lm2598sx-12 National Semiconductor Corporation, lm2598sx-12 Datasheet - Page 26

• Manufacturer Part Number: lm2598sx-12
• Description: Simple Switcher Power Converter 150 Khz 1a Step-down Voltage Regulator, With Features
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
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
22 show the LM2598T junction temperature rises above
ambient 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 numbers are all approximate and there are many factors
that can affect these temperatures. Higher ambient tempera-
tures require 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
Capacitors
Inductor
Diode
PC board
Capacitors
Inductor
Diode
PC board
FIGURE 22. Junction Temperature Rise, TO-220
FIGURE 23. 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 (T)
Package (S)
(Continued)
DS012593-39
26
soldered to should be at least 0.4 in
have 2 or more square inches of 2 oz. (0.0028) in) copper.
Additional copper area improves the thermal characteristics,
but with copper areas greater than approximately 3 in
small improvements in heat dissipation are realized. If fur-
ther thermal improvements are needed, double sided or
multilayer PC-board with large copper areas are recom-
mended.
The curves shown in Figure 23 show the LM2598S (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
junction 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 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.
SHUTDOWN /SOFT-START
The circuit shown in Figure 26 is a standard buck regulator
with 24V in, 12V out, 280 mA load, and using a 0.068 µF
Soft-start capacitor. The photo in Figure 24 and Figure 25
show the effects of Soft-start on the output voltage, the input
current, with, and without a Soft-start capacitor. Figure 24
also shows the error flag output going high when the output
voltage reaches 95% of the nominal output voltage. The
reduced input current required at startup is very evident
when comparing the two photos. The Soft-start feature re-
duces the startup current from 1A down to 240 mA, and
delays and slows down the output voltage rise time.
This reduction in start up current is useful in situations where
the input power source is limited in the amount of current it
can deliver. In some applications Soft-start can be used to
replace undervoltage lockout or delayed startup functions.
If a very slow output voltage ramp is desired, the Soft-start
capacitor can be made much larger. Many seconds or even
minutes are possible.
If only the shutdown feature is needed, the Soft-start capaci-
tor can be eliminated.
2
, and ideally should
2
, only