 Absolute Maximum Ratings
    2. 8.2 ESD Ratings
    3. 8.3 Recommended Operating Conditions
    4. 8.4 Thermal Information
    5. 8.5 Electrical Characteristics
    6. 8.6 Electrical Characteristics Temperature Lookup Table (LUT)
    7. 8.7 Switching Characteristics
    8. 8.8 Typical Performance Characteristics
  9. Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Temperature Analog Output (TAO)
        1. 9.3.1.1 LMT70 Output Transfer Function
          1. 9.3.1.1.1 First Order Transfer Function
          2. 9.3.1.1.2 Second Order Transfer Function
          3. 9.3.1.1.3 Third Order Transfer Function
        2. 9.3.1.2 LMT70A TAO Matching
        3. 9.3.1.3 TAO Noise Considerations
        4. 9.3.1.4 TAO Capacitive Loads
      2. 9.3.2 TON Digital Input
      3. 9.3.3 Light Sensitivity
    4. 9.4 Device Functional Modes
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Temperature Algorithm Selection
        2. 10.2.2.2 ADC Requirements
      3. 10.2.3 Finer Resolution LUT
      4. 10.2.4 Application Curves
    3. 10.3 System Examples
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
      1. 12.1.1 Mounting and Temperature Conductivity
    2. 12.2 Layout Example
  13. 13Device and Documentation Support
    1. 13.1 Related Links
    2. 13.2 Documentation Support
      1. 13.2.1 Related Documentation
    3. 13.3 Community Resources
    4. 13.4 Trademarks
    5. 13.5 Electrostatic Discharge Caution
    6. 13.6 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

10.1 Application Information

The LMT70 analog output temperature sensor is an ideal device to connect to an integrated 12-Bit ADC such as that found in the MSP430 microcontroller family.

Applications for the LMT70 included but are not limited to: IoT based temperature sensor nodes, medical fitness equipment (e.g. thermometers, fitness/smart bands or watches, activity monitors, human body temperature monitor), Class AA or lower RTD replacement, precision NTC or PTC thermistor replacement, instrumentation temperature compensation, metering temperature compensation (e. g. heat cost allocator, heat meter).

10.2 Typical Application

LMT70 LMT70A Schem_03_SNIS187.gifFigure 26. Typical Application Schematic

Most CMOS ADCs found in microcontrollers and ASICs have a sampled data comparator input structure. When the ADC charges the sampling cap, it requires instantaneous charge from the output of the analog source such as the LMT70 temperature sensor and many op amps. This requirement is easily accommodated by the addition of a capacitor (CFILTER) or the extension of the ADC acquisition time thus slowing the ADC sampling rate. The size of CFILTER depends on the size of the sampling capacitor and the sampling frequency. Since not all ADCs have identical input stages, the charge requirements will vary. The general ADC application shown in Figure 27 is an example only. The application in Figure 26 was actually tried and the extension of the MSP430 12-Bit ADC acquisition time was all that was necessary in order to accommodate the LMT70's output stage drive capability.

LMT70 LMT70A Schem_04_SNIS187.gifFigure 27. Suggested Connection to a Sampling Analog-to-Digital Converter Input Stage

10.2.1 Design Requirements

The circuit show in Figure 26 will support the design requirements as shown in Table 3.

Table 3. Design Requirements

PARAMETER TARGET SPECIFICATION
Temperature Range -40°C to +150°C LMT70, -40°C to +85°C for MSP430
Accuracy ±0.2°C typical over full temperature range
VDD 2.2V to 3.6V with typical of 3.0V
IDD 12µA

