2 Analysis of the Floating-Input Pre-bias Output Operation

From the topology of Buck converter shown in Figure 1-1, it can be seen as that output is clamped by a bias voltage source V_bias. Because of the existing of body diode of MOSFET, the input voltage V_in will be clamped by V_bias through the body diode of the high side MOSFET, initially. Input voltage is shown in Equation 1. Actually, most of TI's IC has two basic functions : Enable function and UVLO (under voltage lock out). Device will shut down, if input voltage is lower than UVLO threshold or logic level at Enable pin is ineffective. If V_bias is a low voltage which can cause V_in to be lower than the UVLO threshold, it is obvious that the device will not start up.

If V_in is higher than UVLO threshold and Enable pin is logic effective, the device will start up and implement its logic. In this case, there’s another way that use a controlled signal to let Enable pin be ineffective, which can avoid the converter to operate to make sure there is no continuous energy flow into input side . But it will need more external components to achieve that or occupy more digital I/O resource if MCU is used.

This application note's discussion focuses on the situation V_bias-V_diode>V_UVLO and Enable pin directly being enabled from V_in. Generally, D-CAP™ structure device can be divided to two types. One type is the PSM (pulse-skip mode) device. PSM device has the logic ZC (zero-cross) detection and will reduce its frequency in light-load condition, which can implement a better light-load efficiency. ZC logic will monitor the inductor current during the low-side MOSFET turn on time, if the inductor current trend to be lower than zero, the low side MOSFET will turn off. It means that the inductor current will not be negative, if ZC logic is implemented. Another is the FCCM (force continuous conductive mode) device without the ZC, which implement a constant steady-state frequency under all load conditions. Because different types device can cause different behaviors, this application note will discuss the operation of PSM and FCCM mode device separately.