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LM2612 Datasheet(Fiches technique) 18 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numéro de pièce LM2612
Description  400mA Sub-miniature, Programmable, Step-Down DC-DC Converter for Ultra Low-Voltage Circuits
Télécharger  19 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
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Application Information (Continued)
Do not use or power-up the LM2612 while subjecting it to
high intensity red or infrared light, otherwise degraded, un-
predictable or erratic operation may result. Examples of light
sources with high red or infrared content include the sun and
halogen lamps. Package the circuit in a case opaque to red
or infrared light.
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. Poor layout can
also result in reflow problems leading to poor solder joints
between the micro SMD package and board pads. Poor
solder joints can result in erratic or degraded performance.
Good layout for the LM2612 can be implemented by follow-
ing a few simple design rules:
1.
Place the LM2612 on 6.7mil pads for micro SMD pack-
age. As a thermal relief, connect to each pad with a 6mil
wide trace (micro SMD), 6mils long or longer, then in-
crementally 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 Assem-
bly and Use).
2.
Place the LM2612, inductor and filter capacitors close
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.
3.
Arrange the components so that the switching current
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.
4.
Connect the ground pins of the LM2612 and filter ca-
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 LM2612 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 caused by resistive losses across
the traces.
6.
Route noise sensitive traces, such as the voltage feed-
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
should be routed away from to noisy components. This
reduces EMI radiated onto the DC-DC converter’s own
voltage feedback trace.
7.
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.
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, such as the LP2966.
www.national.com
18




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