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Dear experts,
Hello Ryo-san,
Thank you for your interest in the UCC28600 flyback controller .
The UCC28600 limits the output power, not specifically the output current, so Iout varies inversely as Vout varies.
Therefore start-up time of Vout is not a simple function of a fixed, maximum Iout.
From the datasheet, equation (7) is the minimum start-up time from Vout=0 to regulated Vout when there is no external load applied. All of the available output power is delivered into the output capacitor Cout only, and Cout charges up the fastest in this condition. In eqn (7), "Plim" is the same as "Pout(max)limit" of eqn (6).
Equation (6) calculates the start-up time when there is a resistive load applied to Cout during start-up. In this case, Rload(ss) draws current away from Cout as Vout rises, which slows down the rise time of Vout. Eqn (6) is exponential because of the Rload*Cout time constant.
For maximum time, I suggest to set deltaVout(step) = 0. Even though eqn (6) is called "tSS(min)", it is really a kind of maximum time "tSS(max)", but it is still called (min) so that it can be used in eqn (5).
After determining the highest tSS(min) value from eqn (6) or eqn(7), whichever applies to your customer's design, select the Css based on Eqn (5). Usually, Css is calculated as a non-standard value, so the user selects the next higher standard value. For example, if Css calculates to be 41nF, choose Css = 47nF. Then rearrange eqn (5) to solve for tSS(min) based on the actual selected Css value.
Note: neither of these times, from eqn (6) or (7), account for the time delay from first applying input power to the time that Vout starts to rise. That time depends on how fast the VDD cap can be charged from 0V to the 13-V start-up threshold to begin the start-up process. That is a separate calculation.
Regards,
Ulrich