Abstract
Spread-spectrum frequency modulation is a technique commonly adopted in DC/DC converters with PWM control to reduce electromagnetic interference induced by switching. It typically requires a dedicated circuit. This paper 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 BCD process with 180nm CMOS is described, together with a comparison between the simulated and the measured spectrum. The measured EMI reduction is 11.2dB. The area occupation of the SSFM circuit is 0.032mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> .
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