Stability analysis of linear-nonlinear control (LnLc) applied to fast transient response DC-DC converters

Linear-nonlinear control (LnLc) was presented like a solution to improve the transient response in low output voltage DC-DC buck converters, used to feed last generation of microprocessors and DSPs. This novel control optimizes the features of the conventional linear control in order to reduce the recovery time of the output voltage drop produced when a load current step occurs. In this paper, the ideal control requirements with respect to fast transient response are defined. Also, it is shown that the features of the linear-nonlinear control fit with the ideal control requirements, doing it better than the most of solutions. Furthermore, the LnLc improves the stability of the DC-DC power supply modifying the open loop gain and phase as function of the load current steps. To show this behavior the control transfer function (G_LnLc) has been deduced. Several experimental results have been obtained to check this behavior in a synchronous rectifier buck converter. Thus, the novel LnLc control has been compared with the conventional voltage control, under different conditions: with a fast voltage linear loop, with a slow voltage linear loop, with a unstable voltage linear loop, without error amplifier compensation, etc.