The response time of this loop can be controlled by varying

the RC time constant of the integrator. Setting this at a low

level will result in fast output settling but can result in ringing

in the output envelope. Settling the RC time constant high

will give the loop good stability but will increase settling time.

Figure 3 shows a typical RF power control loop realized by

using the National’s LMV243 with integrated RF detector.

The RF signal from the PA passes through a directional

coupler on its way to the antenna. Directional couplers are

characterized by their coupling factor which is in the 10dB to

30dB range, typical 20dB. Because the coupled output must

in its own right deliver some power (in this case to the

detector), the coupling process takes some power from the

main output. This manifests itself as insertion loss, the inser-

tion loss being higher for lower coupling factors.

3.0 Attenuation between coupler and LMV243 detector

It is very important to choose the right attenuation between

PA output and detector input, i.e. the total of coupling factor

and extra attenuation, in order to achieve power control over

the full output power range of the PA. A typical value for the

output power of the PA is +35.5 dBm for GSM and +30 dBm

for PCS/DCS. In order to accommodate these levels into the

LMV243 detection range the minimum required total attenu-

ation is about 35 dBm (please refer to typical performance

characteristics in the datasheet). A typical coupler factor is

20dB. An extra attenuation of about 15 dB should be in-

serted.

Extra attenuation Z between the coupler and the RF input of

the LMV243 can be achieved by 2 resistors R

according to

Figure 3, where

Z = 20 log [R

e.g. R

To prevent reflection back to the coupler the impedance

seen by the coupler should be 50

Ω. The impedance R

O

consists of R

the formula:

R

R

e.g. with R

4.0 Components of a Power Amplifier Loop

Figure 3 shows the basics of a typical LMV243 quad-band

application.

The key components are:

•

The LMV243

•

One power amplifier, usually for the GSM and PCN/DCS

bands

•

A single two channel RF coupler is used instead of the

two RF couplers

•

A dual or quad-band antenna.

Figure 1 shows the LMV243’s internal architecture. The

LMV243 contains an RF detector, error amplifier, a ramp V/I

converter and an output driver. The LMV243 input interface

consists of an RF input, Ramp voltage, and a digital input to

perform the function ’Shutdown/Transmit Enable’.

5.0 Analog and Digital Input Signals of the LMV243

The LMV243 has the following inputs:

–V

–TX_EN is a digital signal (performs the function ‘Shutdown/

Transmit Enable’).

5.1 V

The actual V

applying a certain mask shape to the ’Ramp in’ pin, the

output voltage level of the LMV243 adjusts the PA control

voltage to get a power level (P

is proportional to the single ramp voltage steps. The recom-

mended V

2.0V. The V

without malfunction or damage. The V

change the output level until the level reaches about 200mV,

so offset voltages in the DAC or amplifier supplying the

Ramp signal will not cause excess RF signal output and

increased power consumption.

6.0 Analog Output

The Output is driven by a rail-to-rail amplifier capable of both

sourcing and sinking. It is able to source and sink 25mA with

less than 200mV voltage drop from either rail over recom-

mended operating conditions. Please refer to the typical

performance

characteristics.

The

output

voltage

vs.

Sourcing/Sinking current show the typical voltage drop from

the rail over temperature. The Sourcing/Sinking current vs.

output

voltage

characteristics

show

the

typical

charging/discharging current, which the output is capable of

delivering at a current voltage. The output is free from

glitches when enabled by TX_EN. When TX_EN is low, the

selected output voltage is fixed or near GND.

7.0 Bandwidth Compensation

To compensate and prevent the closed loop arrangement

from oscillations and overshoots at the output of the RF

detector/error amplifier LMV243, the system can be adjusted

by means of external RC components connected between

Comp and Out . Exact values heavily depend on PA charac-

teristics. A good starting point isR=0

Ω and C = 68pF. The

vast combinations of PA’s and couplers available preclude a

generalized formula for choosing these component. Please

contact National Semiconductor for additional assistance.

8.0 Evaluation Board

An evaluation board in available for the LMV243. Please

contact your local distributor or National Semiconductor

sales office.

20029038

FIGURE 3. PA Control Loop With Extra Attenuation

www.national.com

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