LTC3124 datasheet(13/36 Pages) AD

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LT3120

Rev. 0

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OPERATION

INTRODUCTION

The LT3120 is a monolithic buck-boost DC/DC converter

that can operate over 2.5V to 26V input voltage and 0.8V

to 24V output voltage range. Integrated low RDS(ON)

N-channel DMOS power switches reduce solution com-

plexity and maximize conversion efficiency. Internal high

side power switch drivers require only two small external

capacitors and further simplify application circuit design.

The LT3120 incorporates many additional features to al-

low for maximum flexibility when designing application

solutions, including an accurate EN/UVLO pin compara-

tor, wide operating frequency range of 400kHz to 2MHz,

external clock synchronization, power good indicator,

and Maximum Power Point Control (MPPC) of the input

voltage for operation from current limited sources such

as photovoltaic arrays.

High side drive and sense circuitry allow for operation to

0V on the output while maintaining current control and

synchronousswitchoperation.Operationtovoltagesbelow

2.5V on the input is also possible when bootstrapping the

VCC pin from VOUT, or other alternate sources, allowing

for maximum extraction of energy from storage devices

such as supercapacitors.

A proprietary ultralow noise PWM switching algorithm

maintains output regulation with input voltages that are

below, equal to, or above the output voltage. Transitions

between buck and boost operating modes occur seam-

lessly without transients or subharmonic switching. The

LT3120 has an internal oscillator that can be configured to

operate over a wide range of frequencies. Using a single

programming resistor, the operating frequency can be

configured between 400kHz and 2MHz allowing for flex-

ibility when optimizing for board area and efficiency. The

internal oscillator can also be synchronized to an external

clock applied to the SYNC/MODE pin in noise sensitive

applications.

Burst Mode operation allows for high efficiency operation

at light loads while automatically transitioning to PWM

mode at heavier loads. At low output currents, Burst Mode

operation is enabled, with a current of only 30µA (typical).

In shutdown the total supply current is further reduced

to 3µA (maximum).

PWM MODE OPERATION

If the SYNC/MODE pin is held high, or the load current

on the converter is high enough to command PWM mode

operation with the SYNC/MODE pin held low, the LT3120

operates in a fixed frequency PWM mode. The operating

frequency is defined by the resistance on the RT pin as

described in the Applications Information section of this

data sheet. PWM mode minimizes output voltage ripple

and yields a low noise switching frequency spectrum. A

proprietary switching algorithm provides seamless tran-

sitions between operating modes and eliminates discon-

tinuities in the average inductor current, inductor ripple

current and loop transfer function throughout all modes of

operation.Theseadvantagesresultinincreasedefficiency,

improved loop stability and lower output voltage ripple in

comparison to a traditional buck-boost converter. Figure 1

shows the topology of the LT3120 power stage which is

comprised of four N-channel DMOS switches and their

associated gate drivers.

Figure 1. Power Stage Schematic

VCC

BST1

CBST1

CBST2

BST2

PVIN

PVOUT

SW1

SW2

VCC

LT3120

GND

3120 F01

In PWM mode operation, both switch pins transition on

every cycle independent of the input and output voltages.

Theinternalaveragecurrentcontrolloopoperatessuchthat

thepulsewidthmodulatorgeneratestheappropriateswitch

duty cycle to maintain regulation of the output voltage.

Oscillator and Phase-Locked Loop

The LT3120 operates from an internal oscillator with a

switching frequency that is configured by a single external

resistor between the RT pin and GND. For noise sensitive

applications, an internal phase-locked loop allows the

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