Fairchild's high efficiency, high power buck converter helps mobile designers reduce power consumption

SAN JOSE, USA: Mobile device designers are faced with changing standards and a need to increase battery life, and extend talk and data transmit time. In order to help designers address these challenges, Fairchild Semiconductor, a leading global supplier of high performance power and mobile solutions, has developed a power management product for the RF portion of mobile handsets and tablets.

As part of its ongoing mobile initiative and in close cooperation with customers, Fairchild developed the FAN5904, a high power, high efficiency synchronous buck converter that supports GSM/GPRS/EDGE, 3G/3.5G and 4G power amplifiers (PA). The device reduces power consumption in wireless-enabled mobile devices extending connection times by more than 60 minutes. Available in a small solution footprint occupying less than 23.5mm2, the FAN5904 also reduces the operating temperature of the PA by at least 20ºC while maintaining RF performance.

The FAN5904 supports high and low power modes for GSM/EDGE PAMs, 3G/3.5G and 4G PAs respectively. In high power mode, GSM Tx power is enabled and the FAN5904 can provide up to 35dBm of output power even when there is a 3:1 VSWR antenna mismatch for GSM/EDGE PAMs. The device operates in PWM mode at 3MHz switching frequency to handle 2.3A of output current while achieving up to 92 percent efficiency. For 3G/3.5G and 4G PAs, FAN5904 will operate in low power mode and enable up to 29dBm of output power including 3:1 VSWR conditions.

The switching frequency of the FAN5904 in low power mode is 6MHz in PWM mode for heavier current loads. Under lighter current loads of less than 100mA the FAN5904 will switch to PFM mode to further optimize efficiency when the PA’s output power is less than 20dBm. Since the FAN5904 operates at 3MHz in high power mode and 6MHz in low power mode, it can be implemented with a 470nH, 2520 inductor and two 4.7µF, 1508 capacitors on the output to achieve less than 20.5mm2 of PCB area. For further area reduction a 470nH, 2016 inductor can be implemented which results in less than 15mm2 occupied by the total FAN5904 solution.

The output voltage of the device may be dynamically varied from 0.4V to 3.5V, proportional to an analog input VCON ranging from 0.16V to 1.40V provided by an external DAC. This allows the PA to be supplied with the voltage that enables maximum power added efficiency. The FAN5904 also offers fast transition times, enabling changes to the output voltage in less than 10µs for power transitions. A current-mode control loop with fast transient response ensures excellent line and load regulation.