Backup problem: "Fatal error: Allowed memory..."

[eluser]ministry[/eluser]
Hi.
I'm trying to backup my entire database, but I get this message:

Fatal error: Allowed memory size of 25165824 bytes exhausted (tried to allocate 10755665 bytes) in /web/htdocs/www.mydomain.it/home/system/libraries/Zip.php on line 160

I did my last working backup yesterday; so it seems the last inserted queries have exceded the allowed database size; at the moment its size is about 2.5mb. How can I increase the limit?

By the way, my code is very simple:

Code:

$this->load->dbutil();
$this->load->helper('download');
$namefile="backup_database.zip";
$prefs = array(
         'tables'      => array(),              // Array of tables to backup.
         'ignore'      => array(),           // List of tables to omit from the backup
         'format'      => 'zip',             // gzip, zip, txt
         'filename'    => $namefile,            // File name - NEEDED ONLY WITH ZIP FILES
         'add_drop'    => TRUE,              // Whether to add DROP TABLE statements to backup file
         'add_insert'  => TRUE,              // Whether to add INSERT data to backup file
         'newline'     => "\n"               // Newline character used in backup file
         );

    $backup = $this->dbutil->backup($prefs);
    force_download($nomefile, $backup);