A resistor and a capacitor (LP filter) will generate an exponential sawtooth (some curvature in the slope of the sawtooth) with a square-wave input. The higher the amplitude of the sawtooth in relation to the square-wave amplitude (determined by the RC time constant), the more slope you will see. If you select the time constant to give a small amplitude sawtooth and then amplify it to the desired level, the slope can be fairly straight.
If you want a perfect sawtooth with straight slopes, you will need a current source driving the cap, or use an integrator circuit (op amp or transistor).