FAN5904: Buck Converter with Bypass Mode for GSM/EDGE PAMs, 3G/3.5G and 4G PAs

The FAN5904 is a high-efficiency, low-noise, synchronous, step-down, DC-DC converter optimized for powering Radio Frequency (RF) Power Amplifiers (PAs) in handsets and other mobile applications. In High-Power Mode, GSM Tx power is enabled. In Low-Power Mode, up to 3.0W is supported, enabling up to 29dBm output power for 3G/3.5G and 4G platforms.

The output voltage may be dynamically adjusted from 0.40V to 3.50V, proportional to an analog input voltage VCON ranging from 0.16V to 1.40V, optimizing power-added efficiency. Fast transition times of less than 10µs are achieved, allowing excellent inter-slot settling.

An integrated bypass FET is automatically enabled when the battery voltage and voltage drop across the DC-DC PMOS device are within a set voltage range of the desired output voltage (VOUT = VBAT - VPMOS - VBP_TH). This dynamic bypass feature enables the FAN5904 to support heavy load currents under the most stringent VSWR conditions while maintaining high efficiency and superior spectral performance. The bypass FET may also be enabled by providing a VCON voltage nominally greater than or equal to 1.5V or by driving BPEN HIGH.

The FAN5904 operates in PWM Mode with a 6MHz switching frequency in Low-Power Mode and at 3MHz in High-Power Mode, which limits high-frequency spur levels. It uses a single, small form factor inductor of 470nH. In addition, PFM operation is allowed in Low-Power Mode to improve efficiency at low load currents.

The FAN5904UC00X option allows PFM Mode only when VOUT is less than 1V, while the FAN5904UC01X permits PFM Mode at higher voltages for applications that can tolerate larger output ripple and that demand optimal low-to-moderate load current efficiency.