In power supply design, the control mode defines how voltage regulation is achieved and how the system maintains a stable output in the face of disturbances. It is an essential concept in switch-mode power supplies (SMPS). Among the primary control modes, voltage-mode control, current-mode control, and Constant On-Time (COT) control are the most commonly used. Each of these control methods offers unique advantages and challenges.

COT control stands out due to its relatively simple design and fast response. However, it does have limitations, such as requiring a larger output ripple. Over time, advancements have addressed issues such as frequency instability and ripple control, and COT control continues to evolve with improvements in integration and efficiency.

Traditional COT Control

The traditional COT control method is a form of frequency-variable control. The control core uses a comparator to monitor the output ripple voltage, triggering a timing mechanism once the feedback voltage (Vfb) drops below a reference voltage (Vref). During the on-time, the MOSFET remains on for a fixed period, after which it turns off. The cycle repeats when Vfb falls again to Vref.

However, this approach has two significant drawbacks:

1. Unfixed Frequency: The switching frequency is not constant.
2. Large Output Ripple: To maintain stability, a relatively large output ripple is required.

Figure 1: Traditional COT Control

Adaptive COT Control

To address the frequency instability issue, engineers developed adaptive COT control. In this method, the on-time duration is determined by the input voltage (VIN) and output voltage (VOUT) of each cycle, resulting in a pseudo-fixed frequency. This adaptation keeps the frequency within a defined range across variations in input voltage, making the control scheme more predictable.

However, adaptive COT control still requires a large ESR (Equivalent Series Resistance) in the output capacitor to generate sufficient ripple voltage for system stability.

Figure 2: Adaptive COT Control

Enhanced COT Control

The Enhanced COT Control mode is an improved version of adaptive COT, designed to allow the use of ceramic output capacitors, which have low ESR. To achieve this, the system replicates a ripple voltage that is in phase with the inductor current. There are two main methods to generate this ripple:

The RCC compensation circuit helps create a ripple voltage similar to the one generated by the standard capacitor ESR. This ripple voltage is then fed back to the feedback pin via a coupling capacitor (Cc), which isolates the DC voltage, allowing the system to operate with a minimal AC ripple.

Figure 3: External Ripple Injection Compensation Circuit

The SCT Enhanced COT Control Products implement the first method, integrating the RCC ripple compensation circuit inside the chip. This approach enhances integration, reduces the need for external components, and lowers system cost while maintaining excellent performance.

However, a known challenge with COT control is that, under Continuous Conduction Mode (CCM), the output voltage can be higher than the reference voltage by half the ripple voltage. This is an inherent characteristic of COT control.

Figure 4: Enhanced COT Control

To address this, Enhanced COT Control incorporates an error amplifier to improve accuracy in CCM and introduces harmonic compensation to minimize jitter. Although this optimization makes the design more complex, it significantly enhances performance in various operating conditions. If interested, further details can be obtained from FAE (Field Application Engineer).

SCT's Commitment to Advancing COT Control

SCT (芯洲科技) is dedicated to providing high-value products to customers. Their COT-controlled products, such as the 100V BUCK series, 18V 3A Buck series, and 30V 6A Buck series, employ advanced Enhanced COT Control techniques. These solutions not only leverage the advantages of COT control but also mitigate its limitations.

One of SCT's latest products, the SCT2A00 (100V, 0.6A), is a small-package, ceramic-output-capacitor-friendly COT control chip that offers excellent cost-performance. It is widely used in industries like photovoltaics, industrial automation, and electric two-wheelers.

Figure 5: SCT2A00 Typical Application