How CodeIgniter close db connections?

I'm wondering how CodeIgniter close db connections. In the doc, it says "CodeIgniter intelligently takes care of closing your database connections". But how does CodeIgniter do this? Seems I didn't find the db driver's "close" method get called in CodeIgniter 3.0.3 source code.

I've also posted in StackOverflow: http://stackoverflow.com/questions/43922...onnections

Thanks!