ML2731CT Micro Linear, ML2731CT Datasheet - Page 8

• Manufacturer Part Number: ML2731CT
• Description: PA Bias Controller
• Manufacturer: Micro Linear

PA SUPPLY DC SWITCH
The ML2731 includes a DC switch for connecting the
supply voltage to an external PA. PAEN activates the DC
switch during TRANSMIT mode. To ensure the external PA
devices power up correctly, the chip has control circuits
preventing the PA supply DC switch from being enabled if
the VBIAS voltage is more positive than –3.0V.
Typically, the PA supply DC switch produces a voltage
drop of 100mV while conducting a 500mA current.
Parameters determining the maximum current capability of
the DC switch are the maximum voltage drop that can be
tolerated by the external PA and the thermal limitations of
the ML2731.
voltage and VPA current values and fall times are determined by
8
voltage and VPA current values and rise times are determined
PA current control loop component values as detailed below)
-1
-2
-3
-4
2
1
0
Figure 7: VPA Current ramp following PAEN enable (VBIAS
Figure 8: VPA Current ramp following PAEN disable (Vbias
FUNCTIONAL DESCRIPTION
-1 0 1 2 3 4 5 6 7 8 9
-0.5
-1.5
-2.5
-3.5
0.5
-1
-2
-3
1
0
by PA current control loop component values)
-10
0
Time ( sec)
Time ( sec)
10
PRELIMINARY DATASHEET
20
140
120
100
80
60
40
20
0
30
120
100
80
60
40
20
0
-20
VBIAS
PAEN Enable Logic
VPA Current
PRELIMINARY
PAEN Enable Signal
V BIAS
VPA Current
PA CURRENT CONTROL LOOP
The current control loop allows the PA current to be set by
the reference current on the ICON pin. This enables the
ML2731 to control PA output power and to compensate
for PA current variations due temperature changes. In
addition, it removes the need for calibration of PA current
in manufacturing because the PA current control loop
adjusts the PA bias be proportional to a reference voltage.
The PA current can typically is between 30 to 500mA.
The current control loop is enabled by the PAEN pin.
PA CURRENT CONTROL LOOP AND PA RAMP
COMPONENT VALUES
The ICON input pin is at virtual ground. The reference
current on the ICON pin is determined by the value of
REXT, connected to the ICON input, the voltage
reference (VREF), and the voltage source impedance.
For the fixed voltage source in the above circuit, the PA
current is determined by the formula:
VPA current = [Vcc(R1/(R2+REXT))*2000] / REXT + Offset
The offset current (minimum PA current) is typically
30mA
A typical REXT resistor value is 12k . Typical reference
voltages are in the range 0 to 3.0V to enable the PA
current to be controlled over the range of 30mA to
500mA.
For slow PA ramp on/off times (e.g. >10 sec) the PA bias
control loop has a sufficiently fast settling time to enable
the ramp to be controlled by ramping the ICON voltage
input. For fast, e.g. <10 sec PA ramp on/off times, the
ramp time is set by the value of the capacitance from
VBIAS to ground and VPA to ground.
The typical control loop bandwidths for typical values of
capacitance on VBIAS are shown in Figure 13.
January, 2000
ML2731