Extra attenuation Z between the coupler and the RF input of

the LMV242 can be achieved by 2 resistors R

according to Figure 3, where

Z = 20 LOG (R

or

e.g. R

Ω results in an attenuation of 16.9 dB.

To prevent reflection back to the coupler the impedance

seen by the coupler should be 50

Ω (R

consists of R

with the formula:

R

R

e.g. with R

Ω,R

Ω → R

Ω.

BASEBAND CONTROL OF THE LMV242

The LMV242 has 3 baseband-controlled inputs:

•

V

•

TX_EN is a digital signal (performs the function

“Shutdown/Transmit Enable”).

•

Band Select (BS)

V

The actual V

applying a certain mask shape to the “Ramp in” pin, the

output voltage level of the LMV242 is adjusting the PA

control voltage to get a power level (P

PA, which is proportional to the single ramp voltage steps.

The recommended V

trol is 0.2V to 2.0V. The V

from 0V to V

input does not change the output level until the level reaches

about 206 mV, so offset voltages in the DAC or amplifier

supplying the R

output and increased power consumption.

Transmit Enable

Power consumption requirements are supported by the

TX_EN function, which puts the entire chip into a power

saving mode to enable maximum standby and talk time while

ensuring the output does not glitch excessively during

Power-up and Power-down. The device will be active in the

case TX_EN = High, or otherwise go to a low power con-

sumption shutdown mode. During shutdown the output is

pulled low to minimize the output voltage.

Band Select

The LMV242 is especially suitable for PA control loops with 2

PA’s. The 2 outputs to steer the V

controlled with the band select pin. When the band select is

LOW output2 is selected, while output1 is selected when

band select is HIGH. The not-selected output is pulled low.

Analog Output

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

sourcing and sinking. Several curves are given in the “Typi-

cal performance characteristics”-section regarding the out-

put. The output voltage vs. sourcing/sinking current curves

show the typical voltage drop from the rail over temperature.

The sourcing/sinking current vs. output voltage characteris-

tics show the typical charging/discharging current, which the

output is capable of delivering at a certain 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.

FREQUENCY COMPENSATION

To compensate and prevent the closed loop arrangement

from oscillations and overshoots at the output of the RF

detector/error amplifier of the LMV242, the system can be

adjusted by means of external RC components connected

between Comp1 and Comp2. Exact values heavily depend

on PA characteristics. A good starting point isR=0

Ω and

C = 68 pF. The vast combination of PA’s and couplers

available preclude a generalized formula for choosing these

components. Additional frequency compensation of the

closed loop system can be achieved by adding a resistor

(and if needed an inductor) between the LMV242’s output

and the V

conductor for additional support.

TIMING DIAGRAM

In order to meet the timemask specifications for GSM, a

good timing between the control signals and the RF signal is

essential. According to the specifications the PA’s RF output

power needs to ramp within 28 µsec with minimum over-

shoot. To achieve this, the output of the PA controller should

ramp at the same time as the RF signal from the Base Band.

The ramp signal sets the controllers output to the required

value, where the loop needs a certain time to set this output.

Therefore the ramp should be set high some time before the

output has to be high. How much time depends on the setup

and the PA used. If the controllers shutdown functionality is

used, the shutdown should be set high about 6 µsec before

the ramp is set high.

20079530

FIGURE 4. Simplified PA Control Loop with Extra

Attenuation

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