how do i ignore the backticks in codeigniter when using active records? i need to order by cases

how do i ignore the backticks in codeigniter when using active records? i
need to order by cases

how do i ignore the backticks in codeigniter when using active records? i
need to order by cases
case when roll_number is null then 1 else 0 end,
which code igniter makes it as
ORDER BY `case` when roll_number is null then 1 else 0 end
which gives me an error. Please help