10.2.2 Detailed Design Procedure

10.2.2.1 Temperature Algorithm Selection

Of the three algorithms presented in this datasheet, linear interpolation, second order transfer function or third order transfer function, the one selected will be determined by the users microcontroller resources and the temperature range that will be sensed. Therefore, a comparison of the expected accuracy from the LMT70 is given here. The following curves show effect on the accuracy of the LMT70 when using each of the different algorithms/equations given in LMT70 Output Transfer Function. The first curve (Figure 28) shows the performance when using linear interpolation of the LUT values shown in Electrical Characteristics Temperature Lookup Table (LUT) of every 10°C and provides the best performance. Linear interpolation of the LUT values shown in Electrical Characteristics Temperature Lookup Table (LUT) is used to determine the LMT70 min/max accuracy limits as shown in the Electrical Characteristics and the red lines of Figure 28. The other lines in the middle of Figure 28 show independent device performance. The green limit lines, shown in the subsequent figures, apply for the specific equation used to convert the output voltage of the LMT70 to temperature. The equations are shown under each figure for reference purposes. The green lines show the min/max limits when set in a similar manner to the red limit lines of Figure 28. The limits shown in red for Figure 28 are repeated in all the figures of this section for comparison purposes.

LMT70 LMT70A C001_SNIS187.png
Temp
(°C)
VTAO (mV) Local Slope
(mV/°C)
MIN TYP MAX
20 994.367 995.050 995.734 -5.171
30 942.547 943.227 943.907 -5.194
40 890.423 891.178 891.934 -5.217
50 838.097 838.882 839.668 -5.241
Figure 28. LMT70 Performance Using LUT and Linear Interpolation
LMT70 LMT70A C017_SNIS187.png
TM = -1.809628E-09 (VTAO)3 – 3.325395E-06 (VTAO)2 – 1.814103E-01(VTAO) + 2.055894E+02
Figure 30. Using Third Order Transfer Function Best Fit -10°C to +110°C
LMT70 LMT70A C015_SNIS187.png
TM = -7.857923E-06 (VTAO)2 – 1.777501E-01 (VTAO) + 2.046398E+02
Figure 32. Using Second Order Transfer Function Best Fit -10°C to 110°C
LMT70 LMT70A C016_SNIS187.png
TM = -1.064200E-09 (VTAO)3 – 5.759725E-06 (VTAO)2 – 1.789883E-01(VTAO) + 2.048570E+02
Figure 29. Using Third Order Transfer Function Best Fit -55°C to +150°C
LMT70 LMT70A C014_SNIS187.png
TM = -8.451576E-06 (VTAO)2– 1.769281E-01 (VTAO) + 2.043937E+02
Figure 31. Using Second Order Transfer Function Best Fit -55°C to 150°C

10.2.2.2 ADC Requirements

The ADC resolution and its specifications as well as reference voltage and its specifications will determine the overall system accuracy that you can obtain. For this example the 12-bit SAR ADC found in the MSP430 was used as well as it's integrated reference. At first glance the specifications may not seem to be precise enough to actually be used with the LMT70 but the MSP430 ADC and integrated reference errors are actually measured during production testing of the MSP430. Values are then provided to user for software calibration. These calibration values are located in the MSP430A device descriptor tag-length-value (TLV) structure and found in the device-specific datasheet. The MSP430 Users Guide includes information on how to use these calibration values to calibrate the ADC reading. The specific values used to calibrate the ADC readings are: CAL_ADC_15VREF_FACTOR, CAL_ADC_GAIN_FACTOR and CAL_ADC_OFFSET.

10.2.3 Finer Resolution LUT

The following table is given for reference only and not meant to be used for calculation purposes.

