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This paper presents a hybrid digital adaptive (HDA) control for fast step-load transient responses in synchronous buck DC-DC converters. The proposed HDA controller results in near-time-optimal step-load transient responses even when the output voltage is sampled using a relatively low-resolution, narrow-range window A/D converter. The controller is a combination of a standard constant-frequency pulsewidth-modulated (PWM) control in the vicinity of steady-state operating point and a bank of switching surface controllers (SSCs) away from the reference. The switching surface slope is adaptively selected by a supervisor based on an inductor current estimate. Furthermore, the controller is capable of taking into account a maximum inductor current limitation. A large-signal stability analysis is presented for all possible cases during transients when one of the SSCs is active. Experimental results demonstrate fast transient responses for a 1.3 V, 10 A synchronous buck converter over a wide range of step-load transients.