3 Effects of C_ff on the Loop

The Optimizing Transient Response of Internally Compensated dc-dc Converters With Feedforward Capacitor application report details the effects of adding a feedforward capacitor in the feedback divider. Figure 3-1 shows feedback divider including C_ff.

C_ff introduces a pair of zero and pole in the converter loop. The frequency of the introduced zero and pole are:

Figure 3-2 shows the effects of the zero and pole introduced by C_ff.

C_ff has both effects on the loop gain and phase. The loop gain is increased to enlarge bandwidth which benefits transient response, and the phase is also boosted to increase the phase margin for system stability.

In application notes such as Optimizing Transient Response of Internally Compensated dc-dc Converters With Feedforward Capacitor, some methods have been proposed to use C_ff for phase margin enhancement. But in those methods, the bode plot results without C_ff are always needed to get the recommended C_ff value. The C_ff selecting method is more applicable in the solution validation process but not in the application design process.

A new method to choose C_ff is proposed in this application report. The bode plot results without C_ff are not needed to get the recommended C_ff value in this method, which makes it more applicable in TPS62933 application design.