The Describing Function approach is used to adjust the parameters of fast-oscillations (chattering) caused by the presence of fast-actuators in Super-Twisting control loops. Estimated parameters, amplitude and frequency of self-excited oscillations, allow to compute the average power needed to maintain the trajectories of the system into real sliding-modes. Through the parametrization of the actuator dynamics by a critically damped second-order system or by a constant delay, sets of STA gains are provided to minimize the amplitude of oscillations or the average power. The results are confirmed by simulations. (C) 2018 Elsevier Ltd. All rights reserved.