FAN5909: Multi-Mode Buck Converter with LDO Assist for GSM / EDGE, 3 G/3.5 G and 4 G PAs

The FAN5909 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. Load currents up to 2.5 A are allowed, which enables GSM / EDGE, 3 G/3.5 G, and 4G platforms under very poor VSWR conditions.

The output voltage may be dynamically adjusted from 0.40 V to 3.60 V, proportional to an analog input voltage VCON ranging from 0.16 V to 1.44 V, optimizing power added efficiency. Fast transition times of less than 6 µs are achieved, allowing excellent inter-slot settling.

An integrated LDO is automatically enabled under heavy load conditions or when the battery voltage and voltage drop across the DC-DC PMOS device are within a set range of the desired output voltage. This LDO assist feature supports heavy load currents under the most stringent battery and VSWR conditions while maintaining high efficiency, low dropout, and superior spectral performance.

The FAN5909 DC-DC operates in PWM Mode with a 2.9 MHz switching frequency and supports a single, smallform-factor inductor ranging from 1.0 µH to 2.2 µH. In addition, PFM operation is allowed at low load currents for output voltages below 1.5 V to maximize efficiency. PFM operation can be disabled by setting MODE pin to LOW.

When output regulation is not required, the FAN5909 may be placed in Sleep Mode by setting VCON below 100 mV nominally. This ensures a very low IQ (<50 μA) while enabling a fast return to output regulation.

FAN5909 is available in a low profile, small form factor, 16 bump, Wafer-Level Chip-Scale Package (WLCSP) that is 1.61 mm x 1.61 mm. Only three external components are required: two 0402 capacitors and one 2016 inductor.

Features

• Up to 95% Efficient Synchronous Operation in High Power Conditions
• 2.9 MHz PWM-Only Mode
• 2.9 MHz PWM Operation at High Power and PFM Operation at Low Power and Low Output Voltage for Maximum Low Current Efficiency