Can I regulate the already regulated buck converter output (5V) to 3.3V to power my microcontroller? If yes, how to connect both?
I think i am connecting it wrong. kindly help me to connect it
  correctly


Look what you have done: -



The LTC3638 has no feedback signal from its output hence, it will produce an unregulated output of virtually unknown or unpredictable values AND you have used that FB node to power your 3.3 volt regulator. Now that just isn't right.

Try connecting the OUT node to the IN node and using the correct ratio of feedback resistors from this newly combined node (if needed for 5 volt operation).