CY28435 Cypress Semiconductor, CY28435 Datasheet - Page 11

• Manufacturer Part Number: CY28435
• Description: Clock Generator
• Manufacturer: Cypress Semiconductor

Document #: 38-07664 Rev. *B
Table 4. Crystal Recommendations
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.
14.31818 MHz
Frequency
(Fund)
Cs1
Figure 2. Crystal Capacitive Clarification
Figure 3. Crystal Loading Example
Ce1
X1
Ci1
Clock Chip
Cut
AT
XTAL
Loading Load Cap
Parallel
Ci2
X2
Ce2
20 pF
Cs2
3 to 6p
33pF
Pin
Trim
Trace
2.8pF
PRELIMINARY
0.1 mW
(max.)
Drive
Shunt Cap
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.)
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.)
(max.)
5 pF
CLe
Total Capacitance (as seen by the crystal)
=
Motional
0.016 pF
(max.)
(
Load Capacitance (each side)
Ce1 + Cs1 + Ci1
Ce = 2 * CL – (Cs + Ci)
1
Tolerance
35 ppm
(max.)
+
1
Ce2 + Cs2 + Ci2
Stability
30 ppm
(max.)
1
CY28435
Page 11 of 23
(max.)
Aging
5 ppm
)