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UCC28C45-EP: UCC28C45 CS Offset Vs. Temperature

Chris Hickam
Intellectual 260 points
Part Number: UCC28C45-EP
Other Parts Discussed in Thread: UCC28C45,

I am designing a feedforward control for a simple DCM flyback. Our circuit will operate over a wide temperature range and I am trying to design a temperature compensation circuit. Is it possible for me to get a definition of the temperature dependent voltage drop across the diode between the comp pin and the voltage divider going into the CS pin? I need something pretty accurate like a spice model which accurately accounts for thermal effects. Measured IV data for the diode at -55, 25, and 125 would work for me too then I could build a model of it. Bottom line is I need more information to do my design than is given in the data sheet. I've looked at figure 7 in the data sheet. I think I would be able to deduce the information I am looking for from the plot if the data were collected for any CS voltage other than 0. Maybe that was intentional taking the data at that point to publish? Not sure, but help would be appreciated.

  • 0
    TI__Mastermind 22145 points

    Hey Chris,

    The UCC28C45 has a model of the device, however this will not be over the temperature range you require.

    At this time, we do not have a model for the UCC28C45-EP over temperature.

    You mention that you would be able to deduce information from Figure 7 if CS was collected at not 0 V. Could you please expand on this?

    Thanks,

    Daniel

  • 0 in reply to Daniel Hartung
    Intellectual 260 points

    Hi Daniel,

      The block diagram shows the output of the error amplifier going into a couple diodes and then into a voltage divider with a ratio of 1/3. With a measurement at CS=0, the voltage into the divider is zero and the measurement represents the change in built in potential of the diode vs. temperature at zero current. I probably shouldn't have said any other voltage though measuring it at CS=0.5 would make more sense to me, what would be ideal is to have this curve at multiple values such as CS=0.1, 0.5, 0.9 so one can deduce the exponential change in the voltage drop due to current across the operating range.

      I guess if I assume the resistance of the divider is very large and the effects of current through the diode are negligible in my model I can just say that CS = (CMP-(Zero current diode drop as a function of Temp))/3. Can you confirm the resistance is large enough that the change in voltage with current is negligible or do I need to take a few parts into the lab?

    Chris 

  • 0 in reply to Chris Hickam
    TI__Mastermind 22145 points
    Hey Chris,

    I have been looking at more detailed diagrams of the part in an effort to get you an idea of what the effects you are describing would look like.

    From your description of the problem you are worried about the effects of the voltage change over temperature from the resistor divider and diodes to the error amplifier that compares the difference between a divided down version of COMP and CS.

    Figure 7 that you mentioned in the data sheet shows how the DC offset voltage from what is the zener diode in the block diagram changes based on temperature. This is not a zener diode in the actual device itself, but is typically close to that value.

    The resistor divider doesn't drain through what is the zener diode in the block diagram and may not have a significant effect on it. I cannot guarantee this because the values of the resistors are not specified.

    I hope I was able to answer the basis of this question.

    Thanks,
    Daniel
  • 0 in reply to Daniel Hartung
    Intellectual 260 points
    Thanks Daniel,
    Taking the Comp to CS offset Vs. Temperature given in the data sheet and applying that as the drop to the divider seems to work pretty well. I have some test data from a prototype now and the results of the new model using this drop are matching very closely at temperature.
    Chris