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This paper investigates a digital voltage-mode controller for dc-dc converters based on hysteresis modulation. The control structure implements a high-bandwidth hysteretic differentiator as its main building block, and realizes a nonconventional structure of PID compensation with performances comparable to analog hysteretic controls, thus breaking the bandwidth and dynamic limitations commonly encountered in typical digital control arrangements. The employment of an asynchronous A/D converter based on the threshold inverter quantization concept dramatically shrinks the average delay time that separates the sampling instant from the corrective control action. Moreover, the hysteretic nature of the derivative action results in an inherent nonlinear response to large signal load variations, which translates into fast control intervention and reduced settling times. The hysteretic differentiator employs a ring-oscillator-based modulator, which ensures resolution up to 390 ps without asking for a high-frequency clock. Both the 6-bit asynchronous A/D converter and the ring-oscillator-based modulator are designed and manufactured in the same IC using a standard 0.35 mum CMOS process. Analytical modeling, computer simulations, and experimental results on a synchronous buck converter confirm the validity of the approach and the dynamic performances achievable by the proposed control architecture.
Date of Publication: Jan. 2009