LTC1758-2 データシートの表示（PDF） - Linear Technology

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LTC1758-2

RF Power Controllers with 250kHz Control Loop Bandwidth and 40dB Dynamic Range

Linear Technology 

LTC1758-1/LTC1758-2

APPLICATIONS INFORMATION

Filter

There is a 350kHz single pole filter included in the PCTL

path.

Protection Features

The RF power amplifier is overcurrent protected by an

internal sense amplifier. The sense amplifier measures the

voltage across an internal 0.090Ω resistor to determine

the RF power amplifier current. VPCA or VPCB is lowered as

this supply current exceeds 2.2A, thereby regulating the

current to about 2.25A. The regulated current limit is

temperature compensated. The 0.090Ω resistor and the

current limit feature can be removed by connecting the PA

directly to VIN.

The RF power amplifier control voltage pins are overvolt-

age protected. The VPC overvoltage clamp regulates VPCA

or VPCB to 2.85V when the gain and PCTL input combina-

tion attempts to exceed this voltage.

The internal thermal shutdown circuit will disable the

LTC1758-2 if the junction temperature exceeds approxi-

mately 150°C. The part will be enabled when the tempera-

ture falls below 140°C.

Modes of Operation

The LTC1758-2 supports three operating modes: shut-

down, autozero and enable.

In shutdown mode (SHDN = Low) the part is disabled and

supply currents will be reduced to <1µA. VPCA and VPCB

will be connected to ground via 100Ω 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 50µs 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.

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

VPCB

START

VOLTAGE

LTC1758-2 Timing Diagram

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

1758 TD

≤ 400mV BEFORE TXEN IS ASSERTED HIGH.

7

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