Data Sheet

LT8349

analog.com

Rev B

13 of 29

THEORY OF OPERATION

The LT8349 is a dual-phase, adjustable frequency synchronous boost converter. Referring to the Block Diagram, the

LT8349 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. The

Switching Logic and Gate Drivers block turns on channel 1’s power switch M1 at the start of each CLK1 cycle and

turns on channel 2’s power switch M3 at the start of each CLK2 cycle. CLK1 and CLK2 are 180° out-of-phase, generated

by the Oscillator. During the M1 switch on-phase, the inductor current IL1flows through M1. A signal ISEN1 proportional

to the M1 switch current is added to a stabilizing slope compensation ramp RAMP1 and the resulting sum is fed into

the positive terminal of the PWM comparator ICOMP1. The voltage at the negative input of ICOMP1, labeled “VC”, is

set by the error amplifier EA and is an amplified version of the difference between the feedback voltage FB and the

reference voltage (1V). During the M1 on-phase, IL1 increases. When the signal at the positive input of ICOMP1 exceeds

VC, ICOMP1 sends out a signal ITRIP1 to the Switching Logic and Gate Drivers block to turn off M1. When M1 turns off,

the synchronous power switch M2 turns on until the next CLK1 cycle begins or inductor current IL1 falls to zero (in

Burst Mode operation only). During the M1 off-phase and the M2 on-phase, IL1decreases. Channel 2 operation follows

Channel 1. IL1 and IL2 are designed to match each other (except they are 180° out-of-phase). Through this repetitive

action, the EA sets the correct IL1 and IL2 peak current level to keep the VOUTin regulation.

Multiphase Operation

The LT8349 uses a dual-phase architecture with two phases equally spaced 180° apart, rather than the conventional

single phase of other boost converters. Although this architecture requires two inductors, rather than a single

inductor, there are several important advantages.



Substantially lower peak inductor current allows the use of smaller inductors.



Significantly reduced output ripple current minimizes output capacitance requirement.



Higher frequency output ripple is easier to filter for low noise applications.



Input ripple current is reduced for lower noise on VIN.

The peak inductor current, reduced nearly by a factor of 2 compared to a single-phase boost converter, is given by:

ILPEAK ≅

1

2

•

IO

1 − D

+

∆IL

2

where IO is the average load current, D is the PWM duty cycle, and ΔIL is the inductor ripple current. With 2-phase

operation, one of the phases is always delivering current to the load whenever VIN is greater than one-half VOUT (duty

cycles less than 50%). As the duty cycle decreases further, load current delivery between the two phases begins to

overlap, occurring simultaneously for a growing portion of each phase as the duty cycle approaches zero. This

significantly reduces both the output ripple current and the peak current in each inductor, when compared with a

single-phase converter. The comparison of output ripple current with single phase and dual phase boost converter

operating at 50% duty cycle is shown graphically in Figure 23.