Ramp Generator using op amp
The standard method for doing what you want is to configure an op-amp as an integrator with a constant input current that sets your ramp rate, then discharge the integrator capacitor when it's time for the ramp to reset back to the start level.

How quickly must the ramp reset back to the start level?  The device that you choose to discharge the integrator capacitor determines the reset time - older designs used a J-FET configured as a switch but the ON-resistance slows down the reset.

There are now modern switches available that will discharge a relatively-large capacitor in very little time.