7.4.2.3 Bootstrap Topology

The internal startup current source (I_{VC_ST}) and control logic implement a bootstrap-type startup as discussed in the Startup and Converter Operation section. The startup current source charges C_VC from V_DD 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 does not load V_B when the converter is off for most situations, however care should be taken in ORing topologies where the output is powered when PoE is off.

The converter shuts 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 is initiated as described in the Startup and Converter Operation section if the converter turns off and there is sufficient V_DD 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.

Below V_CUV, the bootstrap control logic disables most of the converter controller circuits except the V_B regulator and internal reference. GATE is low when the converter is disabled.

The bootstrap source provides reliable startup from widely varying input voltages, and eliminates the continual power loss of external resistors. The startup current source does not charge above the maximum recommended V_VC if the converter is disabled and there is sufficient V_DD to charge higher.