How can I add a StyleSheet in a joomla-module backend

Question

I´m developing a simple joomla 3.x module. I need to add the font-awesome.min.css to display some symbols in the backend. For the frontend i use

$doc->addStyleSheet( JUri::base( true ) . ‚/modules/xxxxx/assets/fonts/font-awesome/css/font-awesome.min.css') 

which works perfectly.

I need the same for the backend.