lm2706 National Semiconductor Corporation, lm2706 Datasheet - Page 14

• Manufacturer Part Number: lm2706
• Description: Miniature, Variable, Step-down Dc-dc Converter With Bypass For Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

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Application Information
3. Arrange the components so that the switching current
4. Connect the ground pins of the LM2706, and filter capaci-
and act as antennas. Following this rule reduces radiated
noise. Place the capacitors and inductor within 0.2 in
(5 mm) of the LM2706.
loops curl in the same direction. During the first half of
each cycle, current flows from the input filter capacitor,
through the LM2706 and inductor to the output filter ca-
pacitor and back through ground, forming a current loop.
In the second half of each cycle, current is pulled up from
ground, through the LM2706 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 rever-
sal between the two half-cycles and reduces radiated
noise.
tors 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
(Continued)
14
5. Use wide traces between the power components and for
6. Route noise sensitive traces, such as the voltage feed-
reduces ground-plane noise by preventing the switching
currents from circulating through the ground plane. It also
reduces ground bounce at the LM2706 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. Because the FB trace of the LM2706 carries
current from the bypass switch, this trace must also be
wide.
back path, away from noisy traces between the power
components. The voltage feedback trace must remain
close to the LM2706 circuit and should be routed directly
from FB to V
routed opposite to noisy components. This reduces EMI
radiated onto the DC-DC converter’s own voltage feed-
back trace.
OUT
at the output capacitor and should be