Digitally Controlled Pulse Width Modulator for On-Chip Power Management

Abstract — A digitally controlled current starved pulse width modulator (PWM) is described in this paper. The current from the power grid to the ring oscillator is controlled by a header circuit. By changing the header current, the pulse width of the switching signal generated at the output of the ring oscillator is dynamically controlled, permitting the duty cycle to vary between 25 % and 90%. A duty cycle to voltage converter is used to ensure the accuracy of the system under process, voltage, and temperature (PVT) variations. A ring oscillator with two header circuits is proposed to control both duty cycle and frequency of the operation. Analytic closed-form expressions for the operation of a PWM are provided. The accuracy and performance of the proposed PWM is evaluated with 22-nm CMOS predictive technology models under PVT variations. An error of less than 3.1 % and 4.4 % in the duty cycle, respectively, with and without constant frequency control is reported for the PWM. A constant operation frequency with less than 1.25 % period variation is demonstrated. The proposed PWM is appropriate for dynamic voltage scaling systems due to the small on-chip area and high accuracy under PVT variations. Index Terms — Current starvation, digital-controlled oscilla-tors, pulse width modulation, ring oscillators