ACPM-7891 데이터 시트보기 (PDF) - HP => Agilent Technologies

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ACPM-7891

Tri-Band Power Amplifier Module EGSM, DCS and PCS Multi-slot GPRS

HP => Agilent Technologies 

Appendix C

LTC1758 Theory of Operation

The LTC1758-2 is a dual band RF

power controller for RF power

amplifiers operating in the

850 MHz to 2 GHz range.

RF power is controlled by driv-

ing the RF amplifier power

control pins and sensing the

resultant RF output power via a

directional coupler. The RF

sense voltage is peak detected

using an on-chip Schottky diode.

This detected voltage is com-

pared to the DAC voltage at the

PCTL pin to control the output

power. The RF power amplifier

is protected against high supply

current and high power control

pin voltages. Internal and exter-

nal offsets are cancelled over

temperature by an autozero con-

trol loop, allowing accurate low

power programming. The shut-

down feature disables the part

and reduces the supply current

to <1_A.

Modes of Operation

The LTC1758-2 supports three

operating modes: shutdown,

autozero and enable.

In shutdown mode (SHDN =

Low) the part is disabled and

supply currents will be reduced

to <1_A. VPCA and VPCB will be

connected to ground via 100_

switches.

In autozero mode (SHDN = High,

TXEN = Low) VPCA and VPCB

will remain connected to ground

and the part will be in the

autozero mode. The part must

remain in autozero for at least

50_s to allow for the autozero

circuit to settle.

In enable mode (SHDN = High,

TXEN = High) the control loop

and protection functions will be

operational. When TXEN is

switched high, acquisition will

begin. The control amplifier will

start to ramp the control voltage

to the RF power amplifier. The

RF amplifier will then start to

turn on. The feedback signal

from the directional coupler and

the output power will be

detected by the LTC1758-2 at the

RF pin. The loop closes and the

amplifier output tracks the DAC

voltage ramping at PCTL. The RF

power output will then follow

the programmed power profile

from the DAC.

The LTC1758 datasheet provides

more detailed description of the

part’s operation and can be

downloaded from Linear

Technology’s website.

TOP VIEW

VIN 1

RF 2

SHDN 3

BSEL 4

GND 5

10 VCC

9 VPCA

8 VPCB

7 TXEN

6 PCTL

MS10 Package

10-Lead Plastic MSOP

Figure C1. LTC-1758-2 Pinout.

MODE

Shutdown

Autozero

Enable

SHDN

Low

High

High

TXEN

Low

Low

High

OPERATION

Disabled

Autozero

Power Control

Shutdown Autozero

SHDN

BSEL

Enable

t1

TXEN

tS

PCTL

VPCA

Start

VPCB

Voltage

t2

Note 1

Start

Voltage

tS: autozero settling time, 50µs minimum

t1: BSEL change prior to TXEN, 200ns typical

t2: BSEL change after TXEN, 200ns typical

Note 1: The external DAC driving the PCTL pin can be enabled during autozero. The autozero system

will cancel the DAC transient. the DAC must be settled to an offset ≥ 400mv before TXEN

is asserted high.

Figure C2. LTC1758-2 Timing Diagram.

22

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