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Design procedure of UCC287XX devices

Other Parts Discussed in Thread: UCC28720

Dear TI people,

Regarding the parameters tON(min) and tDMAG(min).

For the UCC28720 a tON(min) specification is given of 300 ns and tDMAG(min) of 1.2 us.

My first question is: Does the calculated value for tON(min) need to be higher or lower than the 300 ns?
My second question is: Does the calculated value for tDMAG(min) need to be higher or lower than the 1.2 us?

I think the answer would be applicable for the other UCC287XX devices as well.

Furthermore I would really appreciate to receive an answer to my earlier question with the following title: “Apply UCC28720 for 1W 5V isolated flyback from 230 AC mains”

My follow up message had some very basic understanding questions; I still hope to get an answer about.

Many thanks,

  • The value you calculate for Ton needs to be higher than the data sheet specified value of Ton(min), which is 300nS.

    The reason for this is that 300nS is the leading edge blanking time on the current sense pin.

    Tdmag(min) is the minimum value of the secondary rectifier conduction time. Its data sheet specified value is 1.2uS.

    Your circuit must have a Tdmag that exceeds this value. The upper value for Tdmag is set by the data sheet specified DMAGCC of 0.425. This would be about 5.3uS if the converter is running at 80kHz

  • Thank you John, this is clear now!