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28 Design Guide
Intel
®
31244 PCI-X to Serial ATA Controller
Routing Guidelines
4.4 Power Distribution and Decoupling
Have ample decoupling to ground, for the power planes, to minimize the effects of the switching
currents. Three types of decoupling are: the bulk, the high-frequency ceramic, and the inter-plane
capacitors.
• Bulk capacitance consist of electrolytic or tantalum capacitors. These capacitors supply large
reservoirs of charge, but they are useful only at lower frequencies due to lead inductance
effects. The bulk capacitors may be located anywhere on the board.
• For fast switching currents, high-frequency low-inductance capacitors are most effective.
Place these capacitors as close to the device being decoupled as possible. This minimizes the
parasitic resistance and inductance associated with board traces and vias.
• Use an inter-plane capacitor between power and ground planes to reduce the effective plane
impedance at high frequencies. The general guideline for placing capacitors is to place
high-frequency ceramic capacitors as close as possible to the module.
4.4.1 Decoupling
Inadequate high-frequency decoupling results in intermittent and unreliable behavior. A general
guideline recommends that you use the largest easily available capacitor in the lowest inductance
package.
4.4.1.1 Intel
®
31244 PCI-X to Serial ATA Controller Decoupling
It is recommended that to decouple the VCC 2.5 V, use at least twelve 0.1 µF capacitors in as close
proximity to the GD31244 VCC pins as possible. When feasible, locate these capacitors on the
back of the board, close to the GD31244 VCC ball.