CY23FS04ZC datasheet(7/12 Pages) CYPRESS

7 / 12 page

CY23FS04

Document #: 38-07304 Rev. *C

Page 7 of 12

XTAL Selection Criteria and Application Example

Choosing the appropriate XTAL will ensure the FailSafe device

will be able to span an appropriate frequency of operation.

Also, the XTAL parameters will determine the holdover

frequency stability. Critical parameters are as follows. Our

recommendation is to choose:

• 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.

C0 is the XTAL shunt capacitance (3 pF–7 pF typ).

C1 is the XTAL motional capacitance (10 fF–30 fF typ).

The capacitive load as “seen” by the XTAL is across its

terminals. It is named Clmin (for minimum value), and Clmax

(for maximum value).These are used for calculating the pull

range.

Please note that the Cl range “center” is approximately 20 pF,

but we may not want a XTAL calibrated to that load. This is

because the pullability is not linear, as represented in the

equation above. Plotting the pullability of the XTAL shows this

expected behavior as shown in Figure 8. In this example,

specifying a XTAL calibrated to 14 pF load provides a

balanced ppm pullability range around the nominal frequency.

Note:

5. The above example shows the maximum range the FailSafe internal capacitor array is capable of (0 to 48.6 pF).Cypress recommends the min/max capacitor

array values be programmed to a narrower range such as 6 pF–30 pF, or 7.5 pF–27 pF. This ensures the XTAL operates between series resonance and

anti-resonance. Please contact Cypress for choosing these range settings.

Clmin = (12 pF IC input cap + 0 pF pulling cap+ 6 pF trace cap on board)/2 = 9 pF

Clmax = (12 pF IC input cap + 48 pF pulling cap+ 6 pF trace cap on board)/2 = 33 pF

Pull Range =(fClmin–fClmax)/fClmin = ((C1)/2)[(1/(C0+Clmin))–(1/(C0+Clmax))]

Pull Range in ppm

= ((C1)/2)[(1/(C0+Clmin))–(1/(C0+Clmax))] × 106

Example:[5]

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