A gm-C Ramp Generator for Voltage Feedforward Control of DC-DC Switching Regulators

A gm-C programmable-slope sawtooth generator for voltage feedforward control of DC-DC switchers is presented. Optimal feedforward is obtained by making the ramp amplitude proportional to the power input voltage PV_IN. The input transistors of the generator operate on triode region, so that its small-signal transconductance depends linearly on their drain-source voltage, which is a scaled replica of PV_IN, whereas showing better performance with respect to a standard differential pair in saturation. A saw-tooth waveform whose amplitude scales directly with PV_IN is then available across the integrating capacitor. As part of a PWM-controlled synchronous buck regulator, the ramp generator is designed according to a SmartMOS process. A range of SPECTREcopy simulations attests the usefulness of the proposed circuit. Design parameters are 3V les PV_IN les9V, 0 les I_load les 2A, 200KHz les f_SW les 1MHz and V_DD = 2.5V. Compatible with low-voltage requirements, the generator can operate at even lower V_DD supply voltages. Following a transition on PV_IN from 3V to 6V, a reduction of 76% and 50% on line regulation occurs as compared to PWM approaches with a fixed ramp and with voltage feedforward using an integrator with a differential pair in saturation, respectively.