3PHASEPWM Datasheet, PDF (2/5 Pages) International Rectifier – 3-Phase Synchronous PWM Controller IC Provides an Integrated Solution for Intel VRM 9.0 Design Guidelines

Power Good signal consistent with TTL DC
levels.
Multiphase Synchronous Buck Converters
Multiphase topologies are a necessary approach
to achieve the power needs of 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 is the N 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 at
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 on 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 at 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 of 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 a straight forward
implementation of an efficient 3 phase converter
delivering 60A at voltage as low as 1.075V (Fig
1).
2