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LM2854 PC Board Layout

Figure 9. LM2854 Pre-BiasedTurn On and Off via Enable

OUT(PRE-BIAS)

= 0.6V

9 LM2854 PC Board Layout

The evaluation board is based on a small 2.0” x 1.2” x 0.062” (51 mm x 31 mm x 1.6 mm) FR4 laminate

PCB with two layers of two ounce copper. The top and bottom side layouts can be seen in Figure 5 and

Figure 6 below. When looking at the top layer, pin 1 of the LM2854 is on the upper left.

Three Textronix oscilloscope probe connectors are provided to facilitate convenient measurement of V

OUT

and V

(switch node). Furthermore, a 50Ω series resistor, assigned reference designator Ra, is

provided that appears in series with the V

OUT

sense trace. This component and associated connectors are

provided to give the user maximum flexibility and enable a convenient point for oscillator signal injection

during loop bode plot measurements.

The PCB layout of the LM2854 evaluation board was designed to occupy as little board space as possible,

while still following sound layout guidelines and techniques. The input capacitor, Cin1, is placed as close

as possible to the PVIN and PGND pins to minimize stray resistance and inductance between Cin1 and

the LM2854. Likewise, the AVIN bypass capacitor is placed as close as possible to the AVIN and AGND

pins. PGND and AGND are connected to each other and the ground plane at a single point, the exposed

pad of the LM2854.

Also, in order to help conduct heat to the ground plane and away from the LM2854, a 3 x 3 via array is

used to electrically and thermally connect the exposed pad to the ground plane (instead of a single via).

Additional ground plane vias are located close to the three PGND pins and in the localized ground plane

emanating away from the exposed pad.

Finally, the FB pin trace is intentionally kept as short as possible and routed away from the SW node to

minimize any EMI pickup.

SNVA323B–March 2008–Revised May 2013 AN-1786 LM2854 500 kHz Buck Regulator Evaluation Board

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