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, special care must be taken as to 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 regarding as to 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.

11.2 Layout Example

C_IN = 10-μF, 35-V, Solid Tantalum AVX, TPS series
C_OUT = 00-μF, 10-V Solid Tantalum AVX, TPS series
D1 = 1-A, 40-V Schottky Rectifier, surface mount
L1 = 100-μH, L20, Coilcraft DO33

Figure 38. Typical Surface-Mount PCB Layout, Fixed Output (2X Size)

C_IN = 10-μF, 35-V, Solid Tantalum AVX, TPS series
C_OUT = 100-μF, 10-V Solid Tantalum AVX, TPS series
D1 = 1-A, 40-V Schottky Rectifier, surface mount
L1 = 100-μH, L20, Coilcraft DO33
R1 = 1 kΩ, 1%
R₂ =  Use formula in Design Procedure
C_FF =  See Table 7

Figure 39. Typical Surface-Mount PCB Layout, Adjustable Output (2X Size)

11.3 Thermal Considerations

The LM2594xx is available in two packages: an 8-pin through-hole PDIP and an 8-pin surface-mount SOIC. Both packages are molded plastic with a copper lead frame. When the package is soldered to the printed-circuit board (PCB), the copper and the board are the heat sink for the LM2594 and the other heat producing components.

For best thermal performance, wide copper traces must be used and all ground and unused pins must be soldered to generous amounts of PCB copper, such as a ground plane (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 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 thermal 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 affects the junction temperature. Some of these factors 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 PCB to dissipate heat also depends on the size, quantity, and spacing of other components on the board. 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.

Circuit Data for Temperature Rise Curve (8-Pin PDIP)
Capacitors	Through hole electrolytic
Inductor	Through hole, Schott, 100 μH
Diode	Through hole, 1-A, 40-V, Schottky
PCB	4 square inches single sided 2 oz. copper (0.0028″)

Figure 40. Junction Temperature Rise, 8-Pin PDIP

Circuit Data for Temperature Rise Curve (Surface Mount)
Capacitors	Surface mount tantalum, molded D size
Inductor	Surface mount, Coilcraft DO33, 100 μH
Diode	Surface mount, 1-A, 40-V, Schottky
PCB	4 square inches single sided 2 oz. copper (0.0028″)

Figure 41. Junction Temperature Rise, 8-Pin SOIC

Figure 40 and Figure 41 show the LM2594 junction temperature rise above ambient temperature with a 500-mA load for various input and output voltages. This data was taken with the circuit operating as a buck switcher with all components mounted on a PCB 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.