LM2599SX-12/NOPB National Semiconductor, LM2599SX-12/NOPB Datasheet - Page 25

• Manufacturer Part Number: LM2599SX-12/NOPB
• Description: IC REG SW 12V 3A STP DN TO-263-7
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM2599SX-12/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 12V
• Current - Output: 3A
• Frequency - Switching: 150kHz
• Voltage - Input: 4.5 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: D²Pak, TO-263 (7 leads + tab)
• For Use With: 551011367-021 - BOARD WEBENCH BUILD IT LM2599
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: *LM2599SX-12; *LM2599SX-12/NOPB; LM2599SX-12

Application Information
but with copper areas greater than approximately 6 in
small improvements in heat dissipation are realized. If fur-
ther thermal improvements are needed, double sided, mul-
tilayer pc-board with large copper areas and/or airflow are
recommended.
The curves shown in Figure 23 show the LM2599S (TO-263
package) junction temperature rise above ambient tempera-
ture with a 2A 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. When load currents higher than 2A are
used, double sided or multilayer pc-boards with large copper
areas and/or airflow might be needed, especially for high
ambient temperatures and high output voltages.
Capacitors
Inductor
Diode
PC board
FIGURE 22. Junction Temperature Rise, TO-220
Circuit Data for Temperature Rise Curve TO-220
Through hole electrolytic
Through hole Renco
Through hole, 5A 40V, Schottky
3 square inches single sided 2 oz. copper
(0.0028")
Package (T)
(Continued)
DS012582-38
2
, only
25
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 20V in, 12V out, 1A load, and using a 0.068 µF Soft-start
capacitor. The photo in Figure 24 Figure 25 show the effects
of Soft-start on the output voltage, the input current, with,
and without a Soft-start capacitor. The reduced input current
required at startup is very evident when comparing the two
photos. The Soft-start feature reduces the startup current
from 2.6A down to 650 mA, and delays and slows down the
output voltage rise time.
Capacitors
Inductor
Diode
PC board
FIGURE 23. Junction Temperature Rise, TO-263
Circuit Data for Temperature Rise Curve TO-263
Surface mount tantalum, molded “D” size
Surface mount, Pulse engineering, 68 µH
Surface mount, 5A 40V, Schottky
9 square inches single sided 2 oz. copper
(0.0028")
Package (S)
DS012582-39
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