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UCC28740: Feedback Loop

Willi Wiens
Prodigy 10 points
Part Number: UCC28740

Hi,

I use the UCC28740 for a flyback converter. 

Now I try to understand the feedback loop.

The Application Note sluaa66 describes where the poles of the optocoupler is. (Equation 4/5).

To determine the pole from the optocoupler I need R(pullup). In the application Note is R(pullup) 15k used. So war so good.

But what is my real R(pullup) in the typical Application Circuit?

The Optocoupler is connected to VDD, so I need an pulldown. But What ist the pulldown resistor? It is R(FB3) with R(FB4) in series. Or R(FB4) parallel with the internal R(FB).

I hope you understand my problem and you can help me.

Best regards

Willi

  • +1
    TI__Genius 17755 points
    Hi Willi,

    Thank you for your patience. The pulldown resistor in Figure 1. Typical Application Circuit of the UCC28740 is the RFB4. There is an internal pulldown resistance in the FB pin as well. I would recommend using the values given in the datasheet for it.

    Best Regards,
    Alexis