The CY2305 and Z9305 are two different parts. Z9305 is obsolete now. An equivalent of the Z9305 would be the CY23S05. The CY2305 and CY23S05 are newer design parts. The only difference between the two would be the Spread Spectrum compatibility: CY2305 is Not Spread Spectrum compatible. CY23S05 is Spread Spectrum compatible with wider loop bandwidth. For your system, the CY23S05 is the choice. You can use a Time Domain Analyzer to check the profile of the spread of the input vs. the output. Or you can also use a Spectrum Analyzer to check as well.