ZHCS754A March 2013 – January 2015 LMT88
PRODUCTION DATA.
10 Layout
10.1 Layout Guidelines
The LMT88 can be applied easily in the same way as other IC temperature sensors. The device can be glued or cemented to a surface. The temperature that the LMT88 is sensing will be within about 0.02°C of the surface temperature to which the leads of LMT88 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 LMT88 die is directly attached to the pin 2 GND pin. The temperatures of the lands and traces to the other leads of the LMT88 will also affect the temperature that is being sensed.
Alternatively, the LMT88 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 LMT88 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 a conformal coating and epoxy paints or dips are often used to ensure that moisture cannot corrode the LMT88 or its connections.
10.2 Layout Example
Figure 10. Layout Used for No Heat Sink Measurements
Figure 11. Layout Used for Measurements With Small Heat Sink
10.3 Thermal Considerations
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. For the LMT88, Equation 14 is used to calculate the rise in the die temperature:
where
- IQ is the quiescent current and ILis the load current on the output.
Because the junction temperature of the LMT88 is the actual temperature being measured, take care to minimize the load current that the LMT88 is required to drive.
Table 4 summarizes the rise in die temperature of the LMT88 without any loading, and the thermal resistance for different conditions.
Table 4. Temperature Rise of LMT88 Due to Self-Heating and Thermal Resistance (θJA)(1)
| SC70-5 | SC70-5 | |||
|---|---|---|---|---|
| NO HEAT SINK | SMALL HEAT SINK | |||
| θJA
(°C/W) |
TJ − TA
(°C) |
θJA
(°C/W) |
TJ − TA
(°C) |
|
| Still air | 412 | 0.2 | 350 | 0.19 |
| Moving air | 312 | 0.17 | 266 | 0.15 |