5.1 Feedback Ripple vs. Transient Response for Transient Step 2.5 to 5A (Slew Rate = 2.5A/3 us)

Figure 5-2. RINJ = 16.2k, CFF = 4.7 nF, ΔVFB = 245 mV, VO Undershoot = 240 mV, Recovery Time = 70 µs
Figure 5-3. RINJ = 36k, CFF = 4.7 nF, ΔVFB = 108 mV, VO Undershoot = 140 mV, Recovery Time = 40 µs
Figure 5-4. RINJ = 68k, CFF = 4.7 nF, ΔVFB = 55 mV, VO Undershoot = 100 mV, Recovery Time = 25 µs
Figure 5-5. RINJ = 82k, CFF = 4.7 nF, ΔVFB = 45 mV, VO Undershoot = 75 mV, Recovery Time = 20 µs
Figure 5-6. RINJ = 150k, CFF = 4.7 nF, ΔVFB = 15 mV, Abnormal Switching and Abnormal Feedback Waveform due to Very Low Ripple at Feedback
Table 5-1. Impact of Ripple Injection Optimization on Load Transient Performance
Figure #Ripple Injection ComponentsZB/ZF RatioΔVFB (mV)VOUT

Undershoot (mV)

Recovery Time (µs)
Figure 5-2RINJ = 16.2k, CINJ = 0.1 uF, CFF = 4.7 nF12924524070
Figure 5-3RINJ = 36k, CINJ = 0.1 uF, CFF = 4.7 nF28610814040
Figure 5-4RINJ = 68k, CINJ = 0.1 uF, CFF = 4.7 nF5415510025
Figure 5-5RINJ = 82k, CINJ = 0.1 uF, CFF = 4.7 nF652457520
Figure 5-7. Graphical Representation of RINJ vs. ΔVFB and ΔVFB vs. VOUT Recovery Time During Transient

Table 5-1 and Figure 5-7 provide a comprehensive summary of how optimizing ripple injection can significantly impact transient performance in ACOT-based converters. Despite ACOT's renowned superiority in transient response compared to its counterparts, there is always room for further optimization and improvement.

As discussed earlier and derived in Equation 1-5, the loop gain of ACOT is heavily influenced by the injection RC impedance (ZB) and the impedance of the feed-forward network (ZF). Hence, the ratio of ZB/ZF plays a crucial role in designing the ripple injection circuit for achieving the best transient response. Because ACOT is a nonlinear ripple-based control method, Bode plot measurements must be interpreted carefully and should not be used as the only validation method. Frequency-domain measurements are useful for understanding loop bandwidth and phase behavior, but final stability validation should always include time-domain tests such as load transient response, feedback ripple observation, and switching behavior.

Important: Design Warning: As illustrated in Figure   5, increasing RINJ increases the ZB/ZF ratio and reduces the feedback ripple ΔVFB, which can improve transient response. However, reducing ΔVFB too much can make the FB comparator more sensitive to noise, jitter, and layout parasitics, potentially causing irregular switching or instability. Therefore, RINJ should not be increased blindly; the selected value must maintain sufficient ΔVFB according to the device data sheet and should be verified through bench testing.