Ripple-based Constant On-time Research Articles

A fast-response RBCOT buck converter with second-order differential and integrator compensation based on FVF

A second-order differential and integration circuit based on a flipped voltage follower (SDI-FVF) is proposed in the paper, providing compensation ripple for the buck convert with ripple-based constant on-time (RBCOT). Additionally, an adaptive conduction time module is designed which stabilizes the operating frequency of the converter in continuous conduction mode (CCM) and guarantees stable conduction time in discontinuous conduction mode (DCM). The proposed RBCOT buck convert has been designed using 180 nm process. It operates with an input voltage range of 2.5 V–5 V and an output voltage range of 0.8 V–5 V, accommodating a load current range of 0 A to 3 A. It operates at a frequency of 4 MHz, exhibiting an output ripple of 1.4 mV, achieving a peak efficiency of 91.5 %. The efficiency exceeds 85 % under a load current of 1 mA. The undershoot and overshoot voltage are only 0.13 mV and 7.1 mV with a load current variation rate of 3 A/μs.

Accurate Loop Gain Model of Ripple-Based Constant on-time Controlled Buck Converters

Ripple based constant on-time (RBCOT) control has been widely used in DC-DC buck converters over past years. However, there has been no accurate loop gain model to link the property of RBCOT controlled buck converter to existing knowledge about basic feedback control principles like crossover frequency (Fc) and phase margin (PM) so far. The previously reported stability criterion for the output capacitor's ESR, i.e., Ton/(2C), is insufficient to avoid subharmonic oscillations over all duty cycle values. This paper presents a new accurate loop gain model for the RBCOT control. Without simplification, the proposed model can accurately predict the system response even beyond the switching frequency. The proposed modeling approach has been extended to two types of external ramp compensation schemes (for solving the subharmonic oscillation issue) and optimum ranges of ramp compensation amplitudes have been derived accordingly. The modeling results are verified by the SIMPLIS simulations and experimental results.

Adaptive On-Time Buck Converter with Wave Tracking Reference Control for Output Regulation Accuracy

A ripple-based constant on-time (RBCOT) buck converter with a virtual inductor current ripple (VICR) control can relax the stability constraint of large equivalent series resistance (ESR) at an output capacitor, but output regulation accuracy deteriorates due to the issue with output DC offset. Thus, this paper proposes a wave tracking reference (WTR) control to improve converter stability with low ESR and concurrently eliminate output DC offset on the regulated output voltage. Moreover, an adaptive on-time (AOT) circuit is presented to suppress the switching frequency variation with load current changes in continuous conduction mode. A prototype chip was fabricated in 0.35 µm CMOS technology for validation. The measurement results demonstrate that the maximum output DC offset is 4.1 mV and the output voltage ripple is as small as 3 mV. Furthermore, the switching frequency variation with the AOT circuit is 11 kHz when load current changes from 50 mA to 500 mA, and the measured maximum efficiency is 90.9% for the maximum output power of 900 mW.

A Fast-Response RBAOT-Controlled Buck Converter With Pseudofixed Switching Frequency and Enhanced Output Accuracy

A ripple-based constant on-time (RBCOT) control scheme is excellent in achieving fast transient response during the load current step. However, it suffers from severe electromagnetic interference (EMI) noise and inherent output voltage offset problem. A monolithic ripple-based adaptive on-time (RBAOT)-controlled buck converter is presented to overcome both drawbacks while retaining the advantage of RBCOT. The on-time is adjusted adaptively by detecting the switching node voltage, and thereby ensuring a pseudofixed switching frequency in a steady state, regardless of the input, output voltage, and load current conditions. A correction voltage generated by a coefficient in a virtual inductor current (VIC) circuit is introduced to counteract the offset voltage and enhance the output accuracy. The proposed scheme is simple to implement and suitable for high-conversion ratio applications. Experimental results show that the switching frequency is centered at 1 MHz with less than 6% variation for an output of 5 V and inputs ranging from 8 to 18 V. The output offset voltage is reduced from 16 to 1 mV for an 18 to 1.05-V conversion. For a load current step of 1 A, the output can be settled within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6~\mu \text{s}$ </tex-math></inline-formula> and the undershoot and overshoot voltages are controlled to be within 20 mV.

