LM2595 datasheet(22/29 Pages) NSC

22 / 29 page

Application Information (Continued)

but with copper areas greater than approximately 3 in

2, only

small improvements in heat dissipation are realized. If fur-

ther thermal improvements are needed, double sided or mul-

tilayer PC-board with large copper areas are recommended.

The curves shown in

Figure 20 show the LM2595S (TO-263

package) junction temperature rise above ambient tempera-

ture with a 1A 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 PC board 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 junc-

tion temperature.

For the best thermal performance, wide copper traces and

generous amounts of printed circuit board copper should be

used in the board layout. (One exception to this is the output

(switch) pin, which should 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 will 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 PC board to dissipate

heat also depends on the size, quantity and spacing of other

components on the board, as well as whether the surround-

ing air is still or moving. Furthermore, some of these compo-

nents such as the catch diode will add heat to the PC board

and the heat can vary as the input voltage changes. For the

inductor, depending on the physical size, type of core mate-

rial 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.

DS012565-34

Circuit Data for Temperature Rise Curve

TO-220 Package (T)

Capacitors

Through hole electrolytic

Inductor

Through hole, Schott, 68 µH

Diode

Through hole, 3A 40V, Schottky

PC board

3 square inches single sided 2 oz. copper

(0.0028")

FIGURE 19. Junction Temperature Rise, TO-220

DS012565-35

Circuit Data for Temperature Rise Curve

TO-263 Package (S)

Capacitors

Surface mount tantalum, molded “D” size

Inductor

Surface mount, Schott, 68 µH

Diode

Surface mount, 3A 40V, Schottky

PC board

3 square inches single sided 2 oz. copper

(0.0028")

FIGURE 20. Junction Temperature Rise, TO-263

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