LM2597 datasheet(27/34 Pages) NSC

27 / 34 page

Application Information (Continued)

THERMAL CONSIDERATIONS

The LM2597/LM2597HV is available in two packages, an

8-pin through hole DIP (N) and an 8-pin surface mount SO-8

(M). Both packages are molded plastic with a copper lead

frame. When the package is soldered to the PC board, the

copper and the board are the heat sink for the LM2597 and

the other heat producing components.

For best thermal performance, wide copper traces should be

used. Pins should be soldered to generous amounts of

printed circuit board copper, (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

even double-sided or multilayer boards provide a better heat

path to the surrounding air. Unless power levels are small,

sockets are not recommended because of the added ther-

mal resistance it adds and the resultant higher junction

temperatures.

Package thermal resistance and junction temperature rise

numbers are all approximate, and there are many factors

that will affect the junction temperature. Some of these fac-

tors include board size, shape, thickness, position, location,

and even board temperature. Other factors are, trace width,

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. Furthermore, some of these com-

ponents 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

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.

The curves shown in

Figure 22 and Figure 23 show the

LM2597 junction temperature rise above ambient tempera-

ture with a 500 mA load for various input and output volt-

ages. The Bias Supply pin was not used (left open) for these

curves. Connecting the Bias Supply pin to the output voltage

would reduce the junction temperature by approximately 5˚C

to 15˚C, depending on the input and output voltages, and the

load current. This data was taken with the circuit operating

as a buck switcher with all components mounted on a PC

board to simulate the junction temperature under actual

operating conditions. This curve is typical, and can be used

for a quick check on the maximum junction temperature for

various conditions, but keep in mind that there are many

factors that can affect the junction temperature.

BIAS SUPPLY FEATURE

The bias supply (V

BS) pin allows the LM2597’s internal

circuitry to be powered from a power source, other than V

IN,

typically the output voltage. This feature can increase effi-

ciency and lower junction temperatures under some operat-

ing conditions. The greatest increase in efficiency occur with

light load currents, high input voltage and low output voltage

(4V to 12V). See efficiency curves shown in

Figure 24 and

Figure 25. The curves with solid lines are with the V

BS pin

connected to the regulated output voltage, while the curves

with dashed lines are with the V

BS pin open.

The bias supply pin requires a minimum of approximately

3.5V at room temperature (4V @ −40˚C), and can be as high

as 30V, but there is little advantage of using the bias supply

feature with voltages greater than 15V or 20V. The current

required for the V

IN pin is typically 4 mA.

DS012440-41

Circuit Data for Temperature Rise Curve (DIP-8)

Capacitors Through hole electrolytic

Inductor

Through hole, Schott, 100 µH

Diode

Through hole, 1A 40V, Schottky

PC board

4 square inches single sided 2 oz. copper

(0.0028")

FIGURE 22. Junction Temperature Rise, DIP-8

DS012440-42

Circuit Data for Temperature Rise Curve (Surface

Mount)

Capacitors Surface mount tantalum, molded “D” size

Inductor

Surface mount, Coilcraft DO33, 100 µH

Diode

Surface mount, 1A 40V, Schottky

PC board

4 square inches single sided 2 oz. copper

(0.0028")

FIGURE 23. Junction Temperature Rise, SO-8

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