LM3203

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SNVS355C – MAY 2005 – REVISED JULY 2005

approximately 6 mil long or longer, as a thermal relief. Then each trace should neck up or down to its optimal

width. The important criterion is symmetry. This ensures the solder bumps on the LM3203 re-flow evenly and that

the device solders level to the board. In particular, special attention must be paid to the pads for bumps B3, C3

result in inadequate re-flow of these bumps.

The Micro SMD package is optimized for the smallest possible size in applications with red or infrared opaque

cases. Because the Micro SMD package lacks the plastic encapsulation characteristic of larger devices, it is

vulnerable to light. Backside metalization 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.

Do not use or power-up the LM3203 while subjecting it to high intensity red or infrared light; otherwise degraded,

unpredictable or erratic operation may result. Examples of light sources with high red or infrared content include

the sun and halogen lamps. Place the device 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, resulting in poor regulation or

instability. Poor layout can also result in re-flow 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

LM3203 can be implemented by following a few simple design rules.

1. Place the LM3203 on 10.82 mil pads. As a thermal relief, 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 symmetry to ensure the solder bumps on the LM3203 re-flow evenly (see Micro SMD

Package Assembly and Use).

2. Place the LM3203, inductor and filter capacitors close together and make the trace short. The traces

between these components carry relatively high switching currents and act as antennas. Following this rule

reduces radiated noise. Place the capacitors and inductor close to the LM3203. The input capacitor should

3. Arrange the components so that the switching current 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 magnetic field reversal between the two half-cycles and reduces radiated noise.

4. Connect the ground pins of the LM3203, and filter capacitors 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 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.

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 feedback 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

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 feedback trace.

connecting it directly to the FB pin for better noise immunity.

Copyright © 2005, Texas Instruments Incorporated

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