
Processor Power Requirement
7
21103G/0—February 1999
AMD-K6
 Processor Power Supply Design
either design. With this approach, a simple bill of material
change is all that is necessary to upgrade to a higher-power
processor. 
One of the key motherboard