Buck feedback loop with high gain to current sense.

[cupoftea]

Hi,
Doing Synch Buck for 27Vin, 13V out, 15A out and 450kHz.

LTpowerCAD is showing the gain Bode plot curving back up to the 0dB line at higher
frequency than the crossover. - Which means instability.
As such, wish to make out own Bode plots.
Can get the Power stage transfer function for a current mode CCM Buck.
Can get the Transfer function for the type 2 transconductance error amplifier.
However, can you confirm that the controller gain means that we will have to multiply in
the factor of 1.4V/0.075V for the attenuation regarding the peak current threshold (75mV)
and the maximum control voltage, which appears to be 1.4V?


Graph on top left of datasheet page 7 appears to show the controller gain, by way of V(ITH) vs Current sense voltage

LTC3891 datasheet

https://www.analog.com/media/en/technical-documentation/data-sheets/3891fa.pdf

 

[cupoftea]

Hi, The LTC3891 datasheet unfortunately doesnt give the current sense gain. ie, the amount by which the current
sense voltage is multiplied, so as to increase it. Due to this, it is not possible to calculate the
transresistance for any Buck converter made with LTC3891...and because of that, its not possible to do a Bode Plot
calculation for the LTC3891, because the transresistance is a needed parameter in the calculation of the control to output transfer
function.
As you know, the transresistance is the voltage coming off the sense resistor divided by the current that caused it by flowing through it.
As such, do you know how we can find out the current sense gain for the LTC3891?

They have LTpowerCAD but sometimes its good to have your own checker...specially since you can extend your own to include any output LC filters added in.