High-frequency decoupling is required for both 1.2-V and 1.8-V PLL supplies and should be provided as close as possible to each of the PLL supply package pins as shown in the example layout in Figure 11-4. TI recommends that decoupling capacitors be placed under the package on the opposite side of the board. High quality, low-ESR, monolithic, surface mount capacitors should be used. Typically 0.1 µF for each PLL supply should be sufficient. The length of a connecting trace increases the parasitic inductance of the mounting and thus, where possible, there should be no trace, allowing the via to butt up against the land itself. Additionally, the connecting trace should be made as wide as possible. Further improvement can be made by placing vias to the side of the capacitor lands or doubling the number of vias.

The location of bulk decoupling depends on the system design. Typically, a good ceramic capacitor in the 10-µF range is adequate.

Figure 11-4 PLL Filter Layout