ML2731CT Micro Linear, ML2731CT Datasheet - Page 10

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

VControl Voltage
VPAout current
VBIAS Voltage
Where ICON is a fixed value (i.e. the VPA current is not
ramped up and down by changing the value of ICON)
then the time taken for the VPA current to ramp up
following PAEN being enabled is determined by the
capacitor values on VBIAS and VPA. The time taken for
the current to ramp down following PAEN being disabled
is determined by the capacitance value on VPA only.
Example timings are:
1 sec ramp-up for 2nF VBIAS capacitance
4 sec ramp-up for 22nF VBIAS capacitance
An example of a 3 step current ramp up and ramp down,
with 3.3nF capacitance on VBIAS, is shown in Figure 12.
RECOMMENDED PA BIAS OPERATION
An example of how the PA bias operates is shown in Figure
11. A three-step reference voltage ramp is generated using
an external baseband circuit and is applied to the ICON
pin via an external resistor. The ML2731 current control
loop uses this input as a reference to set the PA current and
therefore generates a current ramp output (and hence PA
power ramp output.) The rate of change of negative bias
and PA current is determined by the capacitance on the
VBIAS pin and on the VPA pin. Note sufficient time should
be allowed for the negative voltage generator to reach a
steady state, prior to enabling the PA.
10
Current
FUNCTIONAL DESCRIPTION
-3V
VNEG
PAEN
3V
0V
0V
PDN
Figure 11: PA Bias Control Loop Operation
>0 sec
Charge VNEG
caps
>200 s
PRELIMINARY DATASHEET
Timeconstant set by
external caps on VBIAS
>0 sec
>0 sec
PRELIMINARY
discharge VNEG
caps
Timeconstant set by
external caps on VPA
Figure 12: VPA output vs VREF (TRANSMIT mode, REXT = 12k ,
-1
-2
-3
-4
-5
-6
-7
1
0
January, 2000
10
Figure 13: Bandwidth of PA Current control loop for 3.3nF,
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
1
0
0
2.2nF and 1.5nF VBIAS capacitance
capacitance on VBIAS = 3.3nF)
3.3nF
5
Time ( sec)
VREF
VPA Current
Frequency(KHz)
10
2.2nF
100
15
1.5nF
ML2731
20
160
140
120
100
80
60
40
20
0
100