CY23FS08 datasheet(8/15 Pages) CYPRESS

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CY23FS08

Document Number: 38-07518 Rev. *F

Page 8 of 15

Figure 12. Phase Offset

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:

■ 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 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 CLOADMIN (for minimum value), and CLOADMAX (for

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

Note that the CLOAD 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 below.

Plotting the pullability of the XTAL shows this expected behavior

as shown in Figure 13. In this example, specifying a XTAL

calibrated to 14 pF load provides a balanced ppm pullability

range around the nominal frequency.

Example:

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

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

Pull Range = (fCLOADMIN – fCLOADMAX) / fCLOADMIN = (C1 / 2) * [(1 / (C0 + CLOADMIN)) – (1 / (C0 + CLOADMAX))]

Pull Range in ppm = (C1 / 2) * [(1 / (C0 + CLOADMIN)) – (1 / (C0 + CLOADMAX))] * 106

REF

DD/2

(

)

DD/2

FBK

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