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LM3242TMX/NOPB Datasheet(PDF) 21 Page - Texas Instruments |
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LM3242TMX/NOPB Datasheet(HTML) 21 Page - Texas Instruments |
21 / 32 page ![]() LM3242 www.ti.com SNOSB48E – OCTOBER 2011 – REVISED AUGUST 2015 10 Layout 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. 10.1.1 PCB Considerations 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. 10.1.1.1 Energy Efficiency Minimize resistive losses by using wide traces between the power components and doubling up traces on multiple layers when possible. 10.1.1.2 EMI By its very nature, any switching converter generates electrical noise, and the circuit board designer’s challenge is to minimize, contain, or attenuate such switcher-generated noise. A high-frequency switching converter, such as the LM3242, 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 LM3242 switcher, its input capacitor, and output filter inductor and capacitor close together, and make 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 LM3242 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 LM3242 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 LM3242 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 LM3242 by giving it a low-impedance ground connection. To minimize coupling to the DC-DC converter’s own voltage feedback trace: • 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. Copyright © 2011–2015, Texas Instruments Incorporated Submit Documentation Feedback 21 Product Folder Links: LM3242 |
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