A resonant dc-dc converter converts dc input voltage to a dc output voltage. The converter includes switching, switching driving, conversion, and disabling circuits. The switching circuit receives the input voltage and generates a square wave voltage oscillating between the input voltage and a low value, at a frequency with a duty cycle. The switching driving circuit drives the switching circuit and includes a timing circuit for setting the frequency and the duty cycle. The timing circuit sets the value of the duty cycle to about 50% when the converter operates in steady state. The conversion circuit generates the output voltage from the square wave voltage based on the frequency and the duty cycle. The disabling circuit temporarily halts the timing circuit after a power on in such a way to temporarily vary the duty cycle of the square wave voltage during a period of the square wave voltage.