Synchronous buck converter with adaptive constant on-time (D-CAP™) control structure is widely used in various electronic systems, because of its fast transient performance, few external components for low cost and small solution size. In some special applications, it needs the D-CAP™ buck converter to operate in the standby state with pre-bias output and floated input. For previous literature about D-CAP™ structure, most of them focused on normal power transfer operation and loop performance. But the research and the assessment of the pre-bias standby operation has not been involved. Based on the basic logic of the D-CAP™ control, this application note will do a deep analysis and risk assessment about the pre-bias standby operation which has an important meaning in time saving and cost reduction.