Datasheet

LM2598

SNVS125C –MARCH 1998–REVISED APRIL 2013

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OPEN CORE INDUCTORS

Another possible source of increased output ripple voltage or unstable operation is from an open core inductor.

Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to

the other end. These magnetic lines of flux will induce a voltage into any wire or PC board copper trace that

comes within the inductor's magnetic field. The strength of the magnetic field, the orientation and location of the

PC copper trace to the magnetic 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

respect 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 operation. If the circuit now operates correctly,

then the magnetic 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” inductors are an inexpensive, simple way of

making a compact efficient inductor, and they are used by the millions in many different applications.

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 39. Junction Temperature Rise, TO-220

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