I don't know if an equation will help you much, but the parametric equations x = t*cos(t), y = t*sin(t), z = t will describe what you want where the tip of the cone is at the xy-plane and flares out from there. If you want to cut off the tip of the cone, so to speak, just start your parameter above 0. If you want to adjust the steepness of the cone's angle, multiply your x and y by a constant.