LTC3113 datasheet(13/32 Pages) AD

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LT3154

Rev. 0

For more information www.analog.com

OPERATION

Introduction

The LT3154 is a 6A synchronous buck‑boost DC/DC con‑

verter optimized for demanding high current applications.

The low RDS(ON), low gate charge synchronous switches

provide, highly efficient, high frequency pulse width

modulation control. The LT3154 utilizes a proprietary low

noise switching algorithm, which allows its output voltage

to be regulated above, below or equal to the input voltage.

Burst Mode capability is included in the LT3154 and is

user‑selectedviatheSYNC/MODEinputpin.InBurstMode

operation, the LT3154 provides exceptional efficiency at

light output loads by operating the converter only when

necessary to maintain voltage regulation. At higher loads,

the LT3154 automatically transitions to fixed frequency

PWM operation. Continuous PWM mode can also be

selected via the SYNC/MODE pin for low noise operation.

The LT3154 features an accurate, resistor programmable

EN/UVLO comparator which allows the buck‑boost DC/

DC converter to turn on and off at user‑selected voltage

thresholds depending on the power source. Other pro‑

grammable features include the soft‑start period, oscilla‑

tor frequency and output voltage. Default values for each

feature can selected by connecting pins to VIN or GND,

reducing external component count.

Power Stage Topology

Theswitchtopologyforthebuck‑boostconverterisshown

in Figure 1. When the input voltage is significantly greater

thantheoutputvoltage,thebuck‑boostconverteroperates

in buck mode. Switch D turns on continuously and switch

Cremainsoff.SwitchesAandBarepulsewidthmodulated

to produce the required duty cycle to support the output

regulation voltage. As the input voltage decreases, switch

A remains on for a larger portion of the switching cycle.

As the input voltage approaches the output voltage, the

period of switch B will shorten and switch C will begin to

turn on to maintain regulation. The voltages where this

transition region occur will vary depending on load cur‑

rent and switching frequency. Additional inductor current

Figure 1. Buck-Boost Switch Topology

PVIN

AGATE

SW1

SW2

BGATE

DGATE

CGATE

3154 F01

PGND

PVOUT

ripple may occur as SW1 and SW2 begin to transition on/

off near the edges of their respective PWM ramps (see

blockdiagram).Astheinputvoltagedropsbelowtheoutput

voltage, switch A remains on continuously while switch

pair CD is pulse width modulated to obtain the desired

output voltage. At this point, the converter is operating

solely in boost mode.

This switching algorithm provides a smooth transition

between operating modes and reduces discontinuities

in average inductor current, inductor current ripple, and

loop transfer function throughout all three operational

modes. These advantages result in increased efficiency

and stability in comparison to the traditional four‑switch

buck‑boost converter.

PWM Mode Operation

If the SYNC/MODE pin is high or if the load current on the

converter is high enough to force PWM mode operation,

the LT3154 operates at a fixed frequency programmed by

theRTpin.PWMmodeminimizesoutputvoltagerippleand

yields a low noise switching frequency spectrum. A pro‑

prietaryswitchingalgorithmprovidesseamlesstransitions

betweenoperatingmodesandeliminatesdiscontinuitiesin

the average inductor current, inductor ripple current and

loop transfer function throughout all modes of operation.

These advantages result in increased efficiency, improved

loop stability, and lower output voltage ripple. In response

to the internal control loop command, an internal pulse

width modulator generates the appropriate switch duty

cycle to maintain regulation of the output voltage.

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