LM2594 datasheet(22/28 Pages) NSC

22 / 28 page

Application Information (Continued)

These magnetic lines of flux will induce a voltage into any

wire or PC board copper trace that comes within the induc-

tor’s magnetic field. The strength of the magnetic field, the

orientation and location of the PC copper trace to the mag-

netic field, and the distance between the copper trace and

the inductor, determine the amount of voltage generated in

the copper trace. Another way of looking at this inductive

coupling is to consider the PC board copper trace as one

turn of a transformer (secondary) with the inductor winding

as the primary. Many millivolts can be generated in a copper

trace located near an open core inductor which can cause

stability problems or high output ripple voltage problems.

If unstable operation is seen, and an open core inductor is

used, it’s possible that the location of the inductor with re-

spect to other PC traces may be the problem. To determine

if this is the problem, temporarily raise the inductor away

from the board by several inches and then check circuit op-

eration. If the circuit now operates correctly, then the mag-

netic flux from the open core inductor is causing the problem.

Substituting a closed core inductor such as a torroid or

E-core will correct the problem, or re-arranging the PC layout

may be necessary. Magnetic flux cutting the IC device

ground trace, feedback trace, or the positive or negative

traces of the output capacitor should be minimized.

Sometimes, locating a trace directly beneath a bobbin in-

ductor will provide good results, provided it is exactly in the

center of the inductor (because the induced voltages cancel

themselves out), but if it is off center one direction or the

other, then problems could arise. If flux problems are

present, even the direction of the inductor winding can make

a difference in some circuits.

This discussion on open core inductors is not to frighten the

user, but to alert the user on what kind of problems to watch

out for when using them. Open core bobbin or “stick” induc-

tors are an inexpensive, simple way of making a compact ef-

ficient inductor, and they are used by the millions in many dif-

ferent applications.

THERMAL CONSIDERATIONS

The LM2594/LM2594HV 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 LM2594 and

the other heat producing components.

For best thermal performance, wide copper traces should be

used and all ground and unused pins should be soldered to

generous amounts of printed circuit board copper, such as a

ground plane (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 resis-

tance) to the surrounding air, and even double-sided or mul-

tilayer boards provide a better heat path to the surrounding

air. Unless power levels are small, sockets are not recom-

mended because of the added thermal 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.

DS012439-35

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 19. Junction Temperature Rise, DIP-8

DS012439-34

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 20. Junction Temperature Rise, SO-8

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