LMV242MDA datasheet(11/16 Pages) NSC

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Application Section

POWER CONTROL PRINCIPLES

The LMV242 is a member of the power loop controller family

of National Semiconductor, for quad-band TDMA/GSM solu-

tions. The typical application diagram demonstrates a basic

approach for implementing the quad-band solution around

an RF Power Amplifier (PA). The LMV242 contains a 50 dB

Logamp detector and interfaces directly with the directional

coupler.

The LMV242 Base Band (control-) interface consists of 3

signals: TX_EN to enable the device, BS to select either

output 1 or output 2 and V

RAMP to set the RF output power to

the specified level. The LMV242 gives maximum flexibility to

meet GSM frequency- and time mask criteria for many dif-

ferent single supply Power Amplifier types like HBT or Mes-

FET in GaAs, SiGe or Si technology. This is accomplished by

the programmable Ramp characteristic from the Base Band

and the TX_EN signal along with the external compensation

capacitor.

POWER AMPLIFIER CONTROLLED LOOP

This section gives a general overview and understanding of

how a typical Power Amplifier control loop works and how to

solve the most common problems confronted in the design.

General Overview

The key benefit of a PA control loop circuit is its immunity to

changes in the PA gain control function. When a PA control-

ler is used, the relationship between gain and gain control

voltage (V

APC) of the PA is of no consequence to the overall

transfer function. It is a function of the controller’s V

RAMP

voltage. Based upon the value of V

RAMP, the PA controller

will set the gain control voltage of the PA to a level that is

necessary to produce the desired output level. Any tempera-

ture dependency in the PA gain control function will be

eliminated. Also, non-linearity’s in the gain transfer function

of the PA do not appear in the overall transfer function (P

OUT

vs. V

RAMP). The only requirement is that the gain control

function of the PA has to be monotonic. To achieve this, it is

crucial, that the LMV242’s detector is temperature stable.

Typical PA Closed Loop Control Setup

A typical setup of PA control loop is depicted in Figure 1.

Beginning at the output of the Power Amplifier (PA), this

signal is fed, usually via a directional coupler, to a detector.

The error between the detector output current I

DET and the

ramp current I

RAMP, representing the selected power setting,

drives the inverting input of an op amp, configured as an

integrator. A reference voltage drives the non-inverting input

of the op amp. Finally the output of the integrator op amp

drives the gain control input of the power amplifier, which

sets the output power. The loop is stabilized when I

DET is

equal to I

RAMP . Lets examine how this circuit works in detail.

20079528

FIGURE 1. PA Control Loop

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