The LMR23615 is internally compensated as shown in Section 7.2. The internal compensation is designed such that the loop response is stable over the entire operating frequency and output voltage range. Depending on the output voltage, the compensation loop phase margin can be low with all ceramic capacitors. An external feedforward capacitor C_FF is recommended to be placed in parallel with the top resistor divider R_FBT for optimum transient performance.

Figure 7-9 Feedforward Capacitor for Loop Compensation

The feedforward capacitor C_FF in parallel with R_FBT places an additional zero before the crossover frequency of the control loop to boost phase margin. The zero frequency can be found by

Equation 7.

An additional pole is also introduced with C_FF at the frequency of

Equation 8.

The zero f_{Z_CFF} adds phase boost at the crossover frequency and improves transient response. The pole f_P-CFF helps maintaining proper gain margin at frequency beyond the crossover. Table 8-1 lists the combination of C_OUT, C_FF and R_FBT for typical applications,  designs with similar C_OUT but R_FBT other than recommended value, adjust C_FF such that (C_FF × R_FBT) is unchanged and adjust R_FBB such that (R_FBT / R_FBB) is unchanged.

Designs with different combinations of output capacitors need different C_FF. Different types of capacitors have different equivalent series resistance (ESR). Ceramic capacitors have the smallest ESR and need the most C_FF. Electrolytic capacitors have much larger ESR than ceramic, and the ESR zero frequency location would be low enough to boost the phase up around the crossover frequency. Designs that use mostly electrolytic capacitors at the output may not need any C_FF. The location of this ESR zero frequency can be calculated with Equation 9:

Equation 9.

The C_FF creates a time constant with R_FBT that couples in the attenuate output voltage ripple to the FB node. If the C_FF value is too large, it can couple too much ripple to the FB and affect V_OUT regulation. Therefore, calculate C_FF based on output capacitors used in the system. At cold temperatures, the value of C_FF might change based on the tolerance of the chosen component. This may reduce its impedance and ease noise coupling on the FB node. To avoid this, more capacitance can be added to the output or the value of C_FF can be reduced.