5 Feedback Ripple vs. Transient Response

Now, let us examine some design examples to validate the preceding analyses and explanations. This analysis aims to comprehend the transient response performance concerning variations in feedback ripple amplitude (ΔV_FB) so that we can optimize the ripple injection components for the desired transient performance.

Figure 5-1 illustrates the schematic diagram of an MIC28515 buck converter design with V_IN = 48V, V_OUT = 5V, F_SW = 266 kHz. Analysis will be performed with a transient load from 2.5A to 5A with a slew rate of 2.5A/3 us. The details of the evaluation board used for testing can be found in the MIC28515 Evaluation Board user guide.

In the reference schematic (Figure 5-1), the ripple injection components employed are R_INJ = 16.2k, C_INJ = 0.1 uF and C_FF = 4.7 nF. Utilizing Equation 3-1, we can compute ΔV_FB. The calculated value is 220 mV, which closely aligns with the test result (slight variations in the duty cycle compensate for losses, leading to slight deviations in the calculated value). Figure 5-2 depicts the transient response performance associated with Figure 5-1 reference schematic, and Figure 5-3 through Figure 5-5 show the transient performance across different ΔV_FB amplitudes achieved by adjusting the ripple injection components.

ZB and ZF are calculated for the given switching frequency of 266 kHz, and their ratio is given in Table 5-1 to correlate its connection to the transient performance of converter. It is worth noting that the increase in impedance ratio reduces the feedback ripple which results in better transient response. However, Figure 5-6 illustrates that too high value of ZB/ZF results in too low value of ΔV_FB, which in turn can cause abnormal switching and lead to instability.