2 Feedback Network With and Without the Feedforward Capacitor

Without a feedforward capacitor, the feedback network of an internally compensated dc-dc converter consists of two feedback resistors used to set the output voltage of the converter, as shown in Figure 2-1. Figure 2-2 shows the corresponding gain and phase plot.

Figure 2-1 Feedback Network Consisting of Two Bias Resistors Used to Set Output Voltage

Figure 2-2 Standard Feedback Divider Transfer Function

Figure 2-3 shows the addition of the feedforward capacitor, C1 (Cff), in the feedback network and Figure 2-4 shows the corresponding gain and phase plot. With the addition of the feedforward capacitor network, the converter can more effectively respond to high-frequency disturbances on the output voltage rail. The bode plots in Figure 2-2 and Figure 2-4 show that the responses of each feedback network are identical at lower frequencies. At mid-to-higher frequencies, disturbances on the output rail are attenuated less as the impendence path through C1 decreases and effectively provides a boost in gain and phase. In a working power supply, the increased gain and phase correlates to the converter responding faster to transient loads because the voltage deviation, sensed at the feedback node, is attenuated less at higher frequencies. The converter reacts by adjusting the duty cycle to more quickly correct the output voltage deviation.

Figure 2-3 Feedback Network With Addition of Feedforward Capacitor

Although Cff introduces a gain boost after its zero frequency, loop phase boost is at a maximum between the zero and pole frequencies; see the following Equation 1 and Equation 2. Increasing the value of Cff shifts the zero and pole in Equation 1 to lower frequencies, and decreasing the value Cff shifts the zero and pole to higher frequencies. The gain at dc is set by R1 and R2. The following equations calculate the pole, zero, and the dc gain of the feedback network as is shown in Figure 2-4.

Equation 1.

Equation 1 calculates the zero frequency based on the feedforward capacitor value and the top bias resistor, R1. fz is shown on the plot in Figure 2-4.

Equation 2.

Equation 2 calculates the pole frequency based on the feedforward capacitor value and both top and bottom bias resistors, R1 and R2. fp is shown in on the plot in Figure 2-4.

Figure 2-4 Standard Feedback Divider With Feedfoward Capacitor Transfer Function

To optimize transient response, a Cff value is chosen such that the gain and phase boost of the feedback increases the bandwidth of the converter, while still maintaining an acceptable phase margin. In general, larger values of Cff provide greater bandwidth improvements. However, if Cff is too large, the feedforward capacitor causes the loop gain to crossover too high in frequency and the Cff phase boost contribution is insufficient, resulting in unacceptable phase margin or instability. Recommended limitations of Cff is discussed later in this document.