SNIS178 October   2017 LMT84-Q1

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 LMT84 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 Applications 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 Examples
  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 LMT84-Q1 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)
LMT84-Q1 FBD_01_SNIS167.gif

8.3 Feature Description

8.3.1 LMT84 Transfer Function

The output voltage of the LMT84-Q1, across the complete operating temperature range, is shown in Table 3. This table is the reference from which the LMT84-Q1 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-Q1 product folder under Tools and Software Models.

Table 3. LMT84-Q1 Transfer Table

TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)		 TEMP
(°C)		 VOUT
(mV)
–50 1299 -10 1088 30 871 70 647 110 419
–49 1294 -9 1082 31 865 71 642 111 413
–48 1289 -8 1077 32 860 72 636 112 407
–47 1284 -7 1072 33 854 73 630 113 401
–46 1278 -6 1066 34 849 74 625 114 396
–45 1273 -5 1061 35 843 75 619 115 390
–44 1268 -4 1055 36 838 76 613 116 384
–43 1263 -3 1050 37 832 77 608 117 378
–42 1257 -2 1044 38 827 78 602 118 372
–41 1252 -1 1039 39 821 79 596 119 367
–40 1247 0 1034 40 816 80 591 120 361
–39 1242 1 1028 41 810 81 585 121 355
–38 1236 2 1023 42 804 82 579 122 349
–37 1231 3 1017 43 799 83 574 123 343
–36 1226 4 1012 44 793 84 568 124 337
–35 1221 5 1007 45 788 85 562 125 332
–34 1215 6 1001 46 782 86 557 126 326
–33 1210 7 996 47 777 87 551 127 320
–32 1205 8 990 48 771 88 545 128 314
–31 1200 9 985 49 766 89 539 129 308
–30 1194 10 980 50 760 90 534 130 302
–29 1189 11 974 51 754 91 528 131 296
–28 1184 12 969 52 749 92 522 132 291
–27 1178 13 963 53 743 93 517 133 285
–26 1173 14 958 54 738 94 511 134 279
–25 1168 15 952 55 732 95 505 135 273
–24 1162 16 947 56 726 96 499 136 267
–23 1157 17 941 57 721 97 494 137 261
–22 1152 18 936 58 715 98 488 138 255
–21 1146 19 931 59 710 99 482 139 249
–20 1141 20 925 60 704 100 476 140 243
–19 1136 21 920 61 698 101 471 141 237
–18 1130 22 914 62 693 102 465 142 231
–17 1125 23 909 63 687 103 459 143 225
–16 1120 24 903 64 681 104 453 144 219
–15 1114 25 898 65 676 105 448 145 213
–14 1109 26 892 66 670 106 442 146 207
–13 1104 27 887 67 664 107 436 147 201
–12 1098 28 882 68 659 108 430 148 195
–11 1093 29 876 69 653 109 425 149 189
150 183

Although the LMT84-Q1 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. LMT84-Q1 ParaEq_G00_SNIS167.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. LMT84-Q1 ParEqSol_SNIS167.gif

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):

Equation 3. LMT84-Q1 equation_1_nis167.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. LMT84-Q1 equation_2_nis167.gif
Equation 5. LMT84-Q1 equation_3_nis167.gif
Equation 6. LMT84-Q1 equation_4_nis167.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 LMT84-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 LMT84 die is directly attached to the GND pin. The temperatures of the lands and traces to the other leads of the LMT84-Q1 will also affect the temperature reading.

Alternatively, the LMT84-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 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-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) 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-Q1 die temperature:

Equation 7. LMT84-Q1 equation_5_nis167.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 = 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-Q1.

8.4.2 Output Noise Considerations

A push-pull output gives the LMT84-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 LMT84 is ideal for this and other applications which require strong source or sink current.

The LMT84-Q1 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-Q1.

8.4.3 Capacitive Loads

The LMT84-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 LMT84-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.

LMT84-Q1 no_decoupling_cap_loads_less_nis167.gif Figure 10. LMT84-Q1 No Decoupling Required for Capacitive Loads Less Than 1100 pF
LMT84-Q1 series_resister_cap_loads_greater_nis167.gif Figure 11. LMT84-Q1 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 LMT84-Q1 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.