Datasheet

Mobile Intel

Celeron

Processor (0.13 µ) in

Micro-FCBGA and Micro-FCPGA Packages Datasheet

98 Datasheet 298517-006

Table 58. PLL Filter Resistor Recommendations

Resistor Part Number Value Tolerance Power

R1 Various

Ω

10% 1/16W

To satisfy damping requirements, total series resistance in the filter (from V

CCT

to the top plate of the

capacitor) must be at least 0.35 Ω. This resistor can be in the form of a discrete component, or routing,

or both. For example, if the picked inductor has minimum DCR of 0.25 Ω, then a routing resistance of

at least 0.10 Ω is required. Be careful not to exceed the maximum resistance rule (2 Ω). For example, if

using discrete R1, the maximum DCR of the L should be less than 2.0 - 1.1 = 0.9 Ω, which precludes

using L2 and possibly L1.

Other routing requirements include:

• The capacitor should be close to the PLL1 and PLL2 pins, with less than 0.1 Ω per route (These

routes do not count towards the minimum damping resistance requirement).

• The PLL2 route should be parallel and next to the PLL1 route (minimize loop area).

• The inductor should be close to the capacitor; any routing resistance should be inserted between

VCCT and the inductor.

• Any discrete resistor should be inserted between V

CCT

and the inductor.

A4. Comments

• A magnetically shielded inductor protects the circuit from picking up external flux noise. This

should provide better timing margins than with an unshielded inductor.

• A discrete or routed resistor is required because the LC filter by nature has an under-damped

response, which can cause resonance at the LC pole. Noise amplification at this band, although

not in the PLL-sensitive spectrum, could cause a fatal headroom reduction for analog circuitry.

The resistor serves to dampen the response. Systems with tight space constraints should consider a

discrete resistor to provide the required damping resistance. Too large of a damping resistance can

cause a large IR drop, which means less analog headroom and lower frequency.

• Ceramic capacitors have very high self-resonance frequencies, but they are not available in large

capacitance values. A high self-resonant frequency coupled with low ESL/ESR is crucial for

sufficient rejection in the PLL and high frequency band. The recommended tantalum capacitors

have acceptably low ESR and ESL.

• The capacitor must be close to the PLL1 and PLL2 pins; otherwise the value of the low ESR

tantalum capacitor is wasted. Note the distance constraint should be translated from the 0.1-Ω

requirement.