Low-Cost, Low-Voltage, PA Power Control
Amplifier for GSM Applications
MAX4473
Applications Information
Gain Resistor Selection
(RG1, RG2, RG3)
For proper operation, do not make the value of external
gain resistors RG1 and RG2 larger than twice the value
of RG3. In most practical applications, choose RG1
smaller than RG3 to limit the voltage drop over RG1
and R
SENSE
. A large voltage drop over R
SENSE
sub-
stantially reduces the voltage applied to the PA, thus
reducing PA output power. Set RG2 equal to RG1 to
compensate for the input bias currents of A3.
Recommended values for RG3 are between 1kΩ and
10kΩ.
•
Accuracy:
A high R
SENSE
value allows lower cur-
rents to be measured more accurately because input
offset voltages become less significant when the
sense voltage is larger. For best performance, select
R
SENSE
to provide approximately 100mV of sense
voltage for the full-scale current in each application.
•
Efficiency and Power Dissipation:
At high current
levels, the I
2
R losses in R
SENSE
are significant. Take
this into consideration when choosing the resistor
value and its power dissipation (wattage) rating.
Also, the sense resistor’s value may drift if it is
allowed to heat up excessively.
Sense Resistor Selection (R
SENSE
)
Choose R
SENSE
based on the following criteria:
•
Voltage Loss:
A high R
SENSE
value causes the
power-source voltage to degrade through IR loss.
For minimal voltage loss, use low R
SENSE
values.
_________________________Test Circuit
R
SENSE
V
CC
0.1μF
RG1
1kΩ
1%
V
CC
SR1
100Ω
0.1%
RG2
1kΩ
1%
SR2
Pin Configuration
TOP VIEW
SR1
SR2
1
2
8
7
V
CC
OUT
SR3
GND
PC
SHDN
GND
MAX4473
OUT
R
L
10kΩ
C
L
300pF
R
E
750Ω
2N3904
MAX4473
SHDN
3
6
5
PC
4
SR3
RG3
2.5kΩ
1%
μMAX/SO
Chip Information
TRANSISTOR COUNT: 348
6
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
