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 CVC from VDD1 when the converter is disabled (either by the PD control or the VC 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 VC and VB must be minimal while CVC charges, otherwise the converter may never start. The optocoupler will not load VB 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 VC 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 VC. The control circuit discharges VC 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 VDD1 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 VB 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 VC 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 (VVDD1 – VVC) falls less than 7 V, but will start the converter from adapters within tST. The lower test voltage for tST 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 VDD1 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 VVC if the converter is disabled and there is sufficient VDD1 to charge higher.