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A fully integrated wide-band PID controller is implemented for a buck DC-DC converter operating in voltage mode. To implement efficiently areal on-chip PID controller, a novel compensator is proposed utilizing AC ripple current of the inductor. The integral controller with an error amplifier and the proportional controller do not require any additional capacitors. The bandwidth of overall loop is about 200 kHz to support fast transient response at 1MHz switching frequency. The stability of the loop is guaranteed by theoretical analysis and measured results are provided. When the load steps for 1A, overshoot /undershoot voltages are within 50 mV and settling time is less than 90 μs. A buck converter with input of 3.6 V and output of 2.5 V for maximum power of 3.125 W is fabricated in 0.35 μm CMOS process, occupying an area of 0.75 mm2.