Digital PFM controller with adaptive on time based on load current estimation

This paper focuses on efficiency characterization and efficiency improvements in digitally controlled DC-DC converters operating at light loads. An automated efficiency characterization system has been designed based on a Labview controlled laboratory setup and communication with an FPGA based digital controller via serial link. Experimental efficiency characterization results, in conjunction with a power loss model, are used to address optimum selection of control parameters in pulse frequency modulation (PFM) mode of operation. A simple load current estimator is proposed to adaptively select the switch on/off times in order to optimize light load efficiency subject to output voltage ripple specifications. Experimental results on a synchronous point of load buck converter show that PFM operation with adaptive on time results in significant efficiency gains.