Differentiable Sawtooth.

I'm wondering if anyone here has a clue as to where to look for a relatively simple function to calculate an approximate sawtooth curve. I need something that is continuous and differentiable or I'd just use something standard like the floor function. I've looked at fourier approximations and they need a lot more terms than I like to approximate a reasonable sawtooth, PLUS they are awfully bumpy.

So far all attempts to look up periodic asymmetrical functions on the net have failed me.