Datasheet

LM3691

www.ti.com

SNVS506I –MAY 2008–REVISED MAY 2013

ground-plane noise by preventing the switching currents from circulating through the ground plane. It also

reduces ground bounce at the buck by giving it a low-impedance ground connection.

5. Use wide traces between the power components and for power connections to the DC-DC converter circuit.

This reduces voltage errors by resistive losses across the traces. Even 1mm of fine trace creates parasitic

inductance that can undesirably affect performance from increased L*di/dt noise voltages.

6. Route noise sensitive traces, such as the voltage feedback path, away from noisy traces between the power

components. The voltage feedback trace must remain close to the buck circuit and should be routed directly

from FB to VOUT at the output capacitor and should be routed opposite to noise components. This reduces

EMI radiated onto the DC-DC converter’s own voltage feedback trace.

DSBGA PACKAGE ASSEMBLY AND USE

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

techniques, as detailed in Texas Instruments Application Note AN-1112 (Literature Number SNVA009). Refer to

the section Surface Mount 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 DSBGA package must be the

NSMD (Non-Solder Mask Defined) type. 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 AN-1112 (Literature Number SNVA009)

for specific instructions how to do this.

The 6-bump package used for LM3691 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 LM3691 re-flow evenly and that the device

solders level to the board. In particular, special attention must be paid to the pads for bumps A2 and C2,

because GND and V

are typically connected to large copper planes.

The DSBGA package is optimized for the smallest possible size in applications with red or infrared opaque

cases. Because the DSBGA 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, DSBGA

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

Product Folder Links: LM3691