Helper- menu and active submenu (codeigniter)

Hello! I'm using the helper menu to include class active in the site menus.

Helper Menu:

<?php 
    defined('BASEPATH') OR exit('No direct script access allowed');
    if(!function_exists('active_link'))
    {
        function menu_ativo($controller)
        {
        //Obtem Instância da classe CI
        $CI = get_instance();
        //Obtem classe ativa.
        $class = $CI->router->fetch_class();
        return ($class == $controller) ? 'active' : '';
       }
    }
?>

And I invoke this function as below

<li class="treeview <?php echo menu_ativo('conta'); ?>">
   <a href="#">
   <i class="fa fa-money"></i>
   <span><?php echo 'Financeiro'; ?></span>
   <i class="fa fa-angle-left pull-right"></i>
   </a>
   <ul class="treeview-menu">
      <li class="<?php echo menu_ativo('categoria'); ?>">
         <a href="<?php echo site_url('app/categoria'); ?>"><i class="fa fa-circle-o"></i>Categoria</a>
      </li>
      <li class="<?php echo menu_ativo('conta'); ?>">
         <a href="<?php echo site_url('app/conta'); ?>"><i class="fa fa-circle-o"></i>Conta</a>
      </li>
      <li class="<?php echo menu_ativo('encargo'); ?>">
         <a href="<?php echo site_url('app/encargo'); ?>"><i class="fa fa-circle-o"></i>Encargo</a>
      </li>
      <li class="<?php echo menu_ativo('lancamento'); ?>">
         <a href="<?php echo site_url('app/lancamento'); ?>"><i class="fa fa-circle-o"></i>Lançamento</a>
      </li>
   </ul>
</li>

My problem is basically in <li class="treeview <?php echo " ???? " ;?>">

Since every <li> within <ul> belongs to a different controller, I want to activate any sub-menu and keep the main menu active <li> . But with the above helper not working, although the submenu is inheriting class active , the main menu is closing.

Can you guide me?