5 Summary

Traditional fixed-frequency PWM controllers, such as Voltage Mode Control (VMC) and Current Mode Control (CMC), regulate the converter by adjusting the duty cycle based on a periodic clock signal. While these methods provide predictable switching frequency and well-established compensation techniques, their transient response is inherently limited by the switching clock.

In contrast, Adaptive Constant-On-Time (ACOT) control eliminates the dependency on a fixed clock for switching cycles. Instead, switching events are triggered directly by the feedback signal, allowing the controller to respond immediately to changes in load or input conditions. This enables faster transient response and often simplifies the compensation network.

However, reliable ACOT operation requires a sufficient ripple component at the feedback node to ensure proper comparator triggering and stable regulation. When the natural ripple from the output capacitor ESR is insufficient—particularly when low-ESR ceramic capacitors are used, an external ripple injection network is typically employed to introduce a controlled ripple signal at the feedback node. Proper design of this ripple injection network ensures reliable ACOT operation while maintaining stable regulation and low output voltage ripple.

Table 5-1 summarizes the key differences between conventional fixed-frequency PWM controls and ACOT control.