Maintaining high-efficiency through wide load ranges is important for switching DC-DC converter designs. By appropriately varying the switching frequencies with various load currents, the power-efficiency can be improved. The optimal relationship of switching frequencies versus load currents can be obtained by evaluating the power-loss models of switching converters. In this paper, a variable-frequency controller (VFC) circuit which can realize the optimal relationship is proposed. The VFC circuit can easily accommodate with various switching converters. A switching DC-DC buck converter with the proposed VFC circuit is designed. Compared to existing converters, variable-frequency control can be achieved with lower cost and lower controller power-loss by employing the novel VFC circuit.