Datasheet

MOSFET
CONTROL
LOGIC
SW
CURRENT
COMP
OSCILLATOR
ERROR
AMPLIFIER
DELAY
LOGIC
V
CON
FB
SHUTDOWN
CONTROL
MAIN CONTROL
EN
FET SIZE
CONTROL
COMP
PV
IN
PGND
V
DD
GND
SMALL
PFET
LARGE
PFET
LM3208
SNVS404B –APRIL 2006–REVISED MARCH 2013 www.ti.com
BLOCK DIAGRAM
Figure 34. Functional Block Diagram
OPERATION DESCRIPTION
The LM3208 is a simple, step-down DC-DC converter optimized for powering RF power amplifiers (PAs) in
mobile phones, portable communicators, and similar battery powered RF devices. It is designed to allow the RF
PA to operate at maximum efficiency over a wide range of power levels from a single Li-Ion battery cell. It is
based on a current-mode buck architecture, with synchronous rectification for high efficiency. It is designed for a
maximum load capability of 650mA when V
OUT
> 1.05V (typ.) and 400mA when V
OUT
< 1.00V (typ.) in PWM
mode.
Maximum load range may vary from this depending on input voltage, output voltage and the inductor chosen.
Efficiency is typically around 95% for a 400mA load with 3.4V output, 3.9V input. The LM3208 has an R
DSON
management scheme to increase efficiency when V
OUT
≤ 1V. The output voltage is dynamically programmable
from 0.8V to 3.6V by adjusting the voltage on the control pin without the need for external feedback resistors.
This prolongs battery life by changing the PA supply voltage dynamically depending on its transmitting power.
Additional features include current overload protection and thermal overload shutdown.
The LM3208 is constructed using a chip-scale 8-pin DSBGA package. This package offers the smallest possible
size, for space-critical applications such as cell phones, where board area is an important design consideration.
Use of a high switching frequency (2MHz, typ.) reduces the size of external components. As shown in Figure 1,
only three external power components are required for implementation. Use of a DSBGA package requires
special design considerations for implementation. (See DSBGA Package Assembly and Use in the Application
Information section.) Its fine bump-pitch requires careful board design and precision assembly equipment. Use of
this package is best suited for opaque-case applications, where its edges are not subject to high-intensity
ambient red or infrared light. In addition, the system controller should set EN low during power-up and other low
supply voltage conditions. (See Shutdown Mode in the Device Information section.)
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