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A switch size control circuit for hybrid mode DC-DC buck converter is presented. Detecting the output of error amplifier by a comparator, the circuit can select appropriate switch size according to load current for high efficiency. The threshold level of the comparator varies with respect to the amount of slope compensation, which maintains a substantially constant switch size changing point. A monolithic DC-DC buck converter with 5A load current using the proposed circuit has been fabricated with a 0.5μm CMOS process for validation. The switch size is 1/4 of the maximum value when load current is below 1A with 500mA hysteresis. The measured efficiency is up to 96%.