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This paper investigates a mixed-signal fixed frequency digital voltage-mode controller for dc-dc converters. Switch turn-on is determined by system clock, while switch turn-off is determined asynchronously by comparing a signal proportional to the derivative of the output voltage and the voltage ramp driven by the Digital-to-Analog Converter (DAC). One of the most important features is that the derivative action of the Proportional-Integral-Derivative (PID) voltage-mode controller is inherently obtained by a combination of the analog front-end and the hard-wired digital logic, without requiring the digital computation of the derivative action nor any analog derivative circuits. This property potentially enables wide-bandwidth controllers with improvement in dynamic performance respect to conventional digital controllers based on Analog-to-Digital Converters (ADCs) and Digital Pulse Width Modulators (DPWMs). The proposed control architecture is also effective from the IC point of view, since it is based on a DAC, a simple analog front-end and low digital signal-processing requirement. Simulation and experimental results on a 1.2 V - 20 A synchronous buck converter confirm the validity of the proposed solution.