A Novel Accurate Adaptive Constant On-Time Buck Converter for a Wide-Range Operation

The ripple-based constant on-time (COT) control with ripple compensation has been widely adopted in buck converters in the recent years because of its simplicity, fast transient response, and high light-load efficiency. However, this control has the potential issues of instability and excessive load transient response with wide input/output voltage range conditions in the power management integrated circuit (PMIC) applications. In this article, an accurate adaptive COT control scheme is proposed to solve these problems. The control scheme achieves accurate output voltage, and nearly constant quality factor and well transient response over a wide-range operation with simple circuit. The small-signal model is also derived based on the describing function (DF) technique for design optimization. Finally, the simulations and experimental verifications were conducted to verify the proposed concepts.

A Novel Adaptive-Ramp Ripple-Based Constant On-Time Buck Converter for Stability and Transient Optimization in Wide Operation Range

The fixed-ramp ripple-based constant on-time (RBCOT) control scheme for buck converters has recently been adopted in many dc–dc conversion applications for its simplicity and high light-load efficiency. However, it cannot overcome the problems with feedback instability and excessive load transient response when the converter is required to cover wide working conditions of input voltage, output voltage, and switching frequency in the application of power management integrated circuit (PMIC). In this paper, an adaptive-ramp RBCOT scheme is proposed to solve such a problem. The control law is derived from theoretical analysis of small-signal models. A simple analog circuitry, which can be easily integrated into an integrated circuit, is proposed to achieve invariant Q factor (or Q value, quality factor) under different working conditions. Simulations and experiments were conducted to verify the proposed concept.

Stability issues and modelling of ripple‐based constant on‐time control schemes operating in discontinuous conduction mode

Ripple-based constant on-time (RBCOT) control schemes have recently received much attention for the DC converters used in many applications, mainly because of the relatively high efficiency under light-load condition while preserving other features such as fast step-load response and small component count. Since the converter is usually operated under discontinuous conduction mode (DCM) under light-load condition, an effort to investigate the RBCOT schemes under DCM is practically important. Rigorous mathematical analyses are used to investigate the feedback stability issues of the buck converters using RBCOT control schemes operating in DCM. Two RBCOT control schemes are considered: the basic RBCOT scheme and the basic RBCOT with virtual-inductor-current signal (VICRBCOT). It is proved that it is always stable for the basic RBCOT converter operating in DCM, but that is not the case for the VICRBCOT converters. A proper design procedure to maintain stability for both the continuous conduction mode and the DCM operations in VICRBCOT converters is proposed and verified.

Subharmonic Stability Limits for the Buck Converter With Ripple-Based Constant On-Time Control and Feedback Filter

A general closed-form subharmonic stability condition is derived for the buck converter with ripple-based constant on-time control and a feedback filter. The turn-on delay is included in the analysis. Three types of filters are considered: low-pass filter (LPF), phase-boost filter (PBF), and inductor current feedback (ICF) which changes the feedback loop frequency response like a filter. With the LPF, the stability region is reduced. With the PBF or ICF, the stability region is enlarged. Stability conditions are determined both for the case of a single output capacitor and for the case of two parallel-connected output capacitors having widely different time constants. The past research results related to the feedback filters become special cases. All theoretical predictions are verified by experiments.

A Novel Quick Response of RBCOT With VIC Ripple for Buck Converter

A novel quick response of ripple-based constant on-time with virtual inductor current ripple for Buck converter is proposed in this paper. The concept uses the capacitor and the resistor in a series to filter out the output voltage at load transient to dynamically change the width of on-time to prevent the Vout from dropping markedly. Finally, 12-V input voltage, 3.3-V output voltage, and 60-W output power with the novel quick response circuit for the IC of the proposed Buck converter are implemented to verify its viability and superiority.

A Ripple-Based Constant On-Time Control With Virtual Inductor Current and Offset Cancellation for DC Power Converters

In recent years, there has been a growing trend of mandating high-power conversion efficiency, for not only the heavy-load but also the light-load conditions. To achieve this purpose, a ripple-based constant on-time (RBCOT) control for dc–dc converters has received wide attentions because of its natural characteristic of switching frequency reduction under the light-load condition. However, a RBCOT control suffers from an output-voltage offset problem and a subharmonic instability problem. In this paper, a modified RBCOT buck converter circuit is proposed to solve both problems. The circuit uses the concept of virtual inductor current to stabilize the feedback, and an offset-cancellation circuit to eliminate the output dc offset. The modified circuit can be fabricated into an integrated circuit (IC) without adding any pin compared to conventional circuits. A control model based on describing function is developed for the modified converter. The small-signal characteristics and design criteria to meet stability are derived. From the model, it is also found out that it is much easier to accomplish adaptive voltage positioning using the proposed modified RBCOT scheme compared to a conventional constant-frequency controller. Simulation and experimental results are given to verify the proposed scheme.