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LM2612 Fiches technique(PDF) 18 Page - National Semiconductor (TI) |
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LM2612 Fiches technique(HTML) 18 Page - National Semiconductor (TI) |
18 / 19 page Application Information (Continued) Do not use or power-up the LM2612 while subjecting it to high intensity red or infrared light, otherwise degraded, un- predictable or erratic operation may result. Examples of light sources with high red or infrared content include the sun and halogen lamps. Package the circuit 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 IC, resulting in poor regulation or instability. Poor layout can also result in reflow 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 LM2612 can be implemented by follow- ing a few simple design rules: 1. Place the LM2612 on 6.7mil pads for micro SMD pack- age. As a thermal relief, connect to each pad with a 6mil wide trace (micro SMD), 6mils long or longer, then in- crementally increase each trace to its optimal width over a span so that the taper extends beyond the edge of the package. The important criterion is symmetry to ensure re-flow occurs evenly (see Micro SMD Package Assem- bly and Use). 2. Place the LM2612, inductor and filter capacitors close together and make the traces 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 within 0.2in (5mm) of the LM2612. 3. Arrange the components so that the switching current loops curl in the same direction. During the first part of each cycle, current flows from the input filter capacitor, through the LM2612 and inductor to the output filter capacitor and back through ground, forming a current loop. In the second part of each cycle, current is pulled up from ground, through the LM2612 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 part-cycles and reduces radiated noise. 4. Connect the ground pins of the LM2612 and filter ca- 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 LM2612 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 feed- back path, away from noisy traces between the power components. The voltage feedback trace must remain close to the LM2612 circuit and should be direct but should be routed away from to noisy components. This reduces EMI radiated onto the DC-DC converter’s own voltage feedback trace. 7. Place noise sensitive circuitry, such as radio IF blocks, away from the DC-DC converter, CMOS digital blocks and other noisy circuitry. Interference with noise-sensitive circuitry in the system can be reduced through distance. In mobile phones, for example, a common practice is to place the DC-DC converter on one corner of the board, arrange the CMOS digital circuitry around it (since this also generates noise), and then place sensitive preamplifiers and IF stages on the diagonally opposing corner. Often, the sensitive circuitry is shielded with a metal pan and power to it is post-regulated to reduce conducted noise, using low-dropout linear regulators, such as the LP2966. www.national.com 18 |
Numéro de pièce similaire - LM2612 |
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Description similaire - LM2612 |
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