Power supply solutions for changing CPU needs

Electronic News, August 12, 1996 by Stephen Bryson, George Hill, James MacDonald, David McIntyre

The market demand for increasing computer performance has set new requirements for today's PC power supplies. Two years ago, the vast majority of PC CPUs required 5V linear supplies. Since then, targets for analog designers of power supply components have been moving at a tremendous rate as voltage requirements decrease.

Supply requirements for CPUs have steadily decreased, from 5V for Intel 80486 microprocessors, to 3.3V for Pentium P54C, to below 3V for Pentium P55C and Pentium Pro. In addition, Pentium Pro CPUs actually require three unique supply voltages: 3.3V for the I/O chipset and memory, between 2.1V and 3.5V of programmable supply for the processor core, and 1.5V for an active I/O bus.

Furthermore, the core supply needs to provide up to 12.4 amperes (A) in order to support processor speeds up to 200MHz, approximately four times the requirement of a state-of-the-art 486 processor of two years ago. This article provides insight for coping with the rapid change in power supply requirements and addresses several methods of implementing integrated power solutions for computers using P54/P55 and Pentium Pro CPUs.

Power supply solutions for the P54 and P55

Intel recently introduced the Pentium P55C multimedia processor that will be released by the end of 1996. System and motherboard manufacturers have been forced to redesign their power supply solutions to address the unique voltage requirements of this new processor. The process technology of the P55C requires a 2.8V CPU core voltage supply in addition to the 3.3V supply requirement inherited from the P54C. To prepare for this new processor, system designers must find a mechanism that addresses the additional voltage supply requirements. PC manufacturers, including Compaq, IBM and Dell, build motherboards to use in their systems. Thus, they can make the necessary system changes to their power supply designs to meet the P55C requirements.

Some of these PC vendors are now specifying a new ATX computer supply known as the Silver Box. This power supply includes the traditional 5V and 12V outputs plus a 3.3V or 3.5V supply to eliminate the need for a dedicated on-board voltage regulator. These motherboards can then be modified to include an additional voltage regulator to convert the P54C supply to the 2.8V required by the P55C CPU core.

Motherboard manufacturers who target their products toward open-ended systems are concerned with backward and forward compatibility and cannot assume the presence of the ATX Silver Box. To facilitate the transition from the P54 to the P55, Intel has provided motherboard manufacturers with a description of the requirements for a flexible motherboard that will accept both P54 and P55 processors.

To implement this flexible motherboard, the power plane under the CPU socket is split into two sections: one for the CPU core and one for the I/O ring. For a P54 processor, the two power planes must be connected to allow even power distribution of the 3.5V to all CPU power pins. For a P55C, one of the power planes provides 2.8V for the CPU core and the other plane provides 3.3V for the I/O ring, chipset and memory.

A method for implementing a power supply solution on the motherboard that covers both of these cases is using Raytheon Electronics' RC5036 Dual Voltage Regulator. This product integrates a DC/DC converter, a linear regulator and logic switches to toggle the DC/DC converter output between 3.5V and 2.8V and to disable the linear regulator output. Using a single logic input-VCC2DET-from the CPU itself, the RC5036 can configure both outputs to match the voltage requirements of both P54 and P55 processors.

A typical flexible Pentium motherboard requires jumpers to configure the power supply voltages, but the RC5036 can be configured to automatically detect the status on the VCC2DET pin and adjust both outputs without external jumpers. Figure 1 illustrates the RC5036 as used in a typical P54/P55 flexible motherboard. When the VCC2DET pin is at logic HIGH, only the switching regulator is enabled to provide 3.5V for the P54 CPU. When a P55C is inserted in the same CPU socket, the VCC2DET pin transitions to logic LOW to switch the DC/DC converter's output to 2.8V and enable the linear regulator to supply the 3.3V required by the I/O ring, chipset and memory. In addition, to support Intel's ever-changing requirements, the RC5036 output voltages can be adjusted from 1.5V to 3.5V using two external resistors.

Power supply solutions for Pentium Pro

During the developing stages of the Pentium Pro processors, and to assist system developers, Intel provided detailed specifications for the P6 power supply. Such specifications outlined four CPU pins that output a distinct voltage identification (VID) code that specifies the voltage required by the CPU. The power supply must then adjust its output to match this voltage requirement.

Earlier DC/DC controllers used external resistors to preset the power supply's output voltage, but now Raytheon Electronics' RC5040 uses an integrated 4-bit digital-to-analog converter (DAC) to read the VID code directly and program the supply's output voltage accordingly. This approach eliminates the requirement of the designer to change the motherboard design each time the CPU voltage changes. Another concern faced by Pentium Pro system designers using the latest CPUs is transient response. In normal operation, the CPU may have to quickly switch from sleep mode to operations that use load currents in excess of 13A. During such transitions, the voltage provided to the CPU must remain within set limits, forcing the designer to compensate by using bulk capacitors to provide instantaneous current.