Fairchild SEMICONDUCTOR RC5040 Hardware User Manual Download Page 1

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Application Note 42

Implementing the RC5040 and RC5042
DC-DC Converters on Pentium

Pro Motherboards

www.fairchildsemi.com

Introduction

This document describes how to implement a switching volt-
age regulator using an RC5040 or an RC5042 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
2.1V to 3.5V. A specific application circuit, design consider-
ations, component selection, PCB layout guidelines and per-
formance evaluation procedures are covered in detail.

In the past 10 years, microprocessors have evolved at such an
exponential 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
processors 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 voltage for the CPU demands the design of a
programmable power supply—a design that is not com-
pletely re-engineered with every change in CPU voltage.

The operational voltage of CPUs has shown a downwards
trend for the past 5 years: from 5V for the x386 and x486, to
3.3V for Pentium, and 3.1V for Pentium Pro. Furthermore,
emerging chip technologies may require operating voltages
as low as 2.5V. With this trend in mind, Raytheon Electron-
ics has designed the RC5040 and RC5042 controllers. These
controllers integrate 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 with a
set of external resistors. In a high volume production envi-
ronment (such as with personal computers), however, a CPU
voltage change requires a CPU board re-design to accommo-
date the new voltage requirement. The integrated 4-bit DAC
in the RC5040 and the RC5042 reads the voltage ID code
from the Pentium Pro microprocessor and configures the sys-
tem to provide the appropriate 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 voltage.

Pentium

Pro and OverDrive

Processor Power Requirements

Use Intel’s AP-523 Application Note, Pentium® Pro
Processor Power Distribution Guidelines, November 1995
(order number 242764-001), as a basic reference. The speci-
fications contained in this document have been modified
slightly from the original Intel document to include updated
specifications for Pentium Pro microprocessors. Please con-
tact Intel Corporation for specific details.

Input Voltages

Available inputs are +5V

5% and +12V

5%. Raytheon

Electronics’ DC-DC converters may use either or both
inputs. Their input voltage requirements are listed in Table 1.

Table 1. Input Voltage Requirements

Pentium Pro DC Power Requirements

Refer to Table 2 for the power supply specifications for
Pentium Pro and Overdrive Processors. For a motherboard
design without a standard Voltage Regulator Module (VRM)
socket, the on-board DC-DC converter must supply a mini-
mum I

P current of 13.9A at 2.5V and 12.4A at 3.3V. For a

flexible motherboard design, the on-board converter must be
able to supply 14.5A maximum I

DC Voltage Regulation

As indicated in Table 2, the voltage level supplied to the
CPU must be within

5% of its nominal setting. Voltage

regulation limits must include:

• Output load ranges specified in Table 2
• Output ripple/noise
• DC output initial voltage set point
• Temperature and warm up drift (A10

C to +60

at full load with a maximum rate of change of 5

C per 10

minutes minimum but no more than 10

C per hour)