11.1 Layout Guidelines

The LM57 can be applied easily in the same way as other integrated-circuit temperature sensors. It can be glued or cemented to a surface. The temperatures of the lands and traces to the other leads of the LM57 will also affect the temperature reading.

Alternatively, the LM57 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 LM57 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 V_TEMP output to ground or V_DD, the V_TEMP output from the LM57 will not be correct. Printed-circuit coatings are often used to ensure that moisture cannot corrode the leads or circuit traces.

11.2 Layout Example

Figure 25. PW (TSSOP) Package Layout Example

The best thermal conductivity to the junction of the LM57 is through the DAP. Make sure it is connected to the surface whose temperature that is being measured.

Figure 26. SD (WSON) Package Layout Example

11.3 Temperature Considerations

The junction temperature of the LM57 is the actual temperature being measured. The thermal resistance junction-to-ambient (R_θJA) is the parameter (from Thermal Information) used to calculate the rise of a device junction temperature due to its power dissipation. Equation 3 is used to calculate the rise in the die temperature of the LM57.

Equation 3.

where

• T_A is the ambient temperature.
• I_Q is the quiescent current.
• I_L is the load current on V_TEMP.
• R_θJA can be found in Thermal Information

For example using an LM57 in the PW (TSSOP) package, in an application where T_A = 30°C, V_DD = 5.5 V, I_DD = 28 μA, J5 gain, V_TEMP = 2368 mV, and I_L = 0 μA, the total temperature rise would be [183°C/W × 5.5 V × 28 μA] = 0.028°C. To minimize self-heating, the load current on V_TEMP should be minimized.