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ACT8325NDEH-T Datasheet(PDF) 10 Page - Active-Semi, Inc |
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ACT8325NDEH-T Datasheet(HTML) 10 Page - Active-Semi, Inc |
10 / 11 page ACT8325 Rev 2, 25-May-11 STEP-DOWN DC/DC CONVERTER Active-Semi Innovative Power TM - 10 - www.active-semi.com Copyright © 2011 Active-Semi, Inc. ActivePMU TM is a trademark of Active-Semi. (1) output voltage option. output voltage option. Select components as follows: Set RFB2 = 51kΩ, then calculate RFB1 using the following equation: where VFBX is 0.625V. Finally choose CFF using the following equation: Where RFB1 = 47kΩ, use 47pF. Inductor Selection REG1 and REG2 utilize current-mode control and a proprietary internal compensation scheme to simul- taneously simplify external component selection and optimize transient performance over their full operating range. These devices were optimized for operation with 3.3µH inductors, although inductors in the 2.2µH to 4.7µH range can be used. Choose an inductor with a low DC-resistance, and avoid inductor saturation by choosing inductors with DC ratings that exceed the maximum output current of the application by at least 30%. PCB Layout Considerations High switching frequencies and large peak current make PC board layout an important part of step- down DC/DC converter design. A good design mini- mizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors. Step- down DC/DCs exhibit discontinuous input current, so the input capacitors should be placed as close as possible to the IC, and avoiding the use of vias if possible. The inductor, input filter capacitor, and output filter capacitor should be connected as close together as possible, with short, direct, and wide traces. The ground nodes for each regulator’s power loop should be connected at a single point in a star-ground configuration, and this point should be connected to the backside ground plane with multiple vias. For fixed output voltage options, con- nect the output node directly to the FBx pin. For adjustable output voltage options, connect the feed- back resistors and feed-forward capacitor to the FBx pin through the shortest possible route. In both cases, the feedback path should be routed to main- tain sufficient distance from switching nodes to pre- vent noise injection. ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − = 1 V V R R FBX OUTX 2 FB 1 FB 1 FB 6 FF R 10 2 . 2 C − × = (2) |
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