LM2598S-ADJ National Semiconductor, LM2598S-ADJ Datasheet - Page 25

• Manufacturer Part Number: LM2598S-ADJ
• Description: SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator/ with Features
• Manufacturer: National Semiconductor

Application Information
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.
(Continued)
DS012593-38
25
THERMAL CONSIDERATIONS
The LM2598 is available in two packages, a 7-pin TO-220
(T) and a 7-pin surface mount TO-263 (S).
The TO-220 package can be used without a heat sink for
ambient temperatures up to approximately 50˚C (depending
on the output voltage and load current). The curves in Figure
21 show the LM2598T junction temperature rises above am-
bient temperature for different input and output voltages. The
data for these curves was taken with the LM2598T (TO-220
package) operating as a switching regulator in an ambient
temperature of 25˚C (still air). These temperature rise num-
bers are all approximate and there are many factors that can
affect these temperatures. Higher ambient temperatures re-
quire some heat sinking, either to the PC board or a small
external heat sink.
The TO-263 surface mount package tab is designed to be
soldered to the copper on a printed circuit board. 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 PC board copper area that the package is
soldered to should be at least 0.4 in
have 2 or more square inches of 2 oz. (0.0028) in) copper.
Capacitors
Inductor
Diode
PC board
Capacitors
Inductor
Diode
PC board
FIGURE 21. Junction Temperature Rise, TO-220
FIGURE 22. Junction Temperature Rise, TO-263
Circuit Data for Temperature Rise Curve TO-220
Circuit Data for Temperature Rise Curve TO-263
Through hole electrolytic
Through hole, Schott, 68 µH
Through hole, 3A 40V, Schottky
3 square inches single sided 2 oz. copper
(0.0028")
Surface mount tantalum, molded “D” size
Surface mount, Schott, 68 µH
Surface mount, 3A 40V, Schottky
3 square inches single sided 2 oz. copper
(0.0028")
Package (S)
Package (T)
2
, and ideally should
DS012593-39
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