JAJU858 December 2022
The capacitance on VDD needs to supply the device operating current until the output of the converter reaches the target minimum operating voltage in CC regulation. The capacitance on VDD must supply the primary-side operating current used during start-up and between low-frequency switching pulses. The largest result of two independent calculations denoted in Equation 36 determines the value of CVDD.
At start-up, when VVDD(on) is reached, CVDD alone supplies the device operating current and MOSFET gate current until the output of the converter reaches the target minimum operating voltage in CC regulation, VOCC. Now the auxiliary winding sustains VDD for the UCC28742 device above UVLO. The total output current available to the load and to charge the output capacitors is the CC-regulation target, IOCC. Equation 36 assumes that all of the output current of the converter is available to charge the output capacitance until VOCC is achieved. For typical applications, Equation 37 includes an estimated qG × fSW(max) of average gate drive current and a 1-V margin added to VVDD.
The current design uses 10-µF and 0.1-µF capacitors.