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 (R_θJA) 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.

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

10.2 Layout Example

Figure 10. Layout Used for No Heat Sink Measurements

Figure 11. Layout Used for Measurements With Small Heat Sink