Spread-Spectrum Frequency Modulation in a DC/DC Converter With Time-Based Control

Spread-spectrum frequency modulation is a technique commonly adopted in dc/dc converters with pulse width modulation (PWM) control to reduce electromagnetic interference induced by switching. It typically requires a dedicated circuit. This article introduces a novel technique to implement spread-spectrum modulation in a dc/dc converter with time-based control, requiring only an additional transconductor. Modulating the bias current of the voltage-controlled oscillators used in the controller via a common-mode signal, the switching frequency of the converter is modulated without affecting the duty cycle. A theoretical analysis providing the link between modulation factor and design parameters is shown, and the circuit implementation in a time-based buck converter for general-purpose applications in a bipolar-CMOS-DMOS (BCD) process with 180 nm CMOS is described, together with a comparison between the simulated and the measured spectrum. The measured EMI reduction is 11.2 dB. The area occupation of the SSFM circuit is 0.032 mm2.