How to define the noise spectral density of LTC6655?
I cannot find the information in the datasheet.
I'm not sure I fully understand your question about the definition of noise spectral density for LTC6655.
We provide the full voltage noise spectral density plots for the LTC6655 1.25V version on p. 7 of the datasheet, and the 2.5V and 5V versions on p. 8 and p. 10 respectively.
The voltage noise spectral density for the other output voltage variants are scaled versions of these plots, so you can interpolate them to estimate where the noise is likely to fall.
Note that the value of output capacitance significantly changes the output bandwidth and thus the noise bandwidth, so the total integrated noise will be lower if the output cap is larger. There is a tradeoff between noise peaking and minimal noise bandwidth, along with the usual concerns about response time, so going with the largest output capacitor may not always be better.
We do not list a voltage noise density in the spec table because we do not provide max/min guarantees for the noise density. These plots are typical values only. However, in my experience, most parts will have noise profiles very similar to these.