Creating a subquery using laravel eloquent

My raw query working fine. but after converting laravel eloquent it's not working

SELECT
    key_vals.`key`,
    key_vals.`value`
FROM
    key_vals
WHERE TIMESTAMPDIFF(MINUTE,
        key_vals.`last_store_time`,
        now()) < (SELECT ttl from ttls) 

via Chebli Mohamed