Temp
(°C)
VTAO
(mV)
Temp
(°C)
VTAO
(mV)
Temp
(°C)
VTAO
(mV)
Temp
(°C)
VTAO
(mV)
Temp
(°C)
VTAO
(mV)
Temp
(°C)
VTAO
(mV)
Temp
(°C)
VTAO
(mV)
Temp
(°C)
VTAO
(mV)
TYP TYP TYP TYP TYP TYP TYP TYP
-30 1250.398 0 1097.987 30 943.227 60 786.360 90 627.490 120 466.760 150 302.785
-29 1244.953 1 1092.532 31 937.729 61 780.807 91 621.896 121 460.936
-28 1239.970 2 1087.453 32 932.576 62 775.580 92 616.603 122 455.612
-27 1234.981 3 1082.370 33 927.418 63 770.348 93 611.306 123 450.280
-26 1229.986 4 1077.282 34 922.255 64 765.113 94 606.006 124 444.941
-55 1375.219 -25 1224.984 5 1072.189 35 917.087 65 759.873 95 600.701 125 439.593
-54 1370.215 -24 1219.977 6 1067.090 36 911.915 66 754.628 96 595.392 126 434.238
-53 1365.283 -23 1214.963 7 1061.987 37 906.738 67 749.380 97 590.079 127 428.875
-52 1360.342 -22 1209.943 8 1056.879 38 901.556 68 744.127 98 584.762 128 423.504
-51 1355.395 -21 1204.916 9 1051.765 39 896.370 69 738.870 99 579.442 129 418.125
-50 1350.441 -20 1199.884 10 1046.647 40 891.178 70 733.608 100 574.117 130 412.739
-49 1345.159 -19 1194.425 11 1041.166 41 885.645 71 728.055 101 568.504 131 406.483
-48 1340.229 -18 1189.410 12 1036.062 42 880.468 72 722.804 102 563.192 132 401.169
-47 1335.293 -17 1184.388 13 1030.952 43 875.287 73 717.550 103 557.877 133 395.841
-46 1330.352 -16 1179.361 14 1025.838 44 870.100 74 712.292 104 552.557 134 390.499
-45 1325.405 -15 1174.327 15 1020.720 45 864.909 75 707.029 105 547.233 135 385.144
-44 1320.453 -14 1169.288 16 1015.596 46 859.713 76 701.762 106 541.905 136 379.775
-43 1315.496 -13 1164.242 17 1010.467 47 854.513 77 696.491 107 536.573 137 374.393
-42 1310.534 -12 1159.191 18 1005.333 48 849.307 78 691.217 108 531.236 138 368.997
-41 1305.566 -11 1154.134 19 1000.194 49 844.097 79 685.937 109 525.895 139 363.587
-40 1300.593 -10 1149.070 20 995.050 50 838.882 80 680.654 110 520.551 140 358.164
-39 1295.147 -9 1143.654 21 989.583 51 833.343 81 675.073 111 514.886 141 351.937
-38 1290.202 -8 1138.599 22 984.450 52 828.141 82 669.803 112 509.557 142 346.508
-37 1285.250 -7 1133.540 23 979.313 53 822.934 83 664.528 113 504.223 143 341.071
-36 1280.291 -6 1128.476 24 974.171 54 817.723 84 659.250 114 498.885 144 335.625
-35 1275.326 -5 1123.407 25 969.025 55 812.507 85 653.967 115 493.542 145 330.172
-34 1270.353 -4 1118.333 26 963.875 56 807.287 86 648.680 116 488.195 146 324.711
-33 1265.375 -3 1113.254 27 958.720 57 802.062 87 643.389 117 482.843 147 319.241
-32 1260.389 -2 1108.170 28 953.560 58 796.832 88 638.094 118 477.486 148 313.764
-31 1255.397 -1 1103.081 29 948.396 59 791.598 89 632.794 119 472.125 149 308.279

10.2.4 Application Curves

The LMT70 performance using the MSP430 with integrated 12-bit ADC is shown in Figure 33. This curve includes the error of the MSP430 integrated 12-bit ADC and reference as shown in the schematic Figure 26. The MSP430 was kept at room temperature and the LMT70 was submerged in a precision temperature calibration oil bath. A calibrated temperature probe was used to monitor the temperature of the oil. As can be seen in Figure 33 the combined performance on the MSP430 and the LMT70 is better than 0.12°C for the entire -40°C to +150°C temperature range. The only calibration performed was with software using the MSP430A device descriptor tag-length-value (TLV) calibration values for ADC and VREF error.

LMT70 LMT70A C023_SNIS187.pngFigure 33. LMT70 with MSP430 typical performance

10.3 System Examples

LMT70 LMT70A Schem_02_SNIS187.gifFigure 34. Multiple LMT70s connected to one 12-bit ADC channel on an MSP430
LMT70 LMT70A Schem_01_SNIS187.gifFigure 35. Multiple LMT70s connected to a slope ADC for high resolution