LM2608 datasheet(17/18 Pages) NSC

17 / 18 page

Application Information (Continued)

MICRO SMD PACKAGE ASSEMBLY AND USE

Use of the Micro SMD package requires specialized board

layout, precision mounting and careful reflow techniques, as

detailed in National Semiconductor Application Note AN-

1112. Refer to the section Surface Mount Technology (SMT)

Assembly Considerations. For best results in assembly,

alignment ordinals on the PC board should be used to

facilitate placement of the device. Since Micro SMD packag-

ing is a new technology, all layouts and assembly means

must be thoroughly tested prior to production. In particular,

proper placement, solder reflow and resistance to thermal

cycling must be verified.

The 10-Bump package used for the LM2608 has 300micron

solder balls and requires 10.82mil (0.275mm) pads for

mounting on the circuit board. The trace to each pad should

enter the pad with a 90˚ entry angle to prevent debris from

being caught in deep corners. Initially, the trace to each pad

should be 6 mil wide, for a section 6 mil long or longer, as a

thermal relief. Then each trace should neck up to its optimal

width over a span of 11 mils or more, so that the taper

extends beyond the edge of the package. The important

criterion is symmetry. This ensures the solder bumps on the

LM2608 re-flow evenly and that the device solders level to

the board. In particular, special attention must be paid to the

pads for bumps D3, C3, B3, A3 and A2. Because PVIN and

PGND are typically connected to large copper planes, inad-

equate thermal reliefs can result in late or inadequate reflow

of these bumps.

The pad style used with Micro SMD package must be the

NSMD (non-solder mask defined) type. This means that the

solder-mask opening is larger than the pad size or 14.7mils

for the LM2608. This prevents a lip that otherwise forms if

the solder-mask and pad overlap. This lip can hold the

device off the surface of the board and interfere with mount-

ing. See Applications Note AN-1112 for specific instructions.

BOARD 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

IC, resulting in poor regulation or instability. Poor layout can

also result in reflow problems leading to poor solder joints

between the Micro SMD package and board pads. Poor

solder joints can result in erratic or degraded performance.

Good layout for the LM2608 can be implemented by follow-

ing a few simple design rules:

Place the LM2608 on 10.82mil pads for Micro SMD

package. As a thermal relief, connect to each pad with a

6mil wide trace (Micro SMD), 6mils long or longer, then

incrementally increase each trace to its optimal width

over a span so that the taper extends beyond the edge

of the package. The important criterion is symmetry to

ensure re-flow occurs evenly (see Micro SMD Package

Assembly and Use).

Place the LM2608, inductor and filter capacitors close

together and make the traces short. The traces between

these components carry relatively high switching cur-

rents and act as antennas. Following this rule reduces

radiated noise. Place the capacitors and inductor within

0.2in (5mm) of the LM2608.

Arrange the components so that the switching current

loops curl in the same direction. During the first part of

each cycle, current flows from the input filter capacitor,

through the LM2608 and inductor to the output filter

capacitor and back through ground, forming a current

loop. In the second part of each cycle, current is pulled

up from ground, through the LM2608 by the inductor, to

the output filter capacitor and then back through ground,

forming a second current loop. Routing these loops so

the current curls in the same direction prevents mag-

netic field reversal between the two part-cycles and

reduces radiated noise.

Connect the ground pins of the LM2608 and filter ca-

pacitors together using generous component-side cop-

per fill as a pseudo-ground plane. Then, connect this to

the ground-plane (if one is used) with several vias. This

reduces ground-plane noise by preventing the switching

currents from circulating through the ground plane. It

also reduces ground bounce at the LM2608 by giving it

a low-impedance ground connection.

Use wide traces between the power components and for

power connections to the DC-DC converter circuit. This

reduces voltage errors caused by resistive losses across

the traces.

Route noise sensitive traces, such as the voltage feed-

back path, away from noisy traces between the power

components. The voltage feedback trace must remain

close to the LM2608 circuit and should be routed away

from noisy components. This reduces EMI radiated onto

the DC-DC converter’s own voltage feedback trace.

Place noise sensitive circuitry, such as radio IF blocks,

away from the DC-DC converter, CMOS digital blocks

and other noisy circuitry. Interference with noise-

sensitive circuitry in the system can be reduced through

distance.

In mobile phones, for example, a common practice is to

place the DC-DC converter on one corner of the board,

arrange the CMOS digital circuitry around it (since this also

generates noise), and then place sensitive preamplifiers and

IF stages on the diagonally opposing corner. Often, the

sensitive circuitry is shielded with a metal pan and power to

it is post-regulated to reduce conducted noise, using low-

dropout linear regulators, such as the LP2966.

LM2608 Datasheet(PDF) 17 Page - National Semiconductor (TI)

LM2608 Datasheet(HTML) 17 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL