LM3205 datasheet(11/18 Pages) NSC

11 / 18 page

Block Diagram

Operation Description

The LM3205 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 in PWM mode.

Maximum load range may vary from this depending on input

voltage, output voltage and the inductor chosen.

Efficiency is typically around 96% for a 400mA load with 3.4V

output, 4.2V input. The output voltage is dynamically pro-

grammable from 0.8V (typ.) to 3.6V (typ.) by adjusting the

voltage on the control pin without the need for external

feedback resistors. This ensures longer battery life by being

able to change the PA supply voltage dynamically depending

on its transmitting power.

Additional features include current overload protection and

thermal overload shutdown.

The LM3205 is constructed using a chip-scale 8-pin mi-

croSMD or 10-pin LLP package. These packages offers the

smallest possible size, for space-critical applications such as

cell phones, where board area is an important design con-

sideration. Use of a high switching frequency (2MHz) re-

duces the size of external components. As shown in Figure

1, only three external power components are required for

implementation. Use of a microSMD package requires spe-

cial design considerations for implementation. (See mi-

croSMD Package Assembly and use in the Applications

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. Also, the system controller should set EN low

during power-up and other low supply voltage conditions.

(See Shutdown Mode in the Device Information section.)

20158035

FIGURE 2. Functional Block Diagram

LM3205 Datasheet(PDF) 11 Page - National Semiconductor (TI)