Step-down converter for Power Amps in WCDMA & CDMA handsets

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. 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 saving enables longer battery life, smaller batteries, and thus smaller and lighter mobile phones.

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 optimises PA efficiency at both low and high transmit levels. To meet stringent WCDMA requirements, the output voltage responds in less than 30 microseconds. To improve efficiency even further, the AAT1171 provides the lowest quiescent current available on the market today under both full and light load conditions. The no load quiescent current is only 45 microamps (typical).

The AAT1171 also maximises efficiency and extends handset talk time by providing an integrated 85mohm 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 2.2µH inductor and 4.7µF capacitor on the output. The AAT1171 also features current-limiting, short-circuit and over-temperature shutdown protection circuitry to ensure device integrity.
Specified over the -40 to +85 degrees C temperature range, the AAT1171 is immediately available in a Pb-free, 12-pin TDFN33 package.