As shown in Figure 2-2, LM5170-Q1 provides COMP pin where type II compensation network can be designed externally. It’s important to select right R_COMP1, C_COMP1, and C_HF1 based on system transfer function. The poles and zeros of current loop system are determined by

where R_S is the entire resistance on the LV port to the loads, and 50 mΩ assumed in this case.

The compensation network’s components are selected based on following guidance. F_SW is switching frequency of LM5170-Q1 and F_{CO_CUR} is the current loop crossover frequency.

1. F_{CO_CUR} is set to 1/10 ~ 1/5 of F_SW
2. The total inner current loop gain is set to unity at F_{CO_CUR}
3. The zero F_{Z_COMP} is placed at approximately close to the power plant pole F_{P_PLANT}
4. The pole F_{P_COMP} is placed at approximately one or two decades higher than F_{Z_COMP}, but lower than F_SW

Select R_COMP1 = 1.15 kΩ, C_COMP1 = 330 nF, and C_HF1 = 3.3 nF which can meet in the following equations.

Figure 2-3 shows bode plot of inner current loop. Type II compensation network boosts the phase in mid-frequency range. In this example, the current loop crossover frequency is at about 20 kHz, and the phase margin is approximately 58 degree, which is higher than the needed minimal 45 degree of a stable system. F_{P_COMP} is placed at 41.9 kHz which is lower than the switching frequency (99.5 kHz). This helps to minimize output ripple caused by MOSFET switching.