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Feedback loop for LT3680 based current mode buck?

treez

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Hi,
We are trying to calculate the open loop small signal feedback loop transfer function for an LT3680 based buck converter.
As you know, this is given by the [Power stage transfer function] x [Error amp transfer function] x [Modulator transfer function].
The Equation 2A-5 in the attached gives the [power stage & modulator transfer function]. However, it contains the term “R/Ri”.
How can we find out what Ri is for the LT3680?
(Ri is the FET source sense resistor; R is the load)

LT3680
 

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mtwieg

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I would give up on trying to analytically derive the loop response of any of LT's controllers (the current mode ones, anyways). Every time I try, it turns out the actual part has some quirks that aren't reflected in the datasheet or LTspice model, and I have to manually adjust things. The culprit is usually their slope compensation, or logic which governs transitions between normal and burst/pulse skipping modes.
 

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