8.2.2.2 Option B: Intermediate V_OUT Ripple Configuration

This configuration generates less ripple at V_OUT than Option A by the addition of capacitor (Cff) as shown in Figure 23.

Because the output ripple is passed by Cff to the FB pin with little or no attenuation, the R1 value and V_OUT ripple can be smaller than in Option A. The minimum value for R1 is calculated from:

Equation 24.

where ΔI is the minimum ripple current amplitude, which occurs at minimum V_IN, and V_RIPPLE is the peak-to-peak ripple voltage injected at the FB pin. The minimum value for Cff is calculated from:

Equation 25.

where t_ON(max) is the maximum on-time (at minimum V_IN), and R_FB1//R_FB2 is the parallel equivalent of the feedback resistors.

Figure 23. Option B – Intermediate Ripple Configuration