Spurious noise reduction by modulating switching frequency in DC-to-DC converter for RF power amplifier

The presence of a spectrum spreading effect in a DC-to-DC converter operating in Pulse-Width Modulation (PWM) with a modulated switching frequency is analyzed. A step-down DC-to-DC converter prototype with a digital PWM ramp signal modulator is implemented in a standard CMOS 0.18-μm process. The step-down DC-to-DC converter has an 6-bit up/down binary counter to vary its switching frequency between 2.2 MHz and 4.4 MHz in 64 steps as means of decreasing spurious noise peaks at the output of the switch converter. The measurement results show that the spurious switch noise peak is reduced by 12 dB when a monotonic frequency stepping with the up/down counter is used, and it is shown that the additional switching frequency modulation functionality does not degrade the overall efficiency and the closed loop operation significantly.