Spurious-noise-free buck regulator for direct powering of analog/RF loads using PWM control with random frequency hopping and random phase chopping

Buck regulators are widely employed in portable devices due to their high power-conversion efficiency. However, due to their spurious output noise, they are not directly used to power sensitive analog/RF modules, and subsequent linear low-dropout regulators (LDOs) are needed to generate secondary low-noise supply rails for these modules. This results in lower efficiency, and increased size and cost. Moreover, as switching frequencies increase to reduce passive components, LDOs become less effective in filtering the switching noise due to their poor power-supply rejection (PSR) beyond 1 MHz. Several techniques for reducing the spurious noise of buck regulators by manipulating their switching behavior have been studied. This includes using ΔΣ or Δ modulators in the control loop, which although reduce the spurs, result in large increases in the noise floor that mandates subsequent LDOs. Other techniques redistribute the power of each spur into multiple smaller ones using random frequency hopping, or periodic monotonic frequency stepping. However, the resulting spectrum continues to be spurious and the reduction reported in the largest spur is limited to 10 to 12 dB. This limited reduction, coupled with the fact that many extra spurs are generated, leaves the load circuitry vulnerable to performance degradation. This paper proposes a pulse-width modulation (PWM) control scheme for buck converters based on combining random frequency hopping with phase chopping. The technique results in full elimination of spurs, elimination of hopping transients, very low noise floor, and minimalist implementation with little overhead on area and power.