SLUSC60B December 2017 – October 2019 UCC28064A
The MOSFET on-time TON depends on value of the selected inductance on load value, represented by COMP pin voltage and by the converter AC input voltage Equation 45. To ensure proper operation, the timing must be set based on the highest boost inductance (L1MAX) and output power (POUT) at minimum operating AC input Voltage. Because the input voltage is sensed by VINAC pin the on time setting needs to take into account of the selected resistor divider that provide voltage at VINAC pin. In this design example, the boost inductor could be as high as 390 µH.
Let's call KBO the ratio:Equation 49.
the Maximum on time at full load (POUT = 300W) and minimum input voltage (85VAC) is given by formula;
The value of the resistor RT connected to TSET pin to set the on time timers is the minimum of RTH and RTL provided by formulas (46) and (47)
Where the values of KTL_min and KTH_min are the minimum values of KTL and KTH parameters reported on the electrical characteristic table at page 7 of this datasheet.
The selected value for RT is 115 kΩ.