JAJSAC3D April 2006 – September 2021 LMH6321
PRODUCTION DATA
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For some applications, a heat sink may be required with the LMH6321. This depends on the maximum power dissipation and maximum ambient temperature of the application. To accomplish heat sinking, the tabs on DDPAK and SO PowerPAD package may be soldered to the copper plane of a PCB for heatsinking (note that these tabs are electrically connected to the most negative point in the circuit, for example,V−).
Heat escapes from the device in all directions, mainly through the mechanisms of convection to the air above it and conduction to the circuit board below it and then from the board to the air. Natural convection depends on the amount of surface area that is in contact with the air. If a conductive plate serving as a heatsink is thick enough to ensure perfect thermal conduction (heat spreading) into the far recesses of the plate, the temperature rise would be simply inversely proportional to the total exposed area. PCB copper planes are, in that sense, an aid to convection, the difference being that they are not thick enough to ensure perfect conduction. Therefore, eventually we will reach a point of diminishing returns (as seen in Figure 6-5). Very large increases in the copper area will produce smaller and smaller improvement in thermal resistance. This occurs, roughly, for a 1 inch square of 1 oz copper board. Some improvement continues until about 3 square inches, especially for 2 oz boards and better, but beyond that, external heatsinks are required. Ultimately, a reasonable practical value attainable for the junction to ambient thermal resistance is about 30 °C/W under zero air flow.
A copper plane of appropriate size may be placed directly beneath the tab or on the other side of the board. If the conductive plane is placed on the back side of the PCB, it is recommended that thermal vias be used per JEDEC Standard JESD51-5.