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I'm trying to design a flyback AC-DC supply with an output of 3.3V upto 1A. I noticed some weird behavior. If I try to get a WEBENCH design from the UCC28722 product page on the TI website it says that minimum Vout is 5V. However, if I run WEBENCH from its own page I can design a 3.3V supply. Is the UCC28722 suitable for a 3.3V supply? If yes, would the attached reference design work? I'm simply asking since I'll need to get a custom transformer made for this (even for prototyping purposes) and if this controller is not suitable it would be wasted expense for me.
Hello,
The device was designed for USB applications which are design for a 5V output. This is because of the constant current, constant voltage operation that generates the VI curve presented in figure 17 of the data sheet. So when you operating at Iocc the output voltage can drop from 5V down to 2V before shutting off.
The UCC28722 PSR controller can achieve an output accuracy of +/-5% steady state but can exceed this during a large transient (>20%). However, since these controllers were design for battery applications with lots of capacitance this is generally not an issue. If I recall correctly 3.3V applications are more critical when it comes to load transients. The UCC28722 may not be the correct device for this application. However, you should be able to design to regulate a 3.3V output if you desire, it should be an adjustment of the transformer turns ratio.
Regards,