LM3677TLX-ADJ National Semiconductor Corporation, LM3677TLX-ADJ Datasheet - Page 17

• Manufacturer Part Number: LM3677TLX-ADJ
• Description: 3mhz, 600ma Miniature Step-down Dc-dc Converter For Ultra Low Voltage Circuits
• Manufacturer: National Semiconductor Corporation

The Micro SMD package is optimized for the smallest possi-
ble size in applications with red or infrared opaque cases.
Because the Micro SMD package lacks the plastic encapsu-
lation 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 sensi-
tivity. 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.
Good layout for the LM3677 can be implemented by following
a few simple design rules, as illustrated in Figure.
1.
2.
3.
Place the LM3677 on 10.82 mil pads. As a thermal relief,
connect to each pad with a 7 mil wide, approximately 7
mil long trace, and then incrementally increase each
trace to its optimal width. The important criterion is
symmetry to ensure the solder bumps on the re-flow
evenly (see Micro SMD Package Assembly and Use).
Place the LM3677, 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
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 LM3677 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 LM3677 by the inductor, to
the output filter capacitor and then back through ground,
forming a second current loop. Routing these loops so
FIGURE 7. Board Layout Design Rules for the LM3677
IN
and GND pin.
17
BOARD LAYOUT CONSIDERATIONS
PC board layout is an important part of DC-DC converter de-
sign. 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, re-
sulting in poor regulation or instability. Poor layout can also
result in re-flow problems leading to poor solder joints be-
tween the Micro SMD package and board pads. Poor solder
joints can result in erratic or degraded performance.
4.
5.
6.
the current curls in the same direction prevents magnetic
field reversal between the two half-cycles and reduces
radiated noise.
Connect the ground pins of the LM3677, 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 LM3677 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
feedback pathaway from noisy traces between the power
components. The voltage feedback trace must remain
close to the LM3677 circuit and should be routed directly
from FB to V
routed opposite to noise components. This reduces EMI
radiated onto the DC-DC converter’s own voltage
feedback trace.
OUT
at the output capacitor and should be
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