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This paper proposes a multimode digitally controlled boost converter with PID autotuning and constant frequency/constant off-time hybrid pulsewidth modulation (PWM) control. Constant off-time PWM control is introduced in this converter for avoiding the limit cycle oscillation and maintaining a constant dc gain of the control-output transfer function for a high conversion ratio. Discrete-time modeling of constant off-time PWM is given and verified. An autotuning PID compensator based on frequency-response measurement is adopted to compensate the output stage with variable gain and variable corner frequency. PFM is also included in this controller to improve light load efficiency. A new method of automatic PWM/PFM mode switching without current sensing is proposed. Sufficient loop stability margin and optimal efficiency are obtained by using these techniques for a boost converter operating over wide ranges of output voltages, load currents, and external components. Experimental results are shown for an asynchronous boost converter with 2.5-V input voltage, 3-12 V, and 1-400 mA output.