LM3203TLX National Semiconductor Corporation, LM3203TLX Datasheet - Page 14

• Manufacturer Part Number: LM3203TLX
• Description: Step-down Dc-dc Converter With Bypass Mode For Cdma / Wcdma Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
1. Place the LM3203 on 10.82 mil pads. As a thermal relief,
2. Place the LM3203, inductor and filter capacitors close
3. Arrange the components so that the switching current
connect to each pad with a 7 mil wide, approximately 7
mil long traces, and when incrementally increase each
trace to its optimal width. The important criterion is sym-
metry to ensure the solder bumps on the LM3203 re-flow
evenly (see Micro SMD Package Assembly and Use).
together and make the trace 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 close
to the LM3203. The input capacitor should be placed
right next to the device between PV
loops curl in the same direction. During the first half of
each cycle, current flows from the input filter capacitor,
through the LM3203 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 LM3203 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.
IN
(Continued)
and PGND pin.
14
4. Connect the ground pins of the LM3203, and filter ca-
5. Use wide traces between the power components and for
6. Route noise sensitive traces, such as the voltage feed-
7. It is recommended to connect BYPOUT pin to V
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 LM3203 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 trace, away from noisy traces and components.
The voltage feedback trace must remain close to the
LM3203 circuit and should be routed directly from FB pin
to V
route the feedback trace on another layer and to have a
ground plane between the top layer and the layer on
which the feedback trace is routed. This reduces EMI
radiation on to the DC-DC converter’s own voltage feed-
back trace.
the output capacitor using a separate trace, instead of
connecting it directly to the FB pin for better noise
immunity.
OUT
at the output capacitor. A good approach is to
OUT
at