Introduction

This application note is a continuation of Part 1 (AN7167), titled “Introduction to Microchip’s Adaptive Constant-On-Time (ACOT™) Control and Ripple Injection Methods”, where the fundamental operating principles of ACOT control and various ripple injection techniques were introduced. ACOT control, also known as Microchip’s Hyper Speed Control® architecture, powers many of Microchip’s high-performance wide-input-voltage DC-DC converters, including devices such as the MIC2129, MIC24097, MIC28515, MIC28516, and other ACOT-based controller devices.

While multiple ripple injection methods are available, modern power supply designs increasingly demand very low output voltage ripple, particularly in applications such as POL converters, FPGAs, processors, and high-speed digital systems. Under these conditions, relying on output capacitor ESR to generate ripple (Type 1) or using moderate ripple through feed-forward techniques (Type 2), becomes impractical.

As a result, the Type 3 ripple injection scheme, in which ripple is derived from the switching node using an RC network along with a feed-forward capacitor has emerged as the most widely adopted approach in practical ACOT designs. This application note focuses on an in-depth analysis of the Type 3 ripple injection method (illustrated in Figure ), with the objective of providing a comprehensive understanding of how ripple injection influences control loop behavior, stability and transient performance.

ACOT control loop design can be approached from two complementary perspectives: a time-domain view and a frequency-domain view. Frequency-domain analysis provides deeper insight into why the system behaves the way it does. By representing the ACOT control loop using an S-domain model, the interactions between the ripple injection network, feedback network, and power stage can be understood in terms of loop gain, poles and zeros. This perspective reveals how ripple injection directly shapes bandwidth, phase margin and overall dynamic performance.

While ACOT converters can be designed successfully without performing a full frequency-domain analysis, understanding the control loop in this domain enables more precise and intentional design. In this application note, the frequency-domain foundation of ACOT control is first established to provide a clear understanding of loop behavior. This is then translated into practical design methodology, followed by real design examples and experimental validation. Through this approach, the document aims to connect theoretical analysis with practical implementation, enabling both intuitive and precise ACOT control loop design.