ZHCSNG6 November 2021 UCC28781-Q1
PRODUCTION DATA
The peak current threshold of the OPP curve is used to initiate the 160-ms timer, while the peak power limit (PPL) determines the highest controllable peak current of the peak current loop, VCST(MAX). Regardless of VVS, the ratio between VCST(MAX) and VCST(OPP) is fixed at approximately 4/3. In other words, this feature provides the highest short duration peak power (PO(MAX)) that the converter can deliver. The line-dependent PPL curve is able to achieve a consistent peak power level over a wide input voltage range. As an example, to supply a highest peak power of 150% of nominal rated load, choose an RCS value that ensures that the peak current threshold at 150% load and VBULK(MIN) must not be above VCST(MAX). Then, the threshold of the OPP power (PO(OPP)) can be programmed to approximately 112% to support 150% peak power design, based on the following equation.
Additionally, before VO reaches steady-state regulation during VO soft-start, the highest VCST can also reach VCST(MAX). The transformer maximum flux density must have sufficient design margin to the high-temperature saturation limit of the core material at the peak current while in PPL.