CY28411-1
Figure 1. Crystal Capacitive Clarification
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.
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
Clock Chip
Ci2
Ci1
Pin
3 to 6p
X2
X1
Cs2
Cs1
Trace
2.8pF
XTAL
Ce1
Ce2
Trim
33pF
Figure 2. Crystal Loading Example
As mentioned previously, the capacitance on each side of the
CL....................................................Crystal load capacitance
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.
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.)
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
Load Capacitance (each side)
Ce = 2 * CL – (Cs + Ci)
Ce..................................................... External trim capacitors
Cs..............................................Stray capacitance (terraced)
Total Capacitance (as seen by the crystal)
Ci ...........................................................Internal capacitance
(lead frame, bond wires etc.)
1
CLe
=
1
Ce2 + Cs2 + Ci2
1
Ce1 + Cs1 + Ci1
(
)
+
Rev 1.0,November 22, 2006
Page 8 of 18
SPECTRALINEAR [ SPECTRALINEAR INC ]
