LTC3871 Error Amplifier Output Impedance & compensation network design

Hello,

LTpowerCAD provides a model for the LTC3871 operating as a buck and as a boost, but not the inverting buck/boost. The LTpowerCAD model for the LTC3896 (150V inverting buck/boost controller) can be used as a rough substitute instead for the small signal modeling. Here are a few points to keep in mind:

• Select the programmed ILIM setting of the LTC3896 LTpowerCAD model which is closest to the Vsense(max) of the LTC3871 ILIM setting multiplied by 4 if the Rsense configuration is used or multiplied by 5 if the DCR sense configuration is used.
• Check the compensation of the LTC3871 inverting buck/boost in LTspice with a load step response or a time domain Bode plot.

Here are answers to your remaining questions:

• The typical ITH output impedance for the LTC3871 buck error amplifier's output (ITHLOW) and the boost error amplifier's output (ITHHIGH) is 2MOhms and 4MOhms, respectively.
• The RHP zero formulas used by the LTC3871 LTpowerCAD boost design tool is basically the same as shown in AN149. The only difference is that the equivalent inductance is used instead of the discrete inductor value. The equivalent inductance value is equal to the discrete L value divided by the # of phases.

Best regards,

Mike