lm2597hvn-5.0 National Semiconductor Corporation, lm2597hvn-5.0 Datasheet - Page 27

• Manufacturer Part Number: lm2597hvn-5.0
• Description: Simple Switcher Power Converter 150 Khz 0.5a Step-down Voltage Regulator, With Features
• Manufacturer: National Semiconductor Corporation

Application Information
Capacitors Through hole electrolytic
Inductor
Diode
PC board
Capacitors Surface mount tantalum, molded “D” size
Inductor
Diode
PC board
Circuit Data for Temperature Rise Curve (Surface
FIGURE 22. Junction Temperature Rise, DIP-8
Circuit Data for Temperature Rise Curve (DIP-8)
FIGURE 23. Junction Temperature Rise, SO-8
Through hole, Schott, 100 µH
Through hole, 1A 40V, Schottky
4 square inches single sided 2 oz. copper
(0.0028")
Surface mount, Coilcraft DO33, 100 µH
Surface mount, 1A 40V, Schottky
4 square inches single sided 2 oz. copper
(0.0028")
Mount)
(Continued)
DS012440-41
DS012440-42
27
THERMAL CONSIDERATIONS
The LM2597/LM2597HV is available in two packages, an
8-pin through hole DIP (N) and an 8-pin surface mount SO-8
(M). Both packages are molded plastic with a copper lead
frame. When the package is soldered to the PC board, the
copper and the board are the heat sink for the LM2597 and
the other heat producing components.
For best thermal performance, wide copper traces should be
used. Pins should be soldered to generous amounts of
printed circuit board copper, (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
even double-sided or multilayer boards provide a better heat
path to the surrounding air. Unless power levels are small,
sockets are not recommended because of the added ther-
mal resistance it adds and the resultant higher junction
temperatures.
Package thermal resistance and junction temperature rise
numbers are all approximate, and there are many factors
that will affect the junction temperature. Some of these fac-
tors include board size, shape, thickness, position, location,
and even board temperature. Other factors are, trace width,
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. 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.
The curves shown in Figure 22 and Figure 23 show the
LM2597 junction temperature rise above ambient tempera-
ture with a 500 mA load for various input and output volt-
ages. The Bias Supply pin was not used (left open) for these
curves. Connecting the Bias Supply pin to the output voltage
would reduce the junction temperature by approximately 5˚C
to 15˚C, depending on the input and output voltages, and the
load current. This data was taken with the circuit operating
as a buck switcher with all components mounted on a PC
board to simulate the junction temperature under actual
operating conditions. This curve is typical, and can be used
for a quick check on the maximum junction temperature for
various conditions, but keep in mind that there are many
factors that can affect the junction temperature.
BIAS SUPPLY FEATURE
The bias supply (V
circuitry to be powered from a power source, other than V
typically the output voltage. This feature can increase effi-
ciency and lower junction temperatures under some operat-
ing conditions. The greatest increase in efficiency occur with
light load currents, high input voltage and low output voltage
(4V to 12V). See efficiency curves shown in Figure 24 and
Figure 25 . The curves with solid lines are with the V
connected to the regulated output voltage, while the curves
with dashed lines are with the V
The bias supply pin requires a minimum of approximately
3.5V at room temperature (4V
as 30V, but there is little advantage of using the bias supply
feature with voltages greater than 15V or 20V. The current
required for the V
IN
pin is typically 4 mA.
BS
) pin allows the LM2597’s internal
@
BS
−40˚C), and can be as high
pin open.
www.national.com
BS
pin
IN
,