On-Chip Buck-Boost Power Supply for Power Amplifiers
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The increasing use of low voltage portable devices and growing requirements of functionalities embedded into such devices, efficient power management techniques are needed for longer battery life. The efficiency of battery-operated portable applications can be improved by increasing the efficiency of power amplifiers. By dynamically changing the supply voltage from the fixed battery supply using a DC-DC converter, the power amplifier can be operated with high efficiency during power back-off. Given the highly variable nature of the batteries (e.g. 0.9 V - 1.8 V for NiHM), to operate the system at their peak performance levels, even when the battery is close to fully discharged, and to achieve high efficiency, a dynamic, noninverting, synchronous buck-boost converter is proposed. The noninverting buck-boost converter is essentially a cascade combination of a buck converter followed by a boost converter, where a single inductor and capacitor are used for both. The voltage is reduced when the duty cycle, D, is less than 0.5, and increased when D > 0.5. The efficiency of the converter is improved by replacing the rectifier diodes with switches, which results in a synchronous converter topology. The converter output voltage is dynamically adjustable from 0.5 V to 2.7 V, while supplying a load current up to 40 mA from an input supply voltage from 0.7 V to 1.8 V. The peak-to-peak output ripple was simulated to be less than 60 mV. The circuit design is mainly optimized for maximum efficiency inside the typical operating output voltage interval, 1.5 V to 2.0 V. Consequently, a relative large inductor value is used. The size of the inductor was first calculated and then simulated to make sure that the inductor value which gives the highest efficiency is employed. The converter obtains efficiency between 89% and 94% inside the typical operating output voltage interval under various load currents, as the load current is changed from 5 mA to 40 mA. The efficiency is slightly reduced as the output voltage is further reduced. As the output voltage is reduced to 0.6 V the efficiency is reduced to minimum 78% for all load currents. As the output voltage is approaching 2.7 V, the efficiency of the converter is degenerated.