LM2612ATL NSC [National Semiconductor], LM2612ATL Datasheet - Page 17

• Manufacturer Part Number: LM2612ATL
• Description: 400mA Sub-miniature, Programmable, Step-Down DC-DC Converter for Ultra Low-Voltage Circuits
• Manufacturer: NSC [National Semiconductor]

Application Information
1. Place the LM2612 on 10.82mil pads for micro SMD
2. Place the LM2612, inductor and filter capacitors close
3. Arrange the components so that the switching current
4. Connect the ground pins of the LM2612 and filter ca-
package. As a thermal relief, connect to each pad with a
6mil wide trace (micro SMD), 6mils long or longer, then
incrementally increase each trace to its optimal width
over a span so that the taper extends beyond the edge
of the package. The important criterion is symmetry, to
ensure re-flow occurs evenly (see Micro SMD Package
Assembly and Use).
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. Place the capacitors and inductor within
0.2in (5mm) of the LM2612.
loops curl in the same direction. During the first part of
each cycle, current flows from the input filter capacitor,
through the LM2612 and inductor to the output filter
capacitor and back through ground, forming a current
loop. In the second part of each cycle, current is pulled
up from ground, through the LM2612 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 part-cycles and
reduces radiated noise.
pacitors together using generous component-side cop-
per fill as a pseudo-ground plane. Then, connect this to
the ground-plane with several vias. This reduces
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17
5. Use wide traces between the power components and for
6. Route noise sensitive traces, such as the voltage feed-
7. Place noise sensitive circuitry, such as radio IF blocks,
In mobile phones, for example, a common practice is to
place the DC-DC converter in 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 in the diagonally opposing corner. Often, the sen-
sitive circuitry is shielded with a metal pan and power to it is
post-regulated to reduce conducted noise, using low-dropout
linear regulators, such as the LP2966.
ground-plane noise by preventing the switching currents
from circulating through the ground plane. It is recom-
mended to have a dedicate ground plan between the
switch signal and feedback traces. It also reduces
ground bounce at the LM2612 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 LM2612 circuit and should be direct but
routed away from noisy components. This reduces EMI
radiated onto the DC-DC converter’s own voltage feed-
back trace.
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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