1 Introduction

Synchronous buck converter with adaptive constant on-time (D-CAP?) structure [1-2] is widely used as a second-stage source in electronic systems to supply various load, such as MCU, FPGA, and another signal device. In some applications, for the purpose of main source off-line operation, there will exist a backup source (such as battery or super capacitor) to support load after the main power supply being cut off. As shown in Figure 1-1, the Buck converter of main power source will encounter to operate with bias output and floating input. Actually, several of TI’s adaptive on-time control converters already can support normal output pre-bias start up [4-5] with input side clamping to a voltage source. However, because of unclear conception about this special conditional with input floating, there is a common way which uses a controlled switch to avoid power flow back to input of Buck converter. But it will increase the cost and the size of the system. Based on the buck converters with the normal pre-bias start up function, this paper will make a deep analysis of the floating-input pre-bias output operation and assess the feasibility of removing the controlled switch for cost saving purpose.