MySQL Case Statement - Unknown column in where

I have a problem with my query, what's wrong?

SELECT 
    CASE 
        WHEN cp_pessoa.score < 100 THEN 1 
        WHEN cp_pessoa.score < 300 THEN 2 
        WHEN cp_pessoa.score >= 300 THEN 3 
    END as id_ranking
FROM cp_pessoa 
    WHERE id_ranking IN (1,2);

I get this error: #1054 - Unknown column 'id_ranking' in 'where clause'

Thank you!