LM3671MF-1.25 National Semiconductor, LM3671MF-1.25 Datasheet - Page 18

• Manufacturer Part Number: LM3671MF-1.25
• Description: DC/DC CONVERTER, 1.25V, SMD, 3671
• Manufacturer: National Semiconductor

Specifications of LM3671MF-1.25

• Primary Input Voltage: 2.25V
• No. Of Outputs: 1
• Output Voltage: 1.25V
• Output Current: 600mA
• Voltage Regulator Case Style: SOT-23
• No. Of Pins: 5
• Operating Temperature Range: -25°C To +85°C
• Svhc: No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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Application Information
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 en-
sures 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
copper planes, inadequate thermal relief can result in late or
inadequate re-flow of these bumps.
The Micro SMD package is optimized for the smallest pos-
sible 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
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.
Good layout for the LM3671 can be implemented by follow-
ing a few simple design rules below. Refer to Figure 9 for top
layer board layout.
1.
2. Arrange the components so that the switching current
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. Special care must be given to place the
input filter capacitor very close to the V
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
Place the LM3671, inductor and filter capacitors close
IN
and GND are typically connected to large
(Continued)
IN
and GND pin.
18
3. Connect the ground pins of the LM3671 and filter ca-
4. Use wide traces between the power components and for
5. Route noise sensitive traces, such as the voltage feed-
6. Place noise sensitive circuitry, such as radio IF blocks,
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.
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 mag-
netic field reversal between the two half-cycles and re-
duces radiated noise.
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 LM3671 by giving it
a low-impedance ground connection.
power connections to the DC-DC converter circuit. This
reduces voltage errors caused by resistive losses across
the traces.
back 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 divid-
ers on the bottom layer.
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.