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3433EFE

High Voltage Step-Up/Step-Down DC/DC Converter

Linear Technology 

LT3433

APPLICATIO S I FOR ATIO

Antislope Compensation

Most current mode switching controllers use slope com-

pensation to prevent current mode instability. The LT3433

is no exception. A slope compensation circuit imposes an

artificial ramp on the sensed current to increase the rising

slope as duty cycle increases. Unfortunately, this addi-

tional ramp corrupts the sensed current value, reducing

the achievable current limit value by the same amount as

the added ramp represents. As such, current limit is

typically reduced as duty cycles increase.

The LT3433 contains circuitry to eliminate the current limit

reduction associated with slope-compensation, or anti-

slope compensation. As the slope compensation ramp is

added to the sensed current, a similar ramp is added to the

current limit threshold reference. The end result is that

current limit is not compromised so the LT3433 can

provide full power regardless of required duty cycle.

Mode Switching

The LT3433 switches between buck and buck/boost modes

of operation automatically. While in buck mode, if the

converter input voltage becomes close enough to the

output voltage to require a duty cycle greater than 75%,

the LT3433 enables a second switch which pulls the

output side of the inductor to ground during the switch-on

time. This “bridged” switching configuration allows volt-

age conversion to continue when VIN approaches or is less

than VOUT.

When the converter input voltage falls to where the duty

cycle required for continuous buck operation is greater

than 75%, the LT3433 enables its ground-referred switch,

changing the converter operation to a dual-switch bridged

configuration. Because the voltage available across the

switched inductor is greater while bridged, operational

duty cycle will decrease. Voltage drops associated with

external diodes and loss terms are estimated internally so

that required operating duty cycle can be calculated re-

gardless of specific operating voltages.

In the simplest terms, a buck DC/DC converter switches

the VIN side of the inductor, while a boost converter

switches the VOUT side of the inductor. The LT3433

bridged topology merges the elements of buck and boost

topologies, providing switches on both sides of the induc-

tor. Operating both switches simultaneously achieves

both step-up and step-down functionality.

Step-Down (VIN > VOUT)

VIN SW

L

VOUT

CIN

D

COUT

Step-Up (VIN < VOUT)

VIN

L

D

VOUT

CIN

SW

COUT

Step-Up/Step-Down (VIN > VOUT or VIN < VOUT)

VIN SW

L

D

VOUT

CIN

D

SW

COUT

3433 F01

Maximum duty cycle capability (DCMAX) gates the dropout

capabilities of a buck converter. As VIN – VOUT is reduced,

the required duty cycle increases until DCMAX is reached,

beyond which the converter loses regulation. With a

second switch bridging the switched inductor between VIN

and ground, the entire input voltage is imposed across the

inductor during the switch-on time, which subsequently

reduces the duty cycle required to maintain regulation.

Using this topology, regulation is maintained as VIN ap-

proaches or drops below VOUT.

Inductor Selection

The primary criterion for inductor value selection in LT3433

applications is the ripple current created in that inductor.

Design considerations for ripple current are converter

output capabilities in bridged mode, output voltage ripple

and the ability of the internal slope compensation wave-

form to prevent current mode instability.

3433f

9

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