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9.2.2.9 Maximum Ambient Temperature

As with any power conversion device, the LMR436x0 dissipates internal power while operating. The effect of this power dissipation is to raise the internal temperature of the converter above ambient. The internal die temperature (T_J) is a function of the ambient temperature, the power loss, and the effective thermal resistance, R_θJA, of the device, and PCB combination. The maximum junction temperature for the LMR436x0 must be limited to 150°C. This establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 12 shows the relationships between the important parameters. It is easy to see that larger ambient temperatures (T_A) and larger values of R_θJA reduce the maximum available output current. The converter efficiency can be estimated by using the curves provided in this data sheet. If the desired operating conditions cannot be found in one of the curves, interpolation can be used to estimate the efficiency. Alternatively, the EVM can be adjusted to match the desired application requirements and the efficiency can be measured directly. The correct value of R_θJA is more difficult to estimate. For more information reference Semiconductor and IC Package Thermal Metrics Application Report.

Equation 12.

I_{O U T |_{M A X} = \frac{(T_{J} - T_{A})}{R_{θ J A}} \times \frac{η}{(1 - η)} \times \frac{1}{V_{O U T}}}

where

η is the efficiency.

The effective R_θJA is a critical parameter and depends on many factors such as the following:

Power dissipation
Air temperature/flow
PCB area
Copper heat-sink area
Number of thermal vias under the package
Adjacent component placement

The IC junction temperature can be estimated for a given operating condition using Equation 13.

Equation 13.

T_{J} ≅ T_{A} + R_{θ J A} \times I C P o w e r L o s s

where

T_J is the IC junction temperature (ºC).
T_A is the ambient temperature (ºC).
R_θJA is the thermal resistance (ºC/W).
IC power loss is the power loss for the IC (W).

The IC Power loss mentioned above is the overall power loss minus the loss that comes from the inductor DC resistance. The overall power loss can be approximated by using WEBENCH for a specific operating condition and temperature.

Use the following resources as guides to optimal thermal PCB design and estimating R_θJA for a given application environment:

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9.2.2.9 Maximum Ambient Temperature