Robust current control for step-down converter

摘要

In a current mode controlled switching power supply, current through an inductor (L1) is sensed to determine when to turn off or on the switching transistors (86, 88), the inductor current (IL) having a higher frequency AC component and a lower frequency DC component. The AC current feedback path (AC PATH), sensing the ramping ripple current, is separate from the DC current path (DC PATH), sensing the lower frequency average current. Separating the current sensing paths allows the signal to noise ratio of the AC sense signal (Vsen(ac)) to be increased and allows the switching noise to be filtered from the DC sense signal (K*Vsen(dc)). The gain (K) of the DC sense signal is adjusted so that the DC sense signal (K*Vsen(dc)) has the proper proportion to the AC sense signal (Vsen(ac)). The AC sense signal (Vsen(ac)) and the DC sense signal (K*Vsen(dc)) are combined by a summing circuit (72). The composite sense signal (Sum) is applied to a PWM comparator (50) to control the duty cycle of the switch.