參數(shù)資料
型號(hào)： LMV243
廠商： National Semiconductor Corporation
英文描述： Single-Channel, Quad-Band GSM Power Controller in micro SMD
中文描述： 單通道，四頻GSM電源控制器在微型SMD
文件頁(yè)數(shù)： 9/13頁(yè)
文件大?。?/td> 885K
代理商： LMV243
Application Information
1.0 The LMV243 as an RF Power Amplifier (PA) Control-
ler
The LMV243 is a member of the power loop controller family
of National Semiconductor, for a quad-band TDMA/GSM
solution. The typical application diagram demonstrates a
basic approach for implementing the quad-band solution
around the RF Power Amplifier. The LMV243 contains a 50
dB Logamp detector and interfaces directly with the direc-
tional coupler.
The LMV243 Base Band (control) interface consists of 2
signals: TX_EN to bring the device out of shutdown status
within 5μs, and V
for the transmit burst characteristic
determining the desired Output Power level. The LMV243
gives maximum flexibility to meet GSM frequency and time
mask criteria for many different single supply Power Ampli-
fier types like HBT or, MesFET in GaAs, SiGe or Si technol-
ogy. This is accomplished by the Programmable Ramp char-
acteristic from the Base Band and the TX_EN signal along
with the external compensation capacitor.
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.
2.0 A Typical GSM Power Amplifier Controlled Loop
This section should give a general overview and understand-
ing of how a typical Power Amplifier control loop works and
how to get rid of some of the most common problems
confronted in the design. Figure 2 shows the generic com-
ponents of such a loop. Beginning at the output of the GSM
Power Amplifier (PA), this signal is fed, usually via a direc-
tional coupler, to a detector. The output current of the detec-
tor Idet 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 op amp inte-
grator drives the gain control input of the power amplifier.
Now to examine how this circuit works, we will assume
initially that the output of the PA is at some low level and that
the V
RAMP
voltage is at 1V. The V/I converter converts the
V
. This current can
only come from the integrator capacitor C. Current flow in
this direction increases the output voltage of the integrator.
This voltage, which drives the PA, increases the gain (we
assume that the PA’s gain control input has a positive sense,
that is, increasing voltage increases gain). The gain will
increase, thereby increasing the amplifier’s output level until
the detector output current equals the ramp current I
.At
that point, the current through the capacitor will decrease to
zero and the integrator output will be held steady, thereby
settling the loop. If capacitor charge is lost over time, the
gain will decrease. However, this leakage will quickly be
corrected by additional integrator current from the newly
reduced detector current.
The key usefulness of this circuit lies in its immunity to
changes in the PA gain control function. From a static per-
spective at least, the relationship between gain and gain
control voltage is of no consequence to the overall transfer
function. Based upon the value of V
RAMP
, the integrator will
set the gain control voltage to whatever level is necessary to
produce the desired output level. Any temperature depen-
dency in the 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 (V
vs. V
). The
only requirement is that the gain control function of the PAbe
monotonic. It is crucial, however, that the detector is tem-
perature stable.
The circuit as described so far, has been designed to pro-
duce a constant output level for varying input levels. The
only requirement is for it to be temperature stable for input
levels that correspond to the setpoint voltage V
. If the
detector used has a higher dynamic range, the circuit to
precisely set PA output levels over a wide dynamic range. To
do this, the integrator reference voltage, V
, is varied.
The voltage range on V
follows directly from the detec-
tor’s transfer function. For example, if the detector delivers
0.5V for an input of 7dBm, a reference voltage of 0.5V will
cause the loop to settle when the detector input is 7dBm
(the PA output will be greater than this amount by whatever
coupling factor exists between PA and detector). The dy-
namic range for the variable RF P
case will be deter-
mined by the device in the circuit with the least dynamic
range (i.e. the gain control range PA or linear dynamic of
detector).
20029037
FIGURE 2. PA Control Loop
L
www.national.com
9
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