Samsung S7 dimming while using Google maps for in car nav

love the Samsung S7 but one problem i am having trouble sorting , when using google maps in the car as a Sat nav all starts well but after the allotted time the screen goes blank ( as it should) after what ever time it is set to , the max option i have is 10 mins and off course i then don't have google maps telling me how to get to my destination

How do fix this please