The LM21305 is designed to achieve high performance in terms of the transient response, audio susceptibility and output impedance, and will typically require only a single resistor R_c and capacitor C_c1 for compensation. However, depending on the power stage, it could require a second capacitor to create a high frequency pole to cancel the output capacitor ESR.

Figure 9-1 LM21305 Compensation Network

To select the compensation components, a desired cross over frequency f_c should be selected first. It is recommended to select f_c equal to or lower than f_s/6. A simplified procedure is given below for R_c and C_c1, assuming the capacitor ESR zero is at least three times higher than f_c. The compensation resistor can be found by:

Equation 7.

C_c1 does not affect the crossover frequency f_c, but it sets the compensator zero f_Zcomp and affects the phase margin of the loop. For a fast design, C_c1 = 4.7 nF gives adequate performance in most LM21305 applications. Larger C_c1 capacitance gives higher phase margin but at the expese of longer transient response settling time. It is recommended to set the compensation zero no higher than f_c/3 to ensure enough phase margin, implying:

Equation 8.

For more details, see the LM21305 5A Adjustable Frequency Synchronous Buck Regulator Data Sheet (SNVS639).