$form['submitted']['some_placeholder']['own_btn'] = array(
       '#type' => 'button',
       '#value' => t('Add'),
       '#name' => 'ob_add_food',
       '#weight' => 19,
       '#prefix' => '<td>',
       '#suffix' => '</td></tr></table>',

       '#limit_validation_errors' => array(),
       '#ajax' => array(
          'callback' => 'callback_js',
          'wrapper' => 'wrapper-div',
          'method' => 'replace',
          'effect' => 'fade',
          'speed' => 'slow',
    ),
);

function _add_own_food_js($form, $form_state) {
     return  $form['submitted']['some_placeholder'];
  }