Fundamental Study on the Influence of Ripple Noise from DC-DC Converter on Bias Fluctuation Noise in Wireless Portable Equipment

The periodic output ripple of switching power supplies poses a significant challenge in noise-sensitive, high-precision signal processing circuitry used in high-performance wireless portable equipment. This paper presents an analysis of the impact of an ultra-high-frequency compact buck-type DC-DC converter on the electronic circuitry of portable devices. A class-A amplifier is used as a representative example of the electronic circuitry. We derive equations to analyze the fluctuations in the amplifier's signal output bias voltage caused by the converter's output ripple, allowing for a theoretical examination of the fundamental characteristics of these fluctuations. The output bias fluctuations of the amplifier are calculated using these equations and measured with an evaluation circuit board. The primary method for mitigating these output bias fluctuations is to reduce the converter output noise. Therefore, decreasing the ratio between the natural frequency of the inductor-capacitor (LC) filter and the switching frequency of the converter is effective. This reduction in ratio also facilitates the miniaturization of the converter. When the LC filter of the converter is miniaturized with values of 10 nH and 10 nF, the natural frequency is approximately 16 MHz. By increasing the switching frequency to 1.6 GHz, the fundamental component of the amplifier output bias voltage is suppressed to below 0.3 mV.