5.3 Component Selection

Knowing the poles, zeros and the appropriate amplification factor, the rest of the components in the compensation network can be calculated.

The gain is set by the ratio between R_comp and R_FBT. As R_FBT and A_VM are set previously, R_comp can be calculated as the product of these:


$R_{c o m p} = A_{V M} \cdot R_{F B T}$

Setting the first zero of the amplifier is done using C_comp and R_comp, while the other is set by C_FF and R_FBT. These will fall on the output filter poles, resulting in the following capacitor values:


$C_{c o m p} = \frac{1}{ω_{0} \cdot R_{c o m p}}$ $C_{F F} = \frac{1}{ω_{0} \cdot R_{F B T}}$

The first pole of the amplifier is set by the combination of C_HF and R_comp, and the second is set using R_FF and C_FF. Setting the pole resulting from C_HF and R_comp equal to half the switching frequency, f_SW, and the pole resulting from C_FF and R_FF equal to the zero generated by the output capacitor and its series resistance gives the following equations for C_HF and R_FF:


$C_{H F} = \frac{1}{2 π \cdot f_{S W} / 2 \cdot R_{c o m p}}$ $R_{F F} = \frac{1}{ω_{Z} \cdot C_{F F}}$

Note: The calculated values typically fall outside standard values for the components. In this case, the closest available standard value may be chosen. Larger deviations from the calculated components can result in slower switching response as the phase of the input and regulation signals fall out of sync. The system will typically still handle the regulation, but lower efficiency and higher output ripple is to be expected.

AN3725

5.3 Component Selection