Fairchild SEMICONDUCTOR RC5040 Hardware User Manual

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

CC

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

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 

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 (Ambient +10

°

C to +60

°

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/

µ

s at the converter pins

Range: 0.3A – I

CC

P Max

(as defined in Table 2).

Part #

Controller

V

CC

MOSFET

Drain

MOSFET

Gate Bias

RC5040

RC5042

+5V 

±

5%

+5V 

±

5%

+5V 

±

5% or

12V 

±

5%

RC5043

+5V 

±

5%

12V 

±

5%

12V 

±

5%

Rev. 1.1.0