Block Diagram

Operation Description

The LM3208 is a simple, step-down DC-DC converter opti-

mized for powering RF power amplifiers (PAs) in mobile

phones, portable communicators, and similar battery pow-

ered 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

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

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 micro

SMD 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 micro SMD package requires special design con-

siderations for implementation. (See Micro SMD Package

Assembly and use in the Applications Information section.)

Its fine bump-pitch requires careful board design and preci-

sion assembly equipment. Use of this package is best suited

for opaque-case applications, where its edges are not sub-

ject 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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FIGURE 2. Functional Block Diagram

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