LM2598 datasheet(39/47 Pages) TI1

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LM2598

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SNVS125D – MARCH 1998 – REVISED MAY 2016

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Thermal Considerations (continued)

The DDPAK surface mount package tab is designed to be soldered to the copper on a printed-circuit board

(PCB). The copper and the board are the heat sink for this package and the other heat producing components,

such as the catch diode and inductor. The PCB copper area that the package is soldered to must be at least 0.4

in2, and ideally must have 2 or more square inches of 2 oz. (0.0028) in) copper. Additional copper area improves

the thermal characteristics, but with copper areas greater than approximately 3 in2, only small improvements in

heat dissipation are realized. If further thermal improvements are required, TI recommends double-sided or

multilayer PCB with large copper areas.

Figure 41 shows the LM2598S (DDPAK package) junction temperature rise above ambient temperature with a 1-

A load for various input and output voltages. This data was taken with the circuit operating as a buck switching

regulator with all components mounted on a PCB to simulate the junction temperature under actual operating

conditions. This curve can be used for a quick check for the approximate junction temperature for various

conditions, but be aware that there are many factors that can affect the junction temperature.

For the best thermal performance, wide copper traces and generous amounts of PCB copper must be used in

the board layout. (One exception to this is the output (switch) pin, which must not have large areas of copper.)

Large areas of copper provide the best transfer of heat (lower thermal resistance) to the surrounding air, and

moving air lowers the thermal resistance even further.

Package thermal resistance and junction temperature rise numbers are all approximate, and there are many

factors that affect these numbers. Some of these factors include board size, shape, thickness, position, location,

and even board temperature. Other factors are trace width, total printed-circuit copper area, copper thickness,

single- or double-sided multilayer board, and the amount of solder on the board. The effectiveness of the PCB to

dissipate heat also depends on the size, quantity, and spacing of other components on the board, as well as

whether the surrounding air is still or moving. Furthermore, some of these components such as the catch diode

adds heat to the PCB and the heat can vary as the input voltage changes. For the inductor, depending on the

physical size, type of core material, and the DC resistance, it could either act as a heat sink taking heat away

from the board, or it could add heat to the board.

LM2598 Datasheet(PDF) 39 Page - Texas Instruments