LMV242_15 Datasheet, PDF (15/25 Pages) Texas Instruments – LMV242 Dual Output, Quad-Band GSM/GPRS Power Amplifier Controller

LMV242
www.ti.com
SNWS014C – APRIL 2004 – REVISED MAY 2013
It is very important to choose the right attenuation between PA output and detector input 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 LMV242 detection range the
minimum required total attenuation is about 35 dBm (please refer to typical performance characteristics in the
datasheet and Figure 26). A typical coupler factor is 20 dB. An extra attenuation of about 15 dB should be
inserted.
Extra attenuation Z between the coupler and the RF input of the LMV242 can be achieved by 2 resistors R_X and
R_Y according to Figure 27, where
Z = 20 LOG (R_IN / [R_IN + R_Y])
(1)
or
R_Y = R_IN ·
¨¨©§^10-
z
20
-1
(2)
e.g. R_Y = 300Ω results in an attenuation of 16.9 dB.
To prevent reflection back to the coupler the impedance seen by the coupler should be 50Ω (R_O). The
impedance consists of R_X in parallel with R_Y + R_IN. R_X can be calculated with the formula:
R_X = [R_O * (R_Y + R_IN)] / R_Y
(3)
R_X = 50 * [1 + (50 / R_Y)]
(4)
e.g. with R_Y = 300Ω, R_IN = 50Ω → R_X = 58Ω.
ANTENNA
COUPLER
PA
50:
OUT2
OUT1
COMP1
LMV242
RF_IN
R_IN
50:
R_Y
COMP2
R_X
TX_EN
V_RAMP
BS
Figure 28. Simplified PA Control Loop with Extra Attenuation
BASEBAND CONTROL OF THE LMV242
The LMV242 has 3 baseband-controlled inputs:
• V_RAMP signal (Base band DAC ramp signal)
• TX_EN is a digital signal (performs the function “Shutdown/Transmit Enable”).
• Band Select (BS)
V_RAMP Signal
The actual V_RAMP input value sets the RF output power. By 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_OUT/dBm) out of
the PA, which is proportional to the single ramp voltage steps. The recommended V_RAMP voltage range for RF
power control is 0.2V to 2.0V. The V_RAMP input will tolerate voltages from 0V to V_DD without malfunction or
damage. The V_RAMP 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_AMP signal will not cause excess RF signal output and increased
power consumption.
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