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Feedback loop calculation for LT3680 based current mode buck?

Flyback

Well-Known Member
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
https://www.analog.com/media/en/technical-documentation/data-sheets/lt3680.pdf
 

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crutschow

Well-Known Member
Most Helpful Member
Need the transfer function model to see where Ri is in that.
What FET?
 

Flyback

Well-Known Member
Thanks, yes ISYP, its a NPN not FET. They dont seem to provide the modulator transfer function which seems unusual. Maybe they expect us to use LTpowerCAD?
 

crutschow

Well-Known Member
Most Helpful Member
Figure 2 on page 13 of the LT3680 data sheet shows a model for loop response with the current gains, which should help your analysis.
 

Flyback

Well-Known Member
Thanks yes, in fig 2 they say the "current mode power stage" has a gm of 5.3mho. But dont say how they derive this. As you know, current mode power stage is dependent on inductance value and cout value (plus a few other things). They also say that this "current mode power stage" function is comprised of modulator and power stage. However, the modulator transfer function is not given either.
 

crutschow

Well-Known Member
Most Helpful Member
The gm is the transfer function of the modulator which is basically independent of the external output inductance and capacitance.
It shows the current output versus the error voltage. (instead of the voltage out versus the error voltage for a voltage output converter).
The external output inductance and capacitance values, of course, go into the compete transfer function for determining the loop response.

I don't believe how they derived that value is needed to perform your calculations.
 

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