A ripple control buck regulator with fixed output frequency

This paper presents a ripple control buck regulator for stepping down high-voltage adaptor or battery supplies to low-voltage CPU core supply in notebook computers. Methods and circuits are proposed to achieve fixed switching frequency operation. The control loop adjusts delay of the controller and thus controls the output frequency. This is done by using a phase-frequency detector (PFD) to lock the frequency of switching signal with the input clock. The delay value is determined automatically according to operating conditions. Simulation results showed fast load and line transient responses and fixed frequency operations in steady states. In addition, stability of the regulator is shown under a very low output-to-input voltage ratio condition that is difficult for conventional PWM controllers.

I. Introduction

Common control methods for switching regulators include 1) fixed frequency control, such as voltage mode pulse width modulation (PWM) control and current mode PWM control [1]; 2) variable frequency control, such as constant on- or off-time control [2] and ripple control [3], [4]; 3) quasifixed frequency control, such as Quick-PWM [5]. Generally, fixed frequency regulators are preferred because the switching frequency is selectable to avoid sensitive regions, which simplifies electro-magnetic interference (EMI) shielding in electronics equipments. Sensorless current mode (SCM) control [6] and two-sided latched PWM control [7] provide the benefits of current mode PWM control but with low noise susceptibility and wide dynamic range. The ripple control is the simplest among all switching regulators. Main advantages of the ripple regulator, like other variable Fig. 1. Ripple control regulator block diagram. frequency regulators, are fast transient response, unconditional stability, and wide range of output/input voltages. But the switching frequency depends on the operating conditions and power filter. Quasifixed frequency control was brought up based on the idea of controlling the on-time that is directly proportional to the output voltage and inversely proportional to the input voltage, thus an approximately fixed switching frequency is achieved. However, the switching frequency can still vary significantly due to second-order effects in the switching regulator. In this paper, methods and circuits are proposed to achieve fixed frequency operation with ripple control regulators. Both advantages of fixed and nonfixed frequency regulators are therefore obtained.