SW52212 datasheet(21/71 Pages) SILABS

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5.3 Calculating Load Capacitors

In addition to the standard external trim capacitors, consider the trace capacitance and pin capacitance to calculate the crystal loading

correctly. 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.

Clock Chip

Ce1

Ce2

Cs1

Cs2

Ci1

Ci2

3 to 6 pF

Trace

2.8 pF

Trim

7-24 pF

XTAL

Figure 5.2. 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 =

(

Ce + Cs1 + Ci1

Ce2 + Cs2 + Ci2 )

• 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.)

Data Sheet

Functional Description

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