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Описание 3-Phase Synchronous PWM Controller IC Provides an Integrated Solution for Intel VRM 9.0 Design Guidelines
Производители International Rectifier
логотип International Rectifier логотип 



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3PHASEPWM Даташит, Описание, Даташиты
3-Phase Synchronous PWM Controller IC Provides an
Integrated Solution for Intel VRM 9.0 Design Guidelines
Odile Ronat
International Rectifier
The fundamental reason for the rapid change and
growth in information technology power
requirements is the increase in volume of data
processed, stored, transmitted, and displayed. As
a result, power requirements have grown very
rapidly. To control the increase in power
dissipation, operating voltages have been
reduced. Power scales as the square of the
voltage: cut the voltage in half, and the power
dissipation is reduced to one fourth. As circuit
output voltages approach the one-volt mark,
operating currents are increasing exponentially.
Today, feeding this much “ultraclean” current at
these low voltages with tremendous transient
response capability is the key technology driver
of power management for IT.
CPUs used in servers, advance desktop
computers and motherboards such as the Intel
Pentium™ 4 or Xeon™ or AMD K7™ typically
require currents up to 60A at output voltages
around 1V, with transient response in the order
of 50A/us. Designers prefer a multiphase DC-
DC converter topology to address the higher
current requirements and faster transient
response problem,
This article will discuss the specific
requirements for DC-DC converters required to
power today’s GHz class CPUs, the benefits of
the multiphase topology, and an example of
implementation of Intel VRM 9.0 using an
integrated 3-phase synchronous PWM controller
IC.
An Overview of Intel VRM 9.0 Design
Guidelines
With today’s microprocessor voltage below 2V,
they cannot be powered directly out of the
conventional silver box that typically provides
5V and 12V. Located in proximity to the CPU,
the voltage regulator module (VRM) delivers
regulated, stepped down voltage to the CPU.
The Intel VRM 9.0 definition is specifically
intended to meet the needs of systems based on
the Intel ® Xeon™ and Pentium ® 4 processors.
Table 1 provides a summary of the output
requirements for VRM 9.0.
Table 1 - Summary of the VRM 9.0 output requirements (Source: Intel VRM9.0 DC-DC converter design guidelines.
Other requirements include VID voltage
identification and Power Good feedback. The
VRM must accept five lines to set the nominal
maximum voltage. Five processor package pins
will have a high-low pattern corresponding to
the voltage required by the individual processor.
The VRM should provide an open collector
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3PHASEPWM Даташит, Описание, Даташиты
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).
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3PHASEPWM Даташит, Описание, Даташиты
Figure 1 – Typical application of IRU3055
The IRU3055 has on-board MOSFET drivers,
making it cost-effective compared to the
majority of multiphase controllers, which
require external MOSFET drivers. When used
with IR’s HEXFET® power MOSFETs like the
IRF3704S in the control FET socket and the
IRF3711S in the synchronous FET socket, the
IRU3055 can provide up to 60A for
microprocessor VCORE requirements at 80%
efficiency or above.
The new controller IC is a fixed-frequency
voltage mode controller with 50kHz to 500kHz
programmable operating frequency by using an
external resistor. The output voltage is selectable
in 25mV steps from 1.075 to 1.85V with up to
1.5% output voltage accuracy using a
programmable VID code to the five-bit digital-
to-analog converter (DAC).
The new device has a number of features to
provide protection and monitoring for the power
system and the microprocessor. It features
under-voltage lockout (UVLO) for the 5V and
12V supplies as well as an externally
programmable soft-start function. Another
feature is the PGOOD monitor circuit, which is
held low if the output of the converter is not
within ±10% of the programmed voltage.
The IRU3055 also provides over-voltage
protection. If the output voltage exceeds the
DAC-programmed voltage by 20%, the low-side
MOSFETs turn on and shunt the output voltage
to ground, protecting the CPU from damage.
The IRU3055 features loss-less inductor current
sensing and sharing while over-current
protection is achieved by synchronous FET
RDS(on) sensing, eliminating the need for a
separate sense resistor. The IRU3055 can also be
easily configured for voltage droop
compensation so that the output voltage will
linearly decrease as load current increases and
vice versa, as required by Intel VRM design
guidelines.
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