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參數(shù)資料
| 型號： | 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 |
| 文件頁數(shù)： | 10/13頁 |
| 文件大?。?/td> | 885K |
| 代理商： | LMV243 |
Application Information
(Continued)
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
X
and R
Y
according to Figure 3 where
Z = 20 log [R
IN
/ (R
IN
+ R
Y
)]
e.g. R
Y
= 300
results in an attenuation of 16.9dB.
To prevent reflection back to the coupler the impedance
seen by the coupler should be 50
. The impedance R
O
consists of R
X
// (R
Y
, R
O
, + R
IN
). R
X
can be calculated with
the formula:
R
X
= [R
O
* (R
Y
+ R
IN
)] / R
Y
R
X
= 50
*
[1 + (50/R
Y
)]
e.g. with R
Y
= 300
, R
IN
= 50
→
R
X
= 58
.
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
RAMP
is an analog signal (Base band DAC ramp signal)
–TX_EN is a digital signal (performs the function ‘Shutdown/
Transmit Enable’).
5.1 V
RAMP
in signal
The actual V
input value sets the RF output power. By
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
/dBm) out of the PA which
is proportional to the single ramp voltage steps. The recom-
mended V
RAMP
voltage range for RF power control is 0.2V to
2.0V. The V
input will tolerate voltages from 0V to V
without malfunction or damage. The V
input does not
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.
Sourcing/Sinking current show the typical voltage drop from
the rail over temperature. The Sourcing/Sinking current vs.
output
voltage
characteristics
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 is R = 0
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.
The
output
voltage
vs.
show
the
typical
20029038
FIGURE 3. PA Control Loop With Extra Attenuation
L
www.national.com
10
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