Emission reduction with spread spectrum clocking for switched capacitor buck converter

The reduction of electromagnetic emissions (EME) in switch mode power supplies (SMPS) has high importance especially in automotive applications. The main methods to influence the switching behaviour of SMPS are external filters, spread spectrum techniques and output driver circuits. This paper presents the application of the spread spectrum technique for a 1.25 V and 1.851 MHz switched capacitor buck converter with a digital control loop to reduce the conducted emissions. The frequency modulated sawtooth and pseudo-random profiles are implemented in the digital controller and discretized by a 100 MHz system digital clock. The work investigates the influence of the profile's modulation frequency and the effect of the pulse width modulation (PWM) resolution limitation on the EME reduction. The improvement using a sawtooth profile of 10 dBuV in comparison to the non-modulated PWM signal is seen and discussed. The comparison between modulated and non modulated output voltage ripple is also presented.