LM3262 Datasheet, PDF (20/41 Pages) Texas Instruments – Miniature, Adjustable, Step-Down DC-DC Converter

LM3262
SNVS875O – AUGUST 2012 – REVISED DECEMBER 2015
10 Layout
www.ti.com
10.1 Layout Guidelines
PC board layout is critical to successfully designing a DC-DC converter into a product. As much as a 20-dB
improvement in RX noise floor can be achieved by carefully following recommended layout practices. A properly
planned board layout optimizes the performance of a DC-DC converter and minimizes effects on surrounding
circuitry while also addressing manufacturing issues that can have adverse impacts on board quality and final
product yield.
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. Erroneous signals could be sent to the DC-DC
converter device, resulting in poor regulation or instability. Poor layout can also result in re-flow problems leading
to poor solder joints between the DSBGA package and board pads. Poor solder joints can result in erratic or
degraded performance of the converter.
10.1.1 Energy Efficiency
Minimize resistive losses by using wide traces between the power components and doubling up traces on
multiple layers when possible.
10.1.2 EMI
By its very nature, any switching converter generates electrical noise, and the design challenge is to minimize,
contain, or attenuate such switcher-generated noise. A high-frequency switching converter, such as the LM3262,
switches Ampere level currents within nanoseconds, and the traces interconnecting the associated components
can act as radiating antennas. The following guidelines are offered to help to ensure that EMI is maintained
within tolerable levels.
To minimize radiated noise:
• Place the LM3262 switcher, input capacitor, output filter inductor, and output filter capacitor close together,
making the interconnecting traces as short as possible.
• 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 internal PFET of the LM3262 and the
inductor, to the output filter capacitor, then back through ground, forming a current loop. In the second half of
each cycle, current is pulled up from ground, through the internal synchronous NFET of the LM3262 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.
• Make the current loop area(s) as small as possible.
To minimize ground-plane noise:
• Reduce the amount of switching current that circulates through the ground plane — connect the ground
bumps of the LM3262 and its input filter capacitor together using generous component-side copper fill as a
pseudo-ground plane. Then connect this copper fill to the system ground-plane (if one is used) with multiple
vias. These multiple vias help to minimize ground bounce at the LM3262 by giving it a low-impedance ground
connection.
To minimize coupling to the voltage feedback trace of the DC-DC converter:
• Route noise sensitive traces, such as the voltage feedback path, as directly as possible from the switcher FB
pad to the VOUT pad of the output capacitor, but keep it away from noisy traces between the power
components.
To decouple common power supply lines, series impedances may be used to strategically isolate circuits:
• Take advantage of the inherent inductance of circuit traces to reduce coupling among function blocks, by way
of the power supply traces.
• Use star connection for separately routing VBATT to PVIN and VBATT_PA.
• Inserting a single ferrite bead in-line with a power supply trace may offer a favorable tradeoff in terms of
board area by allowing the use of fewer bypass capacitors.
20
Submit Documentation Feedback
Product Folder Links: LM3262
Copyright © 2012–2015, Texas Instruments Incorporated