11.1 Layout Guidelines

As in any switching regulator, layout is very important. Rapidly switching currents associated with wiring inductance can generate voltage transients which can cause problems. For minimal inductance and ground loops, the wires indicated by heavy lines must be wide printed-circuit traces and must be kept as short as possible. For best results, external components must be placed as close to the switcher lC as possible using ground plane construction or single point grounding.

If open core inductors are used, take special care to select the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback, lC groundpath and C_OUT wiring can cause problems.

When using the adjustable version, take special care to select the location of the feedback resistors and the associated wiring. Physically place both resistors near the IC, and route the wiring away from the inductor, especially an open core type of inductor (see Open Core Inductors for more information).

11.2 Layout Examples

C_IN = 470-μF, 50-V, aluminum electrolytic Panasonic HFQ Series
C_OUT = 330-μF, 35-V, aluminum electrolytic Panasonic HFQ Series
D1 = 5-A, 40-V Schottky rectifier, 1N5825
L1 = 47-μH, L39, Renco through hole
R_{PULL UP} = 10k
C_DELAY = 0.1 μF
C_SD/SS = 0.1 μF
Thermalloy heat sink #7020

Figure 46. Typical Through-Hole PCB Layout, Fixed Output (1x Size), Double-Sided

C_IN = 470-μF, 50-V, aluminum electrolytic Panasonic, HFQ Series
C_OUT = 220-μF, 35-V, aluminum electrolytic Panasonic, HFQ Series
D1 = 5-A, 40-V Schottky Rectifier, 1N5825
L1 = 47-μH, L39, Renco, through-hole
R₁ = 1 kΩ, 1%
R₂ = Use formula in Detailed Design Procedure
C_FF = See Figure 35
R_FF = See Feedforward Capacitor (CFF) for Adjustable Output Voltage Version Only
R_{PULL UP} = 10k
C_DELAY = 0.1 μF
C_SD/SS= 0.1 μF
Thermalloy heat sink #7020

Figure 47. Typical Through-Hole PCB Layout, Adjustable Output (1x Size), Double-Sided

11.3 Thermal Considerations

The LM2599 is available in two packages, a 7-pin TO-220 and a 7-pin surface-mount TO-263.

The TO-220 package needs a heat sink under most conditions. The size of the heat sink depends on the input voltage, the output voltage, the load current and the ambient temperature. The curves in Figure 48 show the LM2599T junction temperature rises above ambient temperature for a 3-A load and different input and output voltages. The data for these curves was taken with the LM2599T (TO-220 package) operating as a buck switching regulator in an ambient temperature of 25°C (still air). These temperature rise numbers are all approximate and there are many factors that can affect these temperatures. Higher ambient temperatures require more heat sinking.

The TO-263 surface-mount package tab is designed to be soldered to the copper on a printed-circuit board. The copper and the board are the heat sink for this package and the other heat producing components, such as the catch diode and inductor. The PCB copper area that the package is soldered to must be at least 0.4 in², and ideally must have 2 or more square inches of 2-oz. (0.0028 in) copper. Additional copper area improves the thermal characteristics, but with copper areas greater than approximately 6 in², only small improvements in heat dissipation are realized. If further thermal improvements are needed, double-sided, multilayer PCBs with large copper areas or airflow are recommended.

The curves shown in Figure 49 show the LM2599S (TO-263 package) junction temperature rise above ambient temperature with a 2-A 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 PCB 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 2 A are used, double-sided or multilayer PCBs with large copper areas or airflow might be needed, especially for high ambient temperatures and high output voltages.

CIRCUIT DATA FOR TEMPERATURE RISE CURVE TO-220 PACKAGE (NDZ)
Capacitors	Through-hole electrolytic
Inductor	Through-hole Renco
Diode	Through-hole, 5-A, 40-V Schottky
PCB	3-square inch, single-sided, 2-oz copper (0.0028″)

Figure 48. Junction Temperature Rise, TO-220

CIRCUIT DATA FOR TEMPERATURE RISE CURVE TO-263 PACKAGE (KTW)
Capacitors	Surface-mount, tantalum molded D size
Inductor	Surface-mount, Pulse engineering, 68 μH
Diode	Surface-mount, 5-A, 40-V, Schottky
PCB	9-square inch, single-sided, 2-oz copper (0.0028″)

Figure 49. Junction Temperature Rise, TO-263

For the best thermal performance, wide copper traces and generous amounts of printed-circuit board copper must be used in the board layout. (One exception to this is the output (switch) pin, which must 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 affects 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 PCB 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 adds heat to the PCB 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.