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UCC28782: Secondary Regulator for Fixed Output Voltage

Jejomar Ildefonso
Expert 8640 points
Part Number: UCC28782
Other Parts Discussed in Thread: LM3411

Hi Team,

Our customer intends to use UCC28782 to make a 5-10W wide input range, 5V output auxiliary power supply. In their case, no USB 3.0 PD controller is needed because they just want to have a fixed 5V output voltage.

They found LM3411 might be appropriate and compatible with UCC28782. But we're not quite sure about its control stability. Can you help us confirm whether our customer could use LM3411 as a compatible secondary voltage controller? Thanks in advance!


Kind Regards,

Jejomar

  • +1
    TI__Mastermind 28250 points

    Hello Jejomar, 

    We appreciate your customer's interest in the UCC28782 and LM3411 devices. 

    The LM3411 appears to be a suitable candidate for secondary-side shunt regulation with the UCC28782 ACF controller.  It can certainly drive the optocoupler with the required amount of diode current (translating to FB current through the CTR) when operating in all modes from AAM to SBP2.  

    Section 9.2.2.8 of the UCC28782 datasheet assumes the use of a TL431-type shunt regulator and provides design equations for loop compensation components based on that structure.  The LM3411 is different in that there is no Rbias1 resistor around which to attach Rdiff and Cdiff components.  However, this section provides that Rdiff and Cdiff are optional components and may not be necessary.   

    With the LM3411 structure, the Rout resistor (from OUT pin to opto-coupler-anode) can be considered the equivalent of Rias1, and if their function is found to be necessary (through transient response testing of the prototype), Rdiff and Cdiff may be placed across Rout (Rbias1).   

    For Cint and Rint design, the feedback impedance Zf(f) from LM3411 COMP to OUT pins can be considered to be the equivalent impedance of Cint or of the combination of Cint + Rint.  And the internal Rf of the LM3411 is the equivalent of Rvo1 of the TL431-type structure. 

    Regards,
    Ulrich