LM3679

www.ti.com

SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

is at the switching frequency of the part.

Table 3. Suggested Capacitors and Their Suppliers

Case Size

Model

Type

Vendor

Voltage Rating

Inch (mm)

C1608X5R0J475

Ceramic, X5R

TDK

6.3V

0603 (1608)

C2012X5R0J475

Ceramic, X5R

TDK

6.3V

0805 (2012)

GRM21BR60J475

Ceramic, X5R

muRata

6.3V

0805 (2012)

GRM185R60J475M (0.5mm height) *

Ceramic, X5R

muRata

6.3V

0603 (1608) *

JMK107BJ475MK (0.5mm Height) *

Ceramic, X5R

Taiyo-Yuden

6.3V

0603 (1608) *

JMK212BJ475

Ceramic, X5R

Taiyo-Yuden

6.3V

0805 (2012)

GRM185R60J475M(0.5mm height) **

Ceramic, X5R

muRata

6.3V

0603 (1608) X 2 **

JMK107BJ475MK(0.5mm height) **

Ceramic, X5R

Taiyo-Yuden

6.3V

0603 (1608) X 2 **

C1608X5R0J106

Ceramic, X5R

TDK

6.3V

0603 (1608)

C2012X5R0J106

Ceramic, X5R

TDK

6.3V

0805 (2012)

GRM21BR60J106

Ceramic, X5R

muRata

6.3V

0805 (2012)

JMK212BJ106

Ceramic, X5R

Taiyo-Yuden

6.3V

0805 (2012)

Note: * For Low Profile Solution

Micro SMD PACKAGE ASSEMBLY AND USE

Use of the Micro SMD package requires specialized board layout, precision mounting and careful re-flow

techniques, as detailed in National Semiconductor Application Note 1112. Refer to the section "Surface Mount

Technology (SMD) Assembly Considerations". For best results in assembly, alignment ordinals on the PC board

should be used to facilitate placement of the device. The pad style used with Micro SMD package must be the

NSMD (non-solder mask defined) typ. This means that the solder-mask opening is larger than the pad size. This

prevents a lip that otherwise forms if the solder-mask and pad overlap, from holding the device off the surface of

the board and interfering with mounting. See Application Note 1112 for specific instructions how to do this. The

5-Bump package used for LM3679 has 300 micron solder balls and requires 10.82 mils 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 7 mil wide, for a section approximately 7 mil

long or longer, as a thermal relief. Then each trace should neck up or down to its optimal width. The important

criteria is symmetry. This ensures the solder bumps on the LM3679 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 and A3, because GND

re-flow of these bumps.

The Micro SMD package 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 metallization and/or epoxy coating, along with front-side shading by the printed

circuit board, reduce this sensitivity. However, the package has exposed die edges. In particular, Micro SMD

devices are sensitive to light, in the red and infrared range, shining on the package’s exposed die edges.

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 re-flow problems leading to poor solder joints between the Micro SMD

package and board pads. Poor solder joints can result in erratic or degraded performance.

Copyright © 2007–2008, Texas Instruments Incorporated

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