Design Of A Voltage-mode Buck Regulator With Fast Transient Response

With the upgrading and development of mobile phones, laptops, digital cameras and other portable electronic products, the requirements of the power management chips are also becoming higher and higher. The traditional linear regulator has been unable to meet the needs of efficient systems which exhibited large power consumption due to the voltage difference. Compared with linear regulator, the switching converter achieves high power efficiency and flexible output voltage. Due to these advantages, switching converter is gradually replacing linear regulator in low power applications.In order to improve power system’s performance when the load rapidly changes, this paper designed a voltage-mode buck regulator with fast transient response. A reference-voltage-selectable-scheme is applied in the voltage-mode buck regulator which includes a reference voltage choose circuit and a threshold voltage control circuit. The regulator was designed using 0.18μm CMOS process, simulation shows that it achieves an output overshoot less than 100 mV and a recovery time of 13 μs, better than conventional PWM voltage mode buck converters.In order to eliminate the inrush current and overshoot voltage due to the error amplifier operating in the non-equilibrium when the power start-up, this paper also designs a novel soft-start circuit. The soft-start circuit uses a pulse VPL from the ramp generator as a control signal for charging the capacitor. On the one hand, VPL charges the soft-start capacitor within low voltage time while remains constant soft-start voltage within high voltage time so that the soft start voltage can rise linearly. On the other hand, the soft-start circuit uses the subtraction of two current sources I1 and I2 to obtain a small soft-start charging current I1-I2. By the above two means, the soft start time is effectively extended. Simulation results show that the soft-start circuit works with a start time of 324μs.