Set camera aspect ratio according to device screen

i am creating a simple fix screen puzzle game with multiple levels. i have to set the camera for all levels because some level need large screen some require small screen space. i have a fix top down view camera without movement.
The problem is that my levels show black part up and down the screen . How to set camera for all screen types, So that it not show black part surrounded . how to set aspect ratio and FOV of camera for all screen size of mobile and tabs. i have set aspect ratio .5625 and FOV 30 of perspective camera.