Fairchild RC5040 specifications Introduction, Input Voltages, Pentium Pro DC Power Requirements

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Application Note 42

Implementing the RC5040 and RC5042

DC-DC Converters on Pentium® Pro Motherboards

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.

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.

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.

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

	Controller	MOSFET	MOSFET
Part #	VCC	Drain	Gate Bias
RC5040	+5V ±5%	+5V ±5%	+5V ±5% or
RC5042			12V ±5%
RC5043	+5V ±5%	12V ±5%	12V ±5%

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 ICCP 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 ICCP.

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/∝s at the converter pins

Range: 0.3A – ICCP Max (as defined in Table 2).

Rev. 1.1.0