Screen auto turn off, passcode question

new 4S owner, and this may be a weird question...

I want passcode enabled so if I choose, I can lock the screen (say while I'm at work or out and about)

BUT I also like the screen turning off after being idle, to save battery

I would like for the screen to turn off after idle, but not lock, and only lock if I manually choose to lock it, say by pressing the sleep button

From what I've seen, this is not possible with stock iOS, but is there an app for this? An app that can shut off the screen after a few minutes of idle, but not lock phone?