LM3207 datasheet(17/19 Pages) NSC

17 / 19 page

Application Information (Continued)

wide, for a section approximately 6mil long or longer, to

provide thermal relief. Each trace should then neck up or

down to its optimal width. The important criterion is symme-

try. This ensures the solder bumps re-flow evenly and that

the device solders level to the board. In particular, special

attention must be paid to the pads for bumps A1, A3 and B3.

Because P

GND and PVIN are typically connected to large

copper planes, inadequate thermal relief’s may result in late

or inadequate re-flow of these bumps. The Micro SMD pack-

age is optimized for the smallest possible size in applications

with red or infrared opaque cases. Because the Micro SMD

package lacks the plastic encapsulation characteristic of

larger devices, it is vulnerable to light. Backside metalization

and/or epoxy coating, along with front-side shading by the

printed circuit board, reduce this sensitivity. However, the

package has exposed die edges. Micro SMD devices are

sensitive to light, in the red and infrared range, shining on the

package’s exposed die edges.

BOARD LAYOUT CONSIDERATIONS

Referring to Figure 4, the LM3207 has two major current

loops where pulse and ripple current flow. The loop shown in

the left hand side is most important, because pulse current

shown in Figure 4 flows in this path. The right hand side is

next. The current waveform in this path is triangular, as

shown in Figure 4. Pulse current has many high-frequency

components due to fast di/dt. Triangular ripple current also

has wide high-frequency components. Board layout and cir-

cuit pattern design of these two loops are the key factors for

reducing noise radiation and stable operation. Other lines,

such as from battery to C1(+) and C2(+) to load, are almost

DC current, so it is not necessary to take so much care. Only

pattern width (current capability) and DCR drop consider-

ations are needed.

20165354

FIGURE 4. Current Loop

LM3207 Datasheet(PDF) 17 Page - National Semiconductor (TI)