Datasheet

LM2599

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SNVS123C –APRIL 1998–REVISED APRIL 2013

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 surrounding air is still or moving. Furthermore, some of these components 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 42 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 40 Figure 41 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.

Figure 40. Output Voltage, Input Current,

at Start-Up, WITH Soft-start

Figure 41. Output Voltage, Input Current,

at Start-Up, WITHOUT Soft-start

Product Folder Links: LM2599