CY23FS08 Cypress Semiconductor, CY23FS08 Datasheet - Page 8

• Manufacturer Part Number: CY23FS08
• Description: Failsafe 2.5 V/3.3 V Zero Delay Buffer
• Manufacturer: Cypress Semiconductor

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XTAL Selection Criteria and Application Example
Selecting the appropriate XTAL ensures the FailSafe device is
able to span an appropriate frequency of operation. Also, the
XTAL parameters determine the holdover frequency stability.
Critical parameters are given here. Cypress recommends that
you choose:
■
■
■
■
Example:
C
C
Pull Range = (fC
Pull Range in ppm = (C1 / 2) * [(1 / (C0 + C
Document Number: 38-07518 Rev. *F
LOADMIN
LOADMAX
Low C0/C1 ratio (240 or less) so that the XTAL has enough
range of pullability.
Low temperature frequency variation
Low manufacturing frequency tolerance
Low aging
= (12 pF IC input cap + 0 pF pulling cap + 6 pF trace cap on board) / 2 = 9 pF
= (12 pF IC input cap + 48 pF pulling cap + 6 pF trace cap on board) / 2 = 33 pF
LOADMIN
– fC
LOADMAX
REF
FBK
) / fC
LOADMIN
LOADMIN
t
)) – (1 / (C0 + C
()
= (C1 / 2) * [(1 / (C0 + C
Figure 12. Phase Offset
V
DD
V
DD
/2
/2
LOADMAX
C0 is the XTAL shunt capacitance (3 pF to 7 pF typ).
C1 is the XTAL motional capacitance (10 fF to 30 fF typ).
The capacitive load as “seen” by the XTAL is across its terminals.
It is named C
maximum value).These are used for calculating the pull range.
Note that the C
we may not want a XTAL calibrated to that load. This is because
the pullability is not linear, as represented in the equation below.
Plotting the pullability of the XTAL shows this expected behavior
as shown in
calibrated to 14 pF load provides a balanced ppm pullability
range around the nominal frequency.
))] * 10
LOADMIN
LOADMIN
Figure
6
LOAD
)) – (1 / (C0 + C
range “center” is approximately 20 pF, but
13. In this example, specifying a XTAL
(for minimum value), and C
LOADMAX
))]
CY23FS08
LOADMAX
Page 8 of 15
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