LM2599 datasheet(25/31 Pages) NSC

25 / 31 page

Application Information (Continued)

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 20V in, 12V out, 1A load, and using a 0.068 µF Soft-start

capacitor. The photo in

Figure 23 Figure 24 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.

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.

Circuit Data for Temperature Rise Curve TO-263

Package (S)

Capacitors

Surface mount tantalum, molded “D” size

Inductor

Surface mount, Pulse engineering, 68 µH

Diode

Surface mount, 5A 40V, Schottky

PC board

9 square inches single sided 2 oz. copper

(0.0028")

FIGURE 22. Junction Temperature Rise, TO-263

DS012582-40

FIGURE 23. Output Voltage, Input Current,

at Start-Up, WITH Soft-start

DS012582-41

FIGURE 24. Output Voltage, Input Current,

at Start-Up, WITHOUT Soft-start

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