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UCC38085: SETTING VALUE OF RSET (pin 2)

JULIAN DE ROSS
Intellectual 450 points
Part Number: UCC38085

Hi FORUM.

I am now working on a DC-DC converter using the 38085 but there are a couple of things in the Data sheet which are not very clear to me.

The value of this resistor is found from the Formula  on the image.
I'm not too clear on how to derive :RAMP VOLTAGE HEIGHT"  ?
Is this the peak voltage developed across the current sense resistor?  or something else.

Thanks

Julian

  • 0
    TI__Genius 13660 points
    Hi Julian,

    RAMP VOLTAGE HEIGHT in equation (2) is the compensation voltage height, which is added on CS pin, not current sense resistor voltage.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    Intellectual 450 points

    Hi Teng

    so, in regard to Equation 2 and the meaning of RAMP VOLTAGE HEIGHT, is this derived from multiplying the current limit value set by 5 ?
    Otherwise, how do we actually establish a real world number for that value?

    Thanks

    J

  • 0 in reply to JULIAN DE ROSS
    TI__Genius 13660 points
    Hi Julian,

    The RAMP VOLTAGE HEIGHT is the slope compensation voltage at CS pin, which is derived from compensation current source charging capacitor CF at CS pin, and the compensation current source value is set by RSET.
    As for that How to define the compensation voltage height, first, you should understand the purpose of slope compensation.
    The purpose of slope compensation is to eliminate the sub-harmonic oscillation when the duty cycle is larger than 50%. Larger slope compensation, more fast attenuation of oscillation, but too larger compensation will cause over compensation. Over compensation will limit the maximum load capacity, and over compensation also will affect the transient performance of system. So we need to chose the slope compensation value with all concerned, and a minimum slope compensation should ensure to eliminate the sub-harmonic oscillation, which can be gotten from actually debugging.

    Regards,
    Teng
  • 0 in reply to Teng Feng
    Intellectual 450 points

    Hi Teng

    I do 'get' why slope compensation is needed, I simply could not find a real value in the data book that I could plug into the formula to make get a value of Rset.
    I love it when engineers go through 3 pages of maths and then say "In the end.. we resort to the 'empirical' method" :)   i.e.. suck and see.. trial and error etc...

    So your 'gotten from debugging' seemed in that category...but I totally am ok with that.. I just needed the guidance to push me in the right direction. 

    cheers

    Julian

  • 0 in reply to JULIAN DE ROSS
    TI__Genius 13660 points

    Hi Julian,

    Practically, you can first set slope compensation voltage height 20% of maximum current sense resistor voltage height, on the basis of which, then regulate the compensation slope to ensure elimination of sub-harmonic oscillation. Thanks.

    Regards,
    Teng