LM2598S-ADJ National Semiconductor, LM2598S-ADJ Datasheet - Page 26

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

www.national.com
Application Information
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 mul-
tilayer PC-board with large copper areas are recommended.
The curves shown in Figure 22 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 junc-
tion 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 compo-
nents 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 mate-
rial 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 25 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 23 and Figure 24
show the effects of Soft-start on the output voltage, the input
current, with, and without a Soft-start capacitor. Figure 23
(Continued)
2
, only
26
also shows the error flag output going high when the output
voltage reaches 95% of the nominal output voltage. The re-
duced input current required at startup is very evident when
comparing the two photos. The Soft-start feature reduces 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.
FIGURE 24. Output Voltage, Input Current, at Start-Up,
FIGURE 23. Output Voltage, Input Current, Error Flag
Signal, at Start-Up, WITH Soft-start
WITHOUT Soft-start
DS012593-41
DS012593-40