ML2731

8

January, 2000

PRELIMINARY DATASHEET

PRELIMINARY

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

-10

0

10

20

30

Time (

µsec)

-20

0

20

40

60

80

100

120

V BIAS

PAEN Enable Signal

VPA Current

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.

Figure 7: VPA Current ramp following PAEN enable (VBIAS

voltage and VPA current values and rise times are determined

by PA current control loop component values)

Figure 8: VPA Current ramp following PAEN disable (Vbias

voltage and VPA current values and fall times are determined by

PA current control loop component values as detailed below)

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

W. 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

msec) 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

msec 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.

FUNCTIONAL DESCRIPTION

-1 0 1 2 3 4 5 6 7 8 9

Time (

µsec)

0

20

40

60

80

100

120

140

VBIAS

PAEN Enable Logic

VPA Current

-4

-3

-2

-1

0

1

2