Hysteretic Mixed-Signal Controller for High-Frequency DC–DC Converters Operating at Constant Switching Frequency

A hysteretic mixed-signal controller for high-frequency switch-mode power supplies that achieves both very fast transient response and constant switching frequency under steady-state operation is presented. The mixed-signal controller combines three control loops: analog nonlinear current loop, digital linear voltage loop, and digital frequency loop. To eliminate the sampling delay associated with conventional digital controllers and improve the transient response performance, the fast nonlinear loop is built with a simple analog hardware. The nonlinear current loop is designed based on the hysteretic control of the output capacitor current. To guarantee the constant switching frequency, under different operating conditions, a digital frequency loop is proposed and implemented. The digital frequency loop eliminates undesirable switching frequency variation, associated with the conventional hysteretic controllers, by digitally adjusting the hysteretic bands around the output voltage reference. The controller operation is experimentally verified with a 5-MHz buck converter. Experimental results demonstrate fast transient response while the constant switching frequency is maintained under steady-state operation.