Codeigniter 3将params传递给库构造，失败了

I'm passing an PDO instance to my library using this syntax

    $host = 'localhost';
    $db   = 'ci';
    $user = 'root';
    $pass = '';
    $charset = 'utf8';

    $dsn = "mysql:host=$host;dbname=$db;charset=$charset";
    $pdo = new PDO($dsn, $user, $pass);

    $params = array('pdo'=>$pdo);
    $this->load->library('phpauth/AuthConfig', $params);

But I'm getting this error:

Argument 1 passed to AuthConfig::__construct() must be an instance of PDO, array given

I've searched the manual and other answers I don't see any wrong in my code, how it happens?

By the way, the library I used needs a PDO instance, I have a few doubts here,

1.Is my way of creating a PDO instance the proper way to do it in codeigniter 3?

2.Can I find this PDO instance after using $this->load->database() in my controller, I tried but I can not find it in controller instance

3.Library is using the raw PDO method, my last resort is to use CI3 built in query wrapper to re-style the library method, is this right?