LM3405

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SNVS429B – OCTOBER 2006 – REVISED MAY 2013

PCB Layout Considerations

When planning layout there are a few things to consider when trying to achieve a clean, regulated output. The

most important consideration when completing the layout is the close coupling of the GND connections of the

input capacitor C1 and the catch diode D1. These ground ends should be close to one another and be connected

to the GND plane with at least two through-holes. Place these components as close to the IC as possible. The

next consideration is the location of the GND connection of the output capacitor C2, which should be near the

GND connections of C1 and D1.

There should be a continuous ground plane on the bottom layer of a two-layer board except under the switching

node island.

The FB pin is a high impedance node and care should be taken to make the FB trace short to avoid noise pickup

that causes inaccurate regulation. The LED current setting resistor R1 should be placed as close as possible to

should be routed away from the inductor and any other traces that are switching.

Radiated noise can be decreased by choosing a shielded inductor.

The remaining components should also be placed as close as possible to the IC. Please see Application Note

AN-1229 for further considerations and the LM3405 demo board as an example of a four-layer layout.

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