Proposal of a low power, 1.6 MHz, 91% efficiency, single inductor, double symmetrical outputs integrated DC-DC converter for CCM and DCM operations

A single inductor, double symmetrical outputs, integrated DC-DC converter with analog control has been designed for supply of low power, output-capacitor-less audio amplifiers. Such amplifiers require positive/negative symmetrical voltage supply for driving headphones without common mode on their output voltages. The power stage of the converter use a single external inductor, 3 external capacitors and five integrated power switches to produce two symmetrical bipolar outputs. This circuit permits to make the economy of using a combination of two separate converters for generating both the positive and the negative output voltage. The benefits are a low number of external components and IOs, and reduced silicon area. The control of the output voltages is accomplished by two feedback loops operating in PWM. A primary prototype has been fabricated in CMOS 130nm process for Continuous Conduction Mode (CCM) and achieves 80% peak efficiency at 360 mW output power and 1.6 MHz switching frequency. This prototype validates the simulation results about the analog control strategy and the integration of the whole power stage and control. Improvements are presented in order to increase the efficiency up to 90% at full load. The operation in Discontinuous Conduction Mode (DCM) and Pulse Skipping (PSK) are introduced to improve the efficiency for low output power. The electrical implementation of DCM and some simulation results are presented. Light-load efficiency is improved to around 60%.