3PHASEPWM datasheet(2/5 Pages) IRF

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Power Good signal consistent with TTL DC

levels.

Multiphase Synchronous Buck Converters

Multiphase topologies are a necessary approach

achieve

the

power

needs

today’s

microprocessors

with

high

efficiency

and

without large, expensive and bulky magnetics

and capacitors. Multiple buck converters are

usually connected in parallel to reduce the power

capability for each individual converter as well

as alleviate the thermal stress on each of the

power devices. Each phase-leg carries 1/N of the

total current supplied, where N is the number of

phases. Each leg operates out-of-phase with all

others allowing the sum of the legs to add up to

a regulated DC level with significantly less

ripple and faster transient response capability

than a single -phase converter. This is achieved

without increasing the switching frequency per

leg since the effective output frequency of a

multiphase

converter

the

times

the

frequency per phase.

A multiphase topology also allows the use of

smaller input and output filters since ripple

currents cancellation. It also produces faster

transient response due to multiple inductors in

parallel. Multiphase converters generally result

in lower cost, smaller footprint or lower profile

due to smaller inductors and capacitors. Finally,

multiphase converters are comparatively more

efficient

than

single -phase

converter

equivalent output current ripple frequency and

output current level. The reduced power loss

from the ESR of the input capacitor and the low

switching losses of the MOSFETs at the

relatively

low

switching

frequencies

helps

achieve high conversion efficiency and provides

even heat distribution.

The designer always faces the dilemma of

choosing the right amount of phases for his

application. More phases operating at a lower

switching

frequency

saves

converter

input/output capacitor cost without reducing

efficiency, but also increases complexity, layout

difficulty, and at some point total solution cost.

With current MOSFET technology, the ideal

current-per-phase ranges from 10 to 30 Amps.

Designs

operating

lower

switching

frequencies, using state-of-the-art MOSFETs,

and having low thermal impedance, such as

using heat sinks, tend to be in the upper end of

this current range. Designs targeting compact

size, minimal input and output capacitors, and

maximum efficiency tend to be in the lower end

of this current range.

One way to determine the optimal number of

phases is by the number of MOSFETs required

to handle the per-phase current. If it is necessary

to use 2 or more MOSFETs for both high and

low side, consider adding an additional phase.

The cost and size of the additional phase is

compensated for by the reduction in input and

output capacitors. Design goals such as current

ripple and transient response will provide further

selection criteria for the determination of the

right number of phases for the application.

Implementation

VRM9.0

compatible

circuit using IRU3055 3-phase multiphase

controller ICs

With power MOSFETs able to efficiently and

cost-effectively deliver 20A per phase, 3-phase

has emerged as the preferred number of phases

to deliver 60A while meeting the design

guidelines of Intel VRM 9.0. The IRU3055 is a

five-bit programmable, three-phase synchronous

PWM controller IC with integrated MOSFET

drivers

that

enables

straight

forward

implementation of an efficient 3 phase converter

delivering 60A at voltage as low as 1.075V (Fig

1).

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