SNVS787I January 2012 – August 2021 LM5018
A type III ripple circuit as described in Section 7.3.11 is required for the Fly-Buck topology. Type I and Type II ripple circuits use series resistance and the triangular inductor ripple current to generate ripple at VOUT and the FB pin. The primary ripple current of a Fly-Buck is the combination or primary and reflected secondary currents as illustrated in Figure 8-6. In the Fly-Buck topology, Type I and Type II ripple circuits suffer from large jitter as the reflected load current affects the feedback ripple.
Selecting the Type III ripple components using the equations from Section 7.3.11 ensures that the FB pin ripple is be greater than the capacitive ripple from the primary output capacitor COUT1. The feedback ripple component values are chosen as shown in Equation 30.
The calculated value for Rr is 66 kΩ. This value provides the minimum ripple for stable operation. A smaller resistance must be selected to allow for variations in TON, COUT1, and other components. For this design, Rr value of 46.4 kΩ is selected.