Datasheet

LM20323

SNVS557C –MAY 2008–REVISED APRIL 2013

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Figure 31. Safe Thermal Operating Areas (I

OUT

= 2.5A)

The dashed lines in the figures above show an approximation of the minimum and maximum duty cycle

limitations; while, the solid lines define areas of operation for a given ambient temperature. This data for the

figure was derived assuming the device is operating at 3A continuous output current on a 4 layer PCB with an

copper area greater than 4 square inches exhibiting a thermal characteristic less than 27 °C/W. Since the

internal losses are dominated by the FETs a slight reduction in current by 500mA allows for much larger regions

of operation, as shown in Figure 31.

Figure 32, shown below, provides a better approximation of the θ

for a given PCB copper area. The PCB used

in this test consisted of 4 layers: 1oz. copper was used for the internal layers while the external layers were

plated to 2oz. copper weight. To provide an optimal thermal connection, a 5 x 4 array of 12 mil thermal vias

located under the thermal pad was used to connect the 4 layers.

Figure 32. Thermal Resistance vs PCB Area (4 Layer Board)

PCB LAYOUT CONSIDERATIONS

PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance

of a DC-DC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss

in the traces. These can send erroneous signals to the DC-DC converter resulting in poor regulation or instability.

Good layout can be implemented by following a few simple design rules.

1. Minimize area of switched current loops. In a buck regulator there are two loops where currents are switched

at high slew rates. The first loop starts from the input capacitor, to the regulator VIN pin, to the regulator SW pin,

to the inductor then out to the output capacitor and load. The second loop starts from the output capacitor

ground, to the regulator GND pins, to the inductor and then out to the load (see Figure 33). To minimize both

loop areas the input capacitor should be placed as close as possible to the VIN pin. Grounding for both the input

and output capacitor should consist of a small localized top side plane that connects to GND and the exposed

pad (EP). The inductor should be placed as close as possible to the SW pin and output capacitor.

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