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Original question:

TPS61178: sloop compensation slew rate Se calculation

TPS61178: Small Signal Model - Slope Compensation

Chun Heng Lee
Prodigy 30 points
Part Number: TPS61178
Other Parts Discussed in Thread: TPS61175

Hi TI team, 

The unit of Sn and Se is not the same. I think the equation for Sn is correct because I saw it here https://www.ti.com/lit/an/slva452/slva452.pdf?ts=1631189220487&ref_url=https%253A%252F%252Fwww.google.com%252F.

But I am not 100% sure of Se. What is the unit Se? Is there a missing unit in the equation? More importantly I want to confirm if the equation is correct. 

Secondly, in the TPS61178 datasheet, it says:
Note that if Sn > Se, e.g., when L is too small, the converter operates as a voltage mode converter and the above model no longer holds.

But I saw the opposite statement on TPS61175 datasheet. Yes, they are different controller but I think the working of the current-mode slope compensation theory to be the same, so I kind of expect the statement should be the same too. Appreciate if you can confirm if the statement in TPS61178 datasheet is correct. 
And if the statement is correct, it doesn't say anything about what model to use if it behaves like voltage-mode controller. I am completely lost there. 

Lastly, if TI has a calculator spreadsheet to calculate the compensation values, I would like to get my hands on it. Thank you.

  • 0
    TI__Expert 4760 points

    Hi Chun,

    Thank you for your finding. In fact, the description in TPS61175 datasheet is right and the conclusion in TPS61178 datasheet is wrong. The slope compensation is often a constant value. If L is very large, the controlled current signal is compared with only compensation signal, it is just like voltage mode converter. If L is very small, the slope compensation is not enough for inductor current and the sub-harmonic can not be fully avoided and there is stability issue.

  • 0
    TI__Expert 4760 points

    Hi Chun,

    Is this clear for you?

  • 0
    Prodigy 30 points

    Hi , 

    It is clear for me that the description in the TPS611781 datasheet is wrong, but I still looking for answers for my other two questions.

    1. Is the equation for Se correct in the TPS61178x datasheet

    2. Is there a design spreadsheet for TPS61178x? This is because the loop gain plot that I get by using the small-signal equations in the datasheet seems to be very different from the WEBENCH simulation. I still prefer to use calculations because I will need to do many iterations and simulating using WEBENCH is too time-consuming for what I want to do. 

    Thanks.

  • +1 in reply to Chun Heng Lee
    TI__Expert 4760 points

    Hi Chun,

    1. The equation should be right. The 0.06 should be a current source and the unit is ignored;

    2. Sorry, we don't have the calculation tool. If you want to design the compensation network, slope compensation can be ignored because its impact is not large.