AN2683 datasheet(21/38 Pages) STMICROELECTRONICS

21 / 38 page

AN2683

Construction

21/38

Construction

With the components dimensioned the construction of the circuit board can be considered.

With this type of high frequency converter separate power and signal grounds are a must.

Additionally, the small board area necessitated component placement on both sides and the

use of additional layers for routing the signal interconnects and providing lower conductor

resistance in the heavy current paths. In Figure 4. below the top layer component placement

is shown. The top layer components are comprised of the power handling ones such as the

MOSFETS and inductors. Along with the power component placement is Figure 5 that

shows the top copper power traces. The first inner layer shown in Figure 6 is a layer that

has redundant power traces to lower resistance in the high current paths. The power ground

and signal ground layers are shown in Figure 7 and 8 respectively. Care must be taken that

they connect at only one point close to pin 14 of the PM6680. The component placement for

the bottom layer is shown in Figure 9, these are the parts that do the signal conditioning and

connect to the PM6680 controller. Of special note on the bottom layer copper shown in

Figure 10, is the square copper island under U1 (the PM6680). This island connects to the

thermal sink contact that is on the bottom of the package. A requirement for proper

operation is that this pad be connected to signal ground. As shown in Figure 11, which is the

fourth inner layer used for additional signal routing, a matrix of nine vias are used to make

the connection to the signal ground layer. The board uses 1-ounce copper on all layers.

While not necessary for the signal traces, keeping the copper weight even on all the layers

reduces the chances of the board warping during the manufacturing process.

Figure 4.

Top layer component placement

AN2683 Datasheet(PDF) 21 Page - STMicroelectronics