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UCD3138: PSFB Peak Current Mode

Gonzalo Bustamante
Prodigy 20 points
Part Number: UCD3138

Hello,

I am using UCD3138 to control a PSFB in Peak Current Mode. Our design is based in UCD3138PSFBEVM-027 Evaluation Module, but with different output voltage and power output. Up to now, we have succeeded in putting the system into regulation, and now we are trying to configure burst mode.

In PCM control, as far as we know, the output of the Filter is connected to FrontEnd 2 DAC. The output of the filter is the initial value of the Slope Compensation Ramp. We are a little confused, as the output of the filter is a 24bit value, and the DAC_VALUE is a 14bit value. How is the output of the filter scaled to the DAC value?

Thank you very much,

  • +1
    TI__Genius 17090 points

    If you look at the UCD3138 Technical Reference Manual Section 4.1.6 Filter Output Stage, you can see how the filter output is taken down to 18 bits and clamped to positive.  For Peak current mode we select one of the KCompx values to multiply by to get the best scaling.  I don't know if it's in the document anywhere, but I am pretty sure that the hardware takes the top 14 bits from the 18 bits of the filter output and puts them into the ramp module to start the compensation ramp.  And the EADCDACCTRL register in the Loop Mux selects which filter is used to drive the DAC.  

  • 0 in reply to Ian Bower
    Prodigy 20 points

    Thank you Ian, that definitely makes sense, I've made some calculations based on the filter output and the DAC value as seen in the registers and seems correct. Thank you.