Pull downs and pull ups in the same session can be diminsihing.
A good back workout would consist of:
a power movement e.g. deadlift [2 sets]
a vertical row e.g. a pull up, chin, pull down etc..[2 sets]
and a horizontal row e.g. DB row, BB row etc...[2 sets]
Then you can throw in 2 sets of DB shrugs for your traps if you find it necessary.