SNIS176A March   2013  – January 2015 LMT89

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 LMT89 Transfer Function
    4. 7.4 Device Functional Modes
  8. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Capacitive Loads
    2. 8.2 Typical Applications
      1. 8.2.1 Full-Range Centigrade Temperature Sensor
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
        3. 8.2.1.3 Application Curve
      2. 8.2.2 Centigrade Thermostat
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
        3. 8.2.2.3 Application Curve
    3. 8.3 System Examples
      1. 8.3.1 Conserving Power Dissipation With Shutdown
      2. 8.3.2 Analog-to-Digital Converter Input Stage
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Trademarks
    2. 11.2 Electrostatic Discharge Caution
    3. 11.3 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

10 Layout

10.1 Layout Guidelines

The LMT89 device can be applied easily in the same way as other integrated-circuit temperature sensors. It can be glued or cemented to a surface. The temperature that the LMT89 device is sensing will be within about 0.02°C of the surface temperature to which the leads of the LMT89 device are attached.

This presumes that the ambient air temperature is almost the same as the surface temperature; if the air temperature were much higher or lower than the surface temperature, the actual temperature measured would be at an intermediate temperature between the surface temperature and the air temperature.

To ensure good thermal conductivity the backside of the LMT89 die is directly attached to the pin 2 GND pin. The temperatures of the lands and traces to the other leads of the LMT89 will also affect the temperature that is being sensed.

Alternatively, the LMT89 device 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 LMT89 device 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. Printed-circuit coatings and varnishes such as Humiseal and epoxy paints or dips are often used to ensure that moisture cannot corrode the LMT89 or its connections.

The thermal resistance junction to ambient (RθJA) is the parameter used to calculate the rise of a device junction temperature due to its power dissipation. Equation 13 is used to calculate the rise in the die temperature.

Equation 13.   TJ = TA + RθJA [(V+ IQ) + (V+ − VO) IL]

where

  • IQ is the quiescent current and ILis the load current on the output. Because the junction temperature of the LMT89 is the actual temperature being measured, take care to minimize the load current that the LMT89 device is required to drive.

Table 4 summarizes the rise in die temperature of the LMT89 device (without any loading), and the thermal resistance for different conditions.

Table 4. Temperature Rise of LMT89 Due to Self-Heating and Thermal Resistance (RJΘA)(1)

SC70-5 SC70-5
NO HEAT SINK SMALL HEAT SINK
RθJA
(°C/W)		 TJ − TA
(°C)		 RθJA
(°C/W)		 TJ − TA
(°C)
Still air 412 0.2 350 0.19
Moving air 312 0.17 266 0.15
(1) See for PCB layout samples.

10.2 Layout Example

layout_used_for_no_heat_sink_meas_nis176.gifFigure 10. Layout Used for No Heat Sink Measurements
layout_used_for_meas_with_small_heat_sink_nis176.gifFigure 11. Layout Used for Measurements With Small Heat Sink