I want to write the output in csv in the format "csvname_timestamp", but when i am doing it, fme is writing all features into different csv files, why its doing like that? I want just 1 csv file with all the accumulated info, please help?

Use a Creator followed by DateTimeStamper and join/merge the timestamp with your data via FeatureMerger/FeatureJoiner (1 to 1)

You could also set the timestamp with a scripted Python parameter.  See Example 2 here.

import fme
from datetime import datetime
curTime = datetime.now().strftime("%Y%m%d_%H%M%S")
return curTime

Then set the name of the CSV feature type to the published parameter.