This invention involves with a low power IC (Integrated Circuit) with high energy efficiency. This invention describes a Buck converter that can track the minimum energy point of the load. It works by estimating input energy of every sensing period, taking advantage of energy consumption curve of IC in sub-threshold. Energy estimation is implemented with counting conducted pulses, while maintaining constant input energy of each pulse by regulating output voltage and ON time with digital control circuit. With digital control circuit, minimum energy point can be tracked with a lookup table stored inside. Most of this invention's control circuit is digital, with benefits of low power consumption and small chip area.