IPad: timer timer in the iOS application to authorize a registered user, even when the application is in the background

I need to set up an ios timer to automatically log out a registered user to my iOS application after a certain period of time (e.g. 20 minutes). Thus, the user enters the application with a username and password and after 20 solid minutes of inactivity (without interacting with the user interface), the user must log out (again, the user name and password are requested). I want this timer to work even if the application is in the background or the iPad is sleeping.