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Fairchild SEMICONDUCTOR RC5050 Hardware User Manual

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Application Note 50

Implementing the RC5050 and RC5051 DC-DC

Converters on Pentium

®

Pro Motherboards

www.fairchildsemi.com

Introduction

This document describes how to implement a switching volt-

age regulator using an RC5050 or 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 consider-

ations, component selection, PCB layout guidelines, and per-

formance evaluations 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.

The 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, and to 1.8V for future processors. With this

trend in mind, Raytheon Electronics has designed the

RC5050 and RC5051 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 5-bit DAC in the

RC5050 and 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 Pro Processor Power

Requirements

Refer to 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 more recent processors. Please contact

Intel Corporation for specific details.

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 Raytheon’s RC5050

and RC5051 DC-DC converters are listed in Table 1.

Table 1. Input Voltage Requirements

Pentium Pro DC Power Requirements

Refer to Table 2, Intel Pentium Pro and OverDrive

®

Proces-

sor Power Specifications. For a motherboard designs without

a standard VRM (Voltage Regulator Module) socket, the

on-board DC-DC converter must supply a minimum of

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

Flexible Motherboard design, the on-board DC-DC con-

verter must supply 14.5A maximum I

CC

P.

DC Voltage Regulation

As indicated in Table 2, the voltage level supplied to the

CPU must be within ±5% 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 +10°C to +50°C

at full load with a maximum rate of change of 5°C per 10

minutes minimum but no more than 10°C per hour)

• Output load transient with:

Slew rate >30A/µs at converter pins

Range: 0.3A - I

CC

P Max (as defined in Table 2).

Part # Vcc for IC

MOSFET

Drain

MOSFET

Gate Bias

RC5050

RC5051

+5V ±5% +5V ±5% 12V ±5% or

+5V ±5%

Rev. 1.1.0