ETC 19197

Low Voltage Power Supply Circuits
Application Note
Am486® 3.3-V CPUs and Am5X86™ CPUs feature a 5-V tolerant signal interface, but require a +3.3-V
input for VCC. For cost effectiveness, AMD® recommends the use of a voltage regulator circuit to generate
the VCC input from a regulated +5-V source. This application note describes two such circuits that provide
a programmable voltage range (3.3 V–3.6 V). The note also includes filtering and routing guidelines for
incorporating these circuits into a motherboard design.
The CS-24 (0.5 micron) and CS-34 (0.35 micron)
manufacturing processes incorporate 5-V I/O tolerance. This means that 3.3-V Am486 processors
(0.5 micron) and Am5X86 processors (0.35 micron)
can accept both 3.3-V and 5-V input signals even
though they require 3.3-V input power. The input
circuit protects the microprocessor from potential
damage caused by the application of 5-V signals.
AMD has developed two circuit designs that can
generate a selectable input voltage range (3.3 V–
3.6 V) for the microprocessor VCC source. The linear
regulator circuit is the lower cost solution. The second option is a switching regulator that provides a
higher efficiency output, but at a significant cost
increase. Design requirements (higher efficiency
vs. lower cost) determine which power solution to
use in a specific system design.
Figure 1
A second basic issue in power supply circuit design
is noise reduction. Excessive noise levels can impair proper operation and contribute to possible
radio frequency transmissions that can affect compliance with national and international agency regulations. This application note provides some basic
guidelines for reducing noise generated by or
transmitted through the power supply circuits.
The lower cost linear regulator power circuit is
shown in Figure 1. The circuit provides an output
of +5 V or 3.3 V–3.6 V depending on the state of
the VOLDET input from the microprocessor. On
AMD processors manufactured using the CS-24 or
CS-34 process, the VOLDET signal is set to the
VSS level used by the microprocessor. The actual
output range produced by the regulator circuit depends on the ratio of the values of R1 and R2. Table
1 shows the recommended resistor sizes used to
produce specific voltage outputs (VOUT).
Variable Linear Regulator Microprocessor Power Supply Circuit
Table 1. R1 and R2
Resistor Values
3.3 V
3.45 V
3.6 V
124 Ω
124 Ω
124 Ω
205 Ω
218 Ω
234 Ω
Note: All resistors are 1%
This document contains information on a product under development at Advanced Micro Devices. The information is
intended to help you evaluate this product. AMD reserves the right to change or discontinue work on this proposed
product without notice.
Document #: 19197 Rev. D Amendment/0
Issue Date: August 1995