AN-53 Datasheet, PDF (1/20 Pages) Fairchild Semiconductor – Implementing an RC5051 DC-DC Converter on Pentium II Motherboards

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Application Note 53
Implementing an RC5051 DC-DC Converter on
Pentium^® II Motherboards
Introduction
This document describes how to implement a switching volt-
age regulator using an RC5051 high speed controller, a
power inductor, a Schottky diode, appropriate capacitors,
and external power MOSFETs. This regulator forms a step
down DC-DC converter that can deliver up to 14.5A of
continuous load current at voltages ranging from 1.3V to
3.5V. A specific application circuit, design considerations,
component selection, PCB layout guidelines, and per-
formance evaluations are covered in detail.
In the past 10 years, microprocessors have evolved at such a
rate that a modern chip can rival the computing power of a
mainframe computer. Such evolution has been possible
because of the increasing numbers of transistors that proces-
sors integrate. Pentium CPUs, for example, integrate well
over 5 million transistors on a single piece of silicon.
To integrate so many transistors on a piece of silicon, their
physical geometry has been reduced to the sub-micron level.
As a result of each geometry reduction, the corresponding
operational voltage for each transistor has also been reduced.
This changing CPU voltage demands the design of a pro-
grammable power supply–a design that is not completely
re-engineered with every change in CPU voltage.
The voltage range of the CPU has shown a downwards trend
for the past 5 years: from 3.3V for the Pentium, to 3.1V for
the Pentium Pro, to 2.8V for the Klamath, and to 2.0V for
the Deschutes processors. With this trend in mind, Fairchild
Semiconductor has designed the RC5051 controller. This
controller integrates the necessary programmability to
address the changing power supply requirements of lower
voltage CPUs.
Previous generations of DC-DC converter controllers were
designed with fixed output voltages adjustable only by
means of a set of external resistors. In a high volume produc-
tion environment (such as with personal computers), how-
ever, a CPU voltage change would require a CPU board re-
design to accommodate the new voltage requirement. The
5-bit DAC in the RC5051 reads the voltage ID code that is
programmed into modern processors and provides the appro-
priate CPU voltage. In this manner, the PC board does not
have to be re-designed each time the CPU voltage changes.
The CPU can thus automatically configure its own required
supply voltage.
Intel Pentium II Processor Power
Requirements
Refer to Intel’s AP-587 Application Note, Slot 1 Processor
Power Distribution Guidelines, May 1997 (order number
243332-001), as a basic reference.
Input Voltages
Available inputs are +12V ±5% and +5V ±5%. Either one or
both of these inputs can be used by the DC-DC converter.
The input voltage requirements for Fairchild’s RC5051 DC-
DC converter are listed in Table 1. See below for detailed
information on how to apply these.
Table 1. Input Voltage Requirements
Part # Vcc for IC
RC5051 +5V ±5%
MOSFET
Drain
+5V ±5% or 12V ±5%
Pentium II DC Power Requirements
Refer to Table 2, Intel Pentium II Processor Power Specifica-
tions. For standard motherboard designs , the on-board DC-DC
converter must supply a minimum of 14.2A at 2.8V for Klamath
or 2.0V for Deschutes. For a Deschutes Flexible Motherboard
design, the on-board DC-DC converter must supply 18.9A.
DC Voltage Regulation
As indicated in Table 2, the voltage level supplied to the
CPU must be within ±3% of its nominal setting. Voltage reg-
ulation limits must include:
• Output load ranges specified in Table 2
• Output ripple/noise
• DC output initial voltage set point
• Temperature and warm up drift (Ambient +0°C to +70°C
at full load with a maximum rate of change of 5°C per 10
minutes but no more than 10°C per hour)
• Output load transient with:
• Slew rate >30A/ms at converter pins Range:
0.3A - ICCP Max (as defined in Table 2).
Pentium is a registered trademark of Intel Corporation.
Rev. 1.0.0