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UC1843B-SP: Ramp compensation design

kirous bachi
Intellectual 260 points
Part Number: UC1843B-SP
Other Parts Discussed in Thread: UCC38C44

In the datasheet of UC1843B-SP, in page 18/27, Section "Sense Resistor and Slope compensation" it says that when calculating the falling slope of the inductor current (Forward topolofy or buck topology) we have to multiply Vout/L by a gain Gcs=3, here i did not understand from where does this gain come ?? (Equation 46 in the datasheet).

I have gessed that this gain comes from the fact that the output of the Error Amplifier goes through a divider by 3 (Vc is divided by 3 than connected the PWM Comparator).

After a while, i have read the datasheet of UCC38C44 which has the same functional block diagram as UC1843B-SP. In page 13/55 it says "the gain of the current sense amplifier is 3V/V", but when it comes to "slope compensation" section in page 30/55, we do not see the effect of this gain as in UC1843B-SP

Please, can someone clarify this point to me??

How this Gain could affect the ramp compensation design?? 

What is the effect of the divider of "Error Amplifer output" to the current sense Input ??  

  • 0
    TI__Genius 17035 points

    It is a holiday weekend in the US.   The expert on this part will respond when he gets back in the office on Tuesday.

  • 0
    TI__Mastermind 21965 points

    Hello Kirous,

    The Gcs = 3 V comes from the datasheet itself.

    Current sense gain is specified in the electrical characteristics from 2.85 to 3.15 V/V. This ends up being 3 V/V typical.

    The compensation ramp doesn't see this as it is added before the divider. Thus the gain has no effect on the slope compensation design.

    The error amplifier voltage changes from 0 to 3 V and the current sense from 0 to 1 V. Thus there needs to be a divide by 3 to allow the voltages to be compared.

    Thanks,

    Daniel