LTC1758-1 データシートの表示（PDF） - Linear Technology

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LTC1758-1

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

Linear Technology 

LTC1758-1/LTC1758-2

APPLICATIONS INFORMATION

Forward

The LTC1758 has a wider dynamic range than the

LTC1757A. The Schottky diode detector dynamic range

has been extended to over 40dB. The start voltage accu-

racy has been improved to ±17%. The autozero hold time

has been increased for applications requiring transmit

times of several hundred milliseconds. The PCTL input

filter bandwidth has been reduced to 350kHz for improved

rejection of DAC noise as well as smoother ramp shaping.

The bandwidth has been reduced to 250kHz to control

slow turn-on RF power amplifiers.

Control Amplifier

The control amplifier supplies the power control voltage to

the RF power amplifier. A portion (typically – 19dB for low

frequencies and –14dB for high frequencies) of the RF

output voltage is sampled, via a directional coupler, to

close the gain control loop. When a DAC voltage is applied

to PCTL, the amplifier quickly servos VPCA or VPCB positive

until the detected feedback voltage applied to the RF pin

matches the voltage at PCTL. This feedback loop provides

accurate RF power control. VPCA or VPCB are capable of

driving a 5.5mA load current and 100pF load capacitor.

Operation

The LTC1758-2 dual band RF power control amplifier

integrates several functions to provide RF power control

over frequencies ranging from 850MHz to 2GHz. The

device also prevents damage to the RF power amplifier

due to overvoltage or overcurrent conditions. These func-

tions include an internally compensated power control,

amplifier to control the RF output power, an autozero

section to cancel internal and external voltage offsets, a

sense amplifier with an internal sense resistor to limit the

maximum RF power amplifier current, an RF Schottky

diode peak detector and amplifier to convert the RF feed-

back signal to DC, a VPCA/B overvoltage clamp, gain

compression, a bandgap reference, a thermal shutdown

circuit and a multiplexer to switch the control amplifier

output to either VPCA or VPCB.

Band Selection

The LTC1758-2 is designed for dual band operation. The

BSEL pin will select output VPCA when low and output

VPCB when high. For example, VPCA could be used to drive

a 900MHz channel and VPCB a 1.8GHz/1.9GHz channel.

BSEL must be established before the part is enabled. The

LTC1758-1 can be used to drive a single RF channel or

dual channel with integral multiplexer.

RF Detector

The internal RF Schottky diode peak detector and ampli-

fier converts the RF feedback voltage from the directional

coupler to a low frequency voltage. This voltage is com-

pared to the DAC voltage at the PCTL pin by the control

amplifier to close the RF power control loop. The RF pin

input resistance is typically 200Ω and the frequency

range of this pin is 850MHz to 2000MHz. The detector

demonstrates excellent efficiency and linearity over a

wide range of input power. The Schottky detector is biased

at about 60µA and drives an on-chip peak detector capaci-

tor of 28pF.

Autozero

An autozero system is included to improve power pro-

gramming accuracy over temperature. This section can-

cels internal offsets associated with the Schottky diode

detector and control amplifier. External offsets associated

with the DAC driving the PCTL pin are also cancelled.

Offset drift due to temperature is cancelled between each

burst. The maximum offset allowed at the DAC output is

limited to 400mV. Autozeroing is performed when the

part is in autozero mode (SHDN = high, TXEN = low).

When the part is enabled (TXEN = high, SHDN = high) the

autozero capacitors are held and the VPCA or VPCB pin is

connected to the control amplifier output. The hold droop

voltage of typically < 1µV/ms provides for accurate offset

cancellation over the normal 1/8 duty cycle associated

with the GSM protocol as well as with multislot protocols.

The part must be in the autozero mode for at least 50µs for

autozero to settle to the correct value.

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