9.2.2.1 Loop Compensation Circuit

A typical compensation circuit could be type II (R_c and C_c) to have a phase margin between 60 and 90 degrees, or type III (R_c, C_c and C_ff) to improve the converter transient response. C_Roll adds a high frequency pole to attenuate high-frequency noise when needed. It may also prevent noise coupling from other rails if there is possibility of cross coupling in between rails when layout is very compact.

Figure 22. Loop Compensation

To calculate the external compensation components use Table 1:

Table 1. Design Guideline for the Loop Compensation

	TYPE II CIRCUIT	TYPE III CIRCUIT
Select switching frequency that is appropriate for application depending on L, C sizes, output ripple, EMI concerns and etc. Switching frequencies between 500 kHz and 1 MHz give best trade off between performance and cost. When using smaller L and Cs, switching frequency can be increased. To optimize efficiency, switching frequency can be lowered.		Type III circuit recommended for switching frequencies higher than 500 kHz.
Select cross over frequency (fc) to be less than 1/5 to 1/10 of switching frequency.	Suggested fc = fs/10	Suggested fc = fs/10
Set and calculate Rc.	Equation 7.	Equation 8.
Calculate Cc by placing a compensation zero at or before the converter dominant pole Equation 9.	Equation 10.	Equation 11.
Add CRoll if needed to remove large signal coupling to high impedance COMP node. Make sure that Equation 12. is at least twice the cross over frequency.	Equation 13.	Equation 14.
Calculate Cff compensation zero at low frequency to boost the phase margin at the crossover frequency. Make sure that the zero frequency (fzff is smaller than soft-start equivalent frequency (1/Tss).	NA	Equation 15.