CY28435OXCT SPECTRALINEAR [SpectraLinear Inc], CY28435OXCT Datasheet - Page 11

• Manufacturer Part Number: CY28435OXCT
• Description: Clock Generator for Intel Grantsdale Chipset
• Manufacturer: SPECTRALINEAR [SpectraLinear Inc]

Rev 1.0, November 20, 2006
Calculating Load Capacitors
In addition to the standard external trim capacitors, trace
capacitance and pin capacitance must also be considered to
correctly calculate crystal loading. As mentioned previously,
the capacitance on each side of the crystal is in series with the
crystal. This means the total capacitance on each side of the
crystal must be twice the specified crystal load capacitance
(CL). While the capacitance on each side of the crystal is in
series with the crystal, trim capacitors (Ce1,Ce2) should be
calculated to provide equal capacitive loading on both sides.
Cs1
Figure 2. Crystal Capacitive Clarification
Figure 3. Crystal Loading Example
Ce1
X1
Ci1
Clock Chip
XTAL
Ci2
X2
Ce2
Cs2
3 to 6p
33pF
Trim
Pin
Trace
2.8pF
As mentioned previously, the capacitance on each side of the
crystal is in series with the crystal. This mean the total capac-
itance on each side of the crystal must be twice the specified
load capacitance (CL). While the capacitance on each side of
the crystal is in series with the crystal, trim capacitors
(Ce1,Ce2) should be calculated to provide equal capacitance
loading on both sides.
Use the following formulas to calculate the trim capacitor
values for Ce1 and Ce2.
CL....................................................Crystal load capacitance
CLe......................................... Actual loading seen by crystal
using standard value trim capacitors
Ce..................................................... External trim capacitors
Cs .............................................. Stray capacitance (terraced)
Ci ...........................................................Internal capacitance
(lead frame, bond wires etc.)
CLe
Total Capacitance (as seen by the crystal)
=
(
Load Capacitance (each side)
Ce1 + Cs1 + Ci1
Ce = 2 * CL – (Cs + Ci)
1
+
1
Ce2 + Cs2 + Ci2
1
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)