export csv in zend framework

I'm trying to export a database table as a .csv downloadable from the browser. My code is zend framework based and I'm almost there with the following action:

public function exportTableAction()
{
    $this->_helper->layout->disableLayout();
    $this->_helper->viewRenderer->setNoRender();

    $fileName = $this->_getParam('fileName');
    $tableName = $this->_getParam('tableName');       

    header('Content-type: application/octet-stream');
    header('Content-Disposition: attachment; filename="'.$fileName.'"');

    echo $this->getCsv($tableName, $fileName);
}

I can download my .csv file containing valid data. However, even if I disabled the layout and the renderer, I still get the output of the header, sidebar, and footer of my page at the end of my .csv file. Is there a way to disable any html output other than the one generated in my exportTableAction? Or can I send the header information and the csv string to the browser in a different way?

BTW: I'm using the action stack plugin to help me render the header and sidebar as follows:

...
$actionStack = $front->getPlugin('Zend_Controller_Plugin_ActionStack');
$actionStack->pushStack($userlogAction);
$actionStack->pushStack($rightcolAction);

Cheers, Adrian