Voltage-Scaling Step-Down Converter for WCDMA & CDMA Power Amplifiers

AnalogicTech has announced the AAT1171, a new fully-integrated voltage-scaling DC/DC step-down converter specifically optimized to control the power to the Power Amplifier (PA) in WCDMA and CDMA handsets. By dynamically controlling the operating voltage of the PA in mobile phones, the AAT1171 increases amplifier efficiency and extends talk time.

"The stereotypical WCDMA handset transmit profile makes the PA operate at approximately +16dBm output power for about 50 to 60 percent of its operating life," said Bill Weiss, Product Line Director for AnalogicTech. "When powered from the AAT1171, instead of being powered directly from the battery, the PA consumes 60 percent less power at this +16dBm level. This tremendous energy savings enables longer battery life, longer talk time, smaller batteries, and thus smaller and lighter weight mobile phones."

Dynamic Control

The AAT1171 operates across a wide voltage input range of 2.7V to 5.5V. Its input voltage is controlled by a real-time output signal from the baseband processor and delivers up to 600mA of continuous load current.

By supplying a variable output voltage of 0.6V to 3.6V, the AAT1171 optimizes PA efficiency at both low and high transmit levels. To meet stringent WCDMA requirements, the output voltage responds in less than 30us. To improve efficiency even further, the AAT1171 provides the lowest quiescent current available on the market today under both full and light load conditions. No load quiescent current is only 45uA (typical).

The AAT1171 also maximizes efficiency and extends handset talk time by providing an integrated 85milliohms bypass MOSFET. When transmitting at full power, the MOSFET switch bypasses the DC/DC converter and output inductor to provide a low resistance path between the battery and the PA, allowing the system to operate very close to the minimum battery voltage. The switch is activated by setting the bypass input high or forcing the baseband DAC voltage output to 1.3V.

Operating at a high 2MHz switching frequency, the AAT1171 allows the system designer to use an extremely small 2.2uH inductor and 4.7uF capacitor on the output. The AAT1171 also features current-limiting, short-circuit and over-temperature shutdown protection circuitry to ensure device integrity."

Posted to the site on 11th July 2006