ML2731ET View Datasheet(PDF) - Micro Linear Corporation

Part Name

Description

Manufacturer

ML2731ET

PA Bias Controller

Micro Linear Corporation 

PRELIMINARY

ML2731

FUNCTIONAL DESCRIPTION

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.

>0µsec

>200µs

PDN

VNEG

>0µsec

>0µsec

PAEN

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0

VREF

VPA Current

5

10

15

160

140

120

100

80

60

40

20

0

20

3V

VControl Voltage

0V

0V

VBIAS Voltage

-3V

Current

VPAout current

Charge VNEG

caps

Time (µsec)

Figure 12: VPA output vs VREF (TRANSMIT mode, REXT = 12kW,

capacitance on VBIAS = 3.3nF)

discharge VNEG

caps

Timeconstant set by

external caps on VPA

Timeconstant set by

external caps on VBIAS

1

Figure 11: PA Bias Control Loop Operation

0

Where ICON is a fixed value (i.e. the VPA current is not

ramped up and down by changing the value of ICON)

-1

then the time taken for the VPA current to ramp up

following PAEN being enabled is determined by the

-2

capacitor values on VBIAS and VPA. The time taken for

the current to ramp down following PAEN being disabled

-3

is determined by the capacitance value on VPA only.

Example timings are:

-4

1msec ramp-up for 2nF VBIAS capacitance

-5

4msec ramp-up for 22nF VBIAS capacitance

-6

An example of a 3 step current ramp up and ramp down,

with 3.3nF capacitance on VBIAS, is shown in Figure 12.

-7

10

100

100

Frequency(KHz)

3.3nF 2.2nF 1.5nF

Figure 13: Bandwidth of PA Current control loop for 3.3nF,

2.2nF and 1.5nF VBIAS capacitance

10

PRELIMINARY DATASHEET January, 2000

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