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Original question:

UCC28C45 CT/RT

UCC28C45: UCC28C45 RT/CT and fsw equation

Yunjing Wang1
Expert 4300 points
Part Number: UCC28C45

Hi team,

Could you pls share the equation of UCC25C45 RT/CT and fsw relationship?

Datasheet figure 1 only showed some configurations but my customer want a detail equation.

I found a equation at below link, https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/546571/ucc28c45-ct-rt?tisearch=e2e-sitesearch&keymatch=UCC28C45

It says 1.72/(Ct*Rt), is it fosc or fsw? Should UCC28C45 fsw=1.72/(2*Ct*Rt)? Any application note about this?

Thanks and best regards

Yunjing

  • 0
    TI__Genius 13660 points

    Hi Yunjing, 

    The equation 1.72/(Ct*Rt) should be from the datasheet of UCx84x. https://www.ti.com/lit/gpn/uc3843, This is an approximate equation, and it is not absolutely same with UCC28C45, since the peak-peak voltage and the upper/under threshold of oscillator are different.

    Anyway, any equation is not accurate, my suggestion is to use the information provided in Figure 15 for accurate maximum duty cycle versus oscillator frequency. Any number of maximum duty cycles can be programmed for a given frequency by adjusting the values of RRT and CCT. Once RRT is selected, the oscillator timing capacitor can be found using the curves in Figure 1.

    If customer still like a equation, they can derive the equation according the datasheet section 8.3.5 themselves.

    Regards, Teng

  • 0 in reply to Teng Feng
    TI__Expert 4300 points

    Teng,

    Understood and thanks for your reply!

    Best regards

    Yunjing