SI52111-B3 Datasheet, PDF (8/20 Pages) Silicon Laboratories – PCI-EXPRESS GEN 2 SINGLE OUTPUT CLOCK GENERATOR

Si52111-B3/B4
2.2. Calculating Load Capacitors
In addition to the standard external trim capacitors, consider the trace capacitance and pin capacitance to calculate
the crystal loading correctly. Again, the capacitance on each side is in series with the crystal. The total capacitance
on both sides is twice the specified crystal load capacitance (CL). Trim capacitors are calculated to provide equal
capacitive loading on both sides.
Figure 2. Crystal Loading Example
Use the following formulas to calculate the trim capacitor values for Ce1 and Ce2.
Load Capacitance (each side)
Ce = 2  CL – Cs + Ci
Total Capacitance (as seen by the crystal)
CLe
=
---------------------------------------------------¹---------------------------------------------------


C^-----e^----1^-----+-----C^---1-s^----1^----+-----C^-----i^--1^-
+
C^-----e^----2^-----+-----C^---1-s^----2^-----+----C^-----i^--2^{- }
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.)
8
Rev 1.1