Detailed Description

The MAX4473 is a voltage-controlled, unidirectional,

high-side current setting amplifier for applications

where accurate control of PA supply current is desired.

This device is intended for wireless TDMA based sys-

tems (GSM, DECT), where tight restrictions over the

PA’s transmit burst and output power require closed-

loop control over the PA’s output power. When used

with a PA, the MAX4473 functions as a voltage-con-

trolled constant current source, accurately setting PA

supply current by varying the gain of the PA. If you

know the output power versus supply current profile for

the PA, you can set the PA’s output power by control-

ling the amount of supply current delivered to the PA.

The MAX4473 is composed of an input buffer (A1), a

voltage-to-current converting amplifier (A2), and a rail-

to-rail output error amplifier (A3) (see

Typical Operating

Circuit). External gain and sense resistors allow pro-

grammability for a wide range of applications.

In the

Typical Operating Circuit, PA supply current

flows from the system supply, through the external cur-

outputs of the error amplifier, A3, adjust the gain of the

voltage drop across external gain resistor, RG1. The

voltage drop across RG1 sets the voltage drop across

current delivered to the PA. The voltage drop across

RG1 is set by A1, A2, and the V-to-I FET, Q1. A voltage

applied to the PC input of the input buffer is divided by

four by a resistor-divider network. A2 forces its inverting

voltage across RG3. The resulting current through RG3

sets the current through RG1. This unique architecture

allows the supply current to be set independent of sup-

ply voltage. Set PA supply current according to the fol-

lowing equation:

Shutdown Mode

When SHDN is a logic-level low (SHDN < 0.4V), ampli-

fiers A1, A2, and A3 are off, Q1 is turned off, and the

output of A3 is actively pulled to ground with an N-

channel FET. Supply current is reduced to less than

1µA in shutdown mode. Typical power-up time is 0.9µs

and typical power-down time is 0.3µs, using the

MAX4473 test circuit.

Applications Information

Gain Resistor Selection

(RG1, RG2, RG3)

For proper operation, do not make the value of external

gain resistors RG1 and RG2 larger than twice the value

of RG3. In most practical applications, choose RG1

smaller than RG3 to limit the voltage drop over RG1

stantially reduces the voltage applied to the PA, thus

reducing PA output power. Set RG2 equal to RG1 to

compensate for the input bias currents of A3.

Recommended values for RG3 are between 1k

Ω and

10k

Ω.

Low-Cost, Low-Voltage, PA Power Control

Amplifier for GSM Applications in 8-Pin µMAX

4

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NAME

FUNCTION

1

SR1

Inverting Input of Error Amplifier and Drain of V-to-I FET, Q1. Connect to supply side of current-sense resis-

2

SR2

resistor RG2. Set RG2 equal to RG1.

PIN

3

SHDN

Shutdown Input. Drive SHDN low to disable all amplifiers, pull OUT to GND, set the gate-to-source voltage

mal operation.

4

PC

Power Control Input. Apply a voltage to PC to set a DC current through the sense resistor to control PA bias.

8

+2.7V to +6.5V Voltage Supply Input. Bypass to ground with a 0.1µF capacitor.

7

OUT

Output of Error Amplifier. Connect to gain control pin of power amplifier in bias control applications.

6

SR3

Inverting Input of V-to-I Converter and Source of V-to-I FET, Q1. Connect to ground through gain resistor

RG3.

5

GND

Ground

Pin Description