7.4.1.10 Bootstrap Topology

The internal start-up current source and control logic implement a bootstrap-type start-up as discussed in Start-Up and Converter Operation. The start-up current source charges C_VC from V_DD1 when the converter is disabled (either by the PD control or the V_C control) to store enough energy to start the converter. Steady-state operating power must come from a converter (bias winding) output or other source. Loading on V_C and V_B must be minimal while C_VC charges, otherwise the converter may never start. The optocoupler will not load V_B when the converter is off for most situations; however take care in ORing topologies where the output is powered when PoE is off.

The converter will shut off when V_C falls below its lower UVLO. This can happen when power is removed from the PD, or during a fault on a converter output rail. When one output is shorted, all the output voltages fall including the one that powers V_C. The control circuit discharges V_C until it hits the lower UVLO and turns off. A restart will initiate, as described in Start-Up and Converter Operation, if the converter turns off and there is sufficient V_DD1 voltage. This type of operation is sometimes referred to as hiccup mode, which provides robust output short protection by providing time-average heating reduction of the output rectifier.

The bootstrap control logic disables most of the converter controller circuits except the V_B regulator and internal reference. Both GATE and GAT2 (assuming GAT2 is enabled) will be low when the converter is disabled. FRS, BLNK, and DT will be at ARTN while the V_C UVLO disables the converter. While the converter runs, FRS, BLNK, and DT will be about 1.25 V.

The start-up current source transitions to a resistance as (V_VDD1 – V_VC) falls less than 7 V, but will start the converter from adapters within t_ST. The lower test voltage for t_ST was chosen based on an assumed adapter tolerance, but is not meant to imply a hard cutoff exists. start-up takes longer and eventually will not occur as V_DD1 decreases below the test voltage. The bootstrap source provides reliable start-up from widely varying input voltages, and eliminates the continual power loss of external resistors. The start-up current source will not charge above the maximum recommended V_VC if the converter is disabled and there is sufficient V_DD1 to charge higher.