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UCC28600 Datasheet

Sam Guo
Intellectual 380 points
Other Parts Discussed in Thread: UCC28600

On page of UCC28600 Datasheet, it says:

• ICS1 is the power limit current that is sourced at the CS pin at low-line voltage(3)

• ICS2 is the power limit current that is sourced at the CS pin at high-line voltage(3)

Where to get Ics1 and Ics2 values to calculate Rcs and Rpl?

 

Note:  

In the UCC28600 Design Calculator, Ics1 and Ics2 are calculated results.  I can not plug them into the Rcs and Rpl equation.

  • 0
    TI__Genius 15910 points

    You have to use the Design Calculator to generate the Ics1 and Ics2 values, they aren't meant to be plugged in.  The power limit current is calculated based upon the Iline current that is internal to the device and sets the power limit offset for the particular operating conditions of the specifc design.  The table in the dat sheet is really just to verify results, or show the user what the equations are (or at least a view of some of the equations that are used in the macros that the design calculator uses).

    The assumption is that the Design calculator is required to generate the components for the design, not the terminal component table in the data sheet.

  • 0 in reply to Lisa Dinwoodie
    Intellectual 380 points

    Thank you for your reply.  I design and test a 21V-36V in, 13V out, 8W flyback converter.  It works very well.  The measured efficiency is 90% for such a small converter.

     

    I used 1:1:1 transformer.  And select

    Rovp(top) = 100K to protest Vin > 45V

    Rovp(bot) = 28K to protest Vout > 16V

     

    I changed Rcs = 200m to 300m.  They all work without difference except V(Rcs) is smaller when Rcs is smaller.  Of cause the over current protection level is different.

     

    When the load is stepped from 0 to 8W, Vout drops only 20mV!

     

    I use Rpl = 1K.  It works well.  But when Rpl = 2K, the current curves up instead of ramps up.  The regulation becomes poor, dropping 500mV!

     

    Question:

     

    1. If I know Rovp(top), Rovp(bot), Rcs, how to calculation power limit based on Rpl?
    2. Why the current curves up when Rpl is too large?