Datasheet

LM3671
LM3671Q
SNVS294Q –NOVEMBER 2004–REVISED NOVEMBER 2013 www.ti.com
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 Technology (DSBGA) 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 5-bump package used for LM3671 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 LM3671 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 V
IN
and GND are typically connected to large
copper planes, inadequate thermal relief can result in late or inadequate re-flow of these bumps.
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.
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.
Good layout for the LM3671 can be implemented by following a few simple design rules below. Refer to
Figure 42 for top layer board layout.
1. Place the LM3671, inductor and filter capacitors close together and make the traces short. The traces
between these components carry relatively high switching currents and act as antennas. Following this rule
reduces radiated noise. Special care must be given to place the input filter capacitor very close to the V
IN
and GND pin.
2. Arrange the components so that the switching current loops curl in the same direction. During the first half of
each cycle, current flows from the input filter capacitor through the LM3671 and inductor to the output filter
capacitor and back through ground, forming a current loop. In the second half of each cycle, current is pulled
up from ground through the LM3671 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 magnetic field reversal between the two half-cycles and reduces radiated noise.
3. Connect the ground pins of the LM3671 and filter capacitors together using generous component-side
copper 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 LM3671 by giving it a low-impedance ground connection.
4. 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.
5. 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 LM3671 circuit and should be direct but
should be routed opposite to noisy components. This reduces EMI radiated onto the DC-DC converter’s own
voltage feedback trace. A good approach is to route the feedback trace on another layer and to have a
ground plane between the top layer and layer on which the feedback trace is routed. In the same manner for
the adjustable part it is desired to have the feedback dividers on the bottom layer.
6. 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.
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