how to receive response from ajax call with sencha touch from codeigniter / ExtJS

i am trying to get a response from codeigniter when I send my ajax request from sencha touch/ Ext with

Ext.Ajax.request({//
      url: formBase.url, //something like 'http://path.to/mobile/Login/ajax_validateLogin',
      success:  function(response, opts) {
                    console.log('Login success!');
                    console.log('data: ' + response.responseText);
                },
      failure:  function(result){
                    console.log('Login Error! ');                   
                },

      method: 'POST',
     开发者_开发技巧 params: form.getValues()
});
form = new Ext.form.FormPanel(formBase);
//...

and in codeigniter something like

function ajax_validateLogin() {

    $username = $this->input->post('username'); 
    $password = $this->input->post('password');
    $data['username'] = 'Arschkarte'; //$username;  
    if($username == "f"){
        $data["success"] = true;
    } else {
        $data["success"] = false;
        $data["errors"]["reason"] = "Login failed. Try again.";
    }
    $this->load->view('mobile/loginMobile', $data);
}

and the view

<?php 
    echo 'username: '.$username;
?>

i allways get the Error "Login Failed" and the response of the POST is allways empty (while parameters are send correctly)

any ideas? THNX!!!