LTC3129 datasheet(10/30 Pages) LINER

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LTC3129

3129fc

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operaTion

INTRODUCTION

TheLTC3129isa1.3µAquiescentcurrent,monolithic,cur-

rent mode, buck-boost DC/DC converter that can operate

overawideinputvoltagerangeof1.92Vto15Vandprovide

up to 200mA to the load. Internal, low RDS(ON) N-channel

power switches reduce solution complexity and maximize

efficiency.Aproprietaryswitchcontrolalgorithmallowsthe

buck-boostconvertertomaintainoutputvoltageregulation

with input voltages that are above, below or equal to the

output voltage. Transitions between the step-up or step-

down operating modes are seamless and free of transients

and sub-harmonic switching, making this product ideal

for noise sensitive applications. The LTC3129 operates

at a fixed nominal switching frequency of 1.2MHz, which

provides an ideal trade-off between small solution size and

high efficiency. Current mode control provides inherent

input line voltage rejection, simplified compensation and

rapid response to load transients.

Burst Mode capability is also included in the LTC3129 and

is user-selected via the PWM input pin. In Burst Mode

operation, the LTC3129 provides exceptional efficiency at

light output loading conditions by operating the converter

only when necessary to maintain voltage regulation. The

Burst Mode quiescent current is a miserly 1.3µA. At higher

loads, the LTC3129 automatically switches to fixed fre-

quencyPWMmodewhenBurstModeoperationisselected.

(Please refer to the Typical Performance Characteristics

curves for the mode transition point at different input and

output voltages.) If the application requires extremely low

noise, continuous PWM operation can also be selected

via the PWM pin.

A MPPC (maximum power point control) function is also

provided that allows the input voltage to the converter to

be servo'd to a programmable point for maximum power

when operating from various non-ideal power sources

such as photovoltaic cells. The LTC3129 also features

an accurate RUN comparator threshold with hysteresis,

allowing the buck-boost DC/DC converter to turn on and

off at user-selected VIN voltage thresholds. With a wide

voltage range, 1.3µA Burst Mode current and program-

mable RUN and MPPC pins, the LTC3129 is well suited

for many diverse applications.

PWM MODE OPERATION

IfthePWMpinishighoriftheloadcurrentontheconverter

is high enough to command PWM mode operation with

PWM low, the LTC3129 operates in a fixed 1.2MHz PWM

mode using an internally compensated average current

mode control loop. PWM mode minimizes output voltage

ripple and yields a low noise switching frequency spec-

trum.Aproprietaryswitchingalgorithmprovidesseamless

transitions between operating modes and eliminates dis-

continuitiesintheaverageinductorcurrent,inductorripple

current and loop transfer function throughout all modes of

operation.Theseadvantagesresultinincreasedefficiency,

improved loop stability and lower output voltage ripple in

comparison to the traditional buck-boost converter.

Figure 1 shows the topology of the LTC3129 power stage

which is comprised of four N-channel DMOS switches and

their associated gate drivers. In PWM mode operation

both switch pins transition on every cycle independent of

the input and output voltages. 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.

Figure 1. Power Stage Schematic

VCC

BST1

CBST1

CBST2

BST2

VIN

VOUT

SW1

SW2

VCC

LTC3129

PGND

3129 F01

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