lu82562ex Intel Corporation, lu82562ex Datasheet - Page 17

• Manufacturer Part Number: lu82562ex
• Description: Dual Footprint
• Manufacturer: Intel Corporation

Figure 4. Crystal Circuit
3.1.1.6
A crystal specified and tested as series resonant oscillates without problem in a parallel-resonant
circuit, but the frequency is higher than nominal by several hundred parts per million. The purpose
of adding load capacitors to a crystal circuit is to establish resonance at a frequency higher than the
crystal’s inherent series resonant frequency.
Figure 4
crystal circuit. The crystal and the capacitors form a feedback element for the internal inverting
amplifier. This combination is called parallel-resonant, because it has positive reactance at the
selected frequency. In other words, the crystal behaves like an inductor in a parallel LC circuit.
The formula for crystal load capacitance is as follows:
An allowance of 3 pF to 7 pF accounts for lumped stray capacitance. The calculated load
capacitance is 16 pF with an estimated stray capacitance of about 5 pF.
Individual stray capacitance components can be estimated and added. For example, surface mount
pads for the load capacitors add approximately 2.5 pF in parallel to each capacitor. This technique
is especially useful if Y1, C1 and C2 must be placed farther than approximately one-half (0.5) inch
from the controller. It is worth noting that thin circuit boards generally have higher stray
capacitance than thick circuit boards.
illustrates a simplified schematic of the 82562EZ(EX) and the 82547GI(EI) controller’s
Load Capacitance
where C1 = C2 = 22 pF (as suggested in most Intel reference designs)
and C
within the Ethernet controller package
stray
= allowance for additional capacitance in pads, traces and the chip carrier
C
L
X1 or Xin
=
Silicon
LAN
82562EZ(EX)/82547GI(EI) Dual Footprint Design Guide
-------------------------
(
(
C1
C1 C2
C1
+
⋅
C2
C2
X2 or Xout
)
)
Silicon
LAN
+
C
stray
9