
M
5GHz Linear Power Amplifiers
6
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Detailed Description
The MAX2840/MAX2841 linear power amplifiers (PAs)
are intended for 5GHz to 6GHz band applications. The
PAs are fully characterized in the 5.15GHz to 5.35GHz
band. The PAs consist of two driver stages and an out-
put stage. The ICs also feature an integrated power
detector and power shutdown mode.
Dynamic Power Control
The MAX2840/MAX2841 are designed to provide opti-
mum power-added efficiency (PAE) in both high- and
low-power applications. For a +3.3V supply at high-out-
put power level, the output power is typically +18dBm
(MAX2841) with an idle current of 165mA, or +15dBm
(MAX2840) with an idle current of 120mA. At low-output
power levels, the DC current can be reduced by an
external DAC to improve PAE while maintaining suffi-
cient ACPR performance. This is achieved by using
external resistors connected to the BIAS pin to set the
bias currents of the driver and output stages. See the
Typical Application Circuit
for resistor values. Typically,
a DAC voltage of 1.9V gives a 260mA bias current for
the MAX2841, a DAC voltage of 1.6V gives a 155mA
bias current for the MAX2840. Increasing the DAC volt-
age decreases the idle current. Similarly, decreasing
the DAC voltage increases the idle current. Users can
set the desired idle current using only two off-chip 1%
resistors: a shunt resistor (R2) from BIAS to ground and
a series resistor (R1) to the DAC voltage, as shown in
the
Typical Application Circuit
. Resistor values R1 and
R2 are determined as follows:
[Equation 1]
V
MAX
= 1.2
(R1 + R2) / R2;
(I
CC
= 0, V
DAC
= V
MAX
)
[Equation 2]
I
MAX
= PART
1.2 / 2000 + (R1
R2) / (R1 + R2);
(I
CC
= I
MAX
= max value, V
DAC
= 0)
[Equation 3]
I
DAC
= (V
DAC
- 0.82) / R1
[Equation 4]
I
MID
= PART
1.2 / (2000 + R2);
(V
DAC
= floating)
[Equation 5]
I
CC
= PART
I
BIAS
where:
V
MAX
I
MAX
I
MID
V
DAC
I
DAC
PART
If no DAC is used and a constant idle current is
desired, use Equation 4 to determine the resistor values
for a given total bias current. Only R2 is required.
For a DAC capable of sourcing and sinking currents,
the full voltage range of the DAC (typically from 0 to
+3V) can be used. By substituting the desired values of
V
MAX
and I
MAX
into Equations 1 and 2, R1 and R2 can
be easily calculated.
For a DAC capable of sourcing current only, use
Equation 4 to determine the value of resistor R2 for the
desired maximum current. Use Equation 1 to determine
the value of resistor R1 for the desired minimum current.
= maximum DAC voltage
= maximum idle current
= idle current with V
DAC
= floating
= DAC voltage
= DAC current
= 1000 for MAX2840, 1600 for MAX2841
Pin Description
PIN
NAME
DESCRIPTION
A1
SHDN
Shutdown Input. Drive logic low to place the PA in shutdown mode; drive logic high for normal
operation.
A2, A4, C2,
C4
GND
Ground. Requires a good connection to the circuit board ground plane with multiple vias for lowest
possible inductance and lowest thermal resistance.
A3
B2
B4
C1
C3
V
CC
RF_IN
RF_OUT
BIAS
P_DET
DC Supply Voltage. Requires external RF bypass capacitor to ground.
Power Amplifier Input. Requires external DC-blocking capacitor.
Power Amplifier Output. Requires external DC-blocking capacitor and pullup inductor.
Bias Control. Connect two external resistors and DAC block to adjust the bias current.
Power Detector Output. This output is a DC voltage indicating the PA output power.