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LTC1340

Overview

The LTC1340 is a monolithic IC that combines a charge

pump and a low noise amplifier to provide a 0V to 5V swing

to drive a varactor diode-based PLL system from a single

3V supply. Traditional PLL frequency synthesizers used in

cellular phones and other portable RF systems use varactor

diodes as the voltage variable element in the VCO. Typical

varactor diodes require at least 4V of control voltage swing

to obtain their full range of capacitance adjustment. Newer

battery-powered systems, operating from low voltage

power supplies, have trouble providing this bias voltage

without an additional step-up circuit.

The LTC1340 design provides a 5V signal swing suitable

for biasing such a varactor diode when powered from a 3V

or higher voltage supply. The internal op amp and feed-

back network with built-in offset provide a gain of 2.3 so

that a 0.35V to 2.5V swing at the noninverting input

provides a 0V to 5V swing at the output. The onboard

charge pump provides the boosted voltage necessary to

drive the varactor and requires only a single 0.1

µF output

filter capacitor to complete the boost circuit. The amplifier

requires one capacitor (typically 1nF) at its output to set

amplifier noise bandwidth and to ensure amplifier stabil-

ity. The performance characteristics of the LTC1340 are

designed to meet the requirements of GSM and similar

cellular phone transceivers without requiring additional

circuitry. The LTC1340’s high level of functional integra-

tion allows it to replace several power supply and regulator

components in a typical PLL synthesizer. This results in

significant space and complexity savings.

Charge Pump

The LTC1340 features a self-contained doubling charge

pump with internal flying capacitors. The charge pump

refreshes the output on each phase of the internal 2MHz

clock, giving an effective 4MHz switching frequency. An

external 0.1

µF capacitor at the CP pin acts as a charge

reservoir and provides filtering to minimize clock

feedthrough to the amplifier section. The CP pin can be

In addition, it can be filtered with an RC or LC network prior

action between the charge pump and the amplifier through

careful internal shielding.

Amplifier

chip feedback network to amplify the input signal to the

gained output level. The amplifier requires an external

capacitor from its output to AGND to provide closed-loop

stability, noise bandwidth limiting and to further reduce

charge pump feedthrough. The – 3dB signal bandwidth of

the amplifier is given by the following equation:

LTC1340

PGND

CP

SHDN

DOUBLER

CHARGE

PUMP WITH

INTERNAL

FLYING

CAPACITOR

IN

50

Ω

PGND

1340 BD

62.3pF

1.15M

0.62V

OUT

±20µA

AGND

0.1

µF

(EXTERNAL)

0.1

µF

(EXTERNAL)

1.5M

47.9pF

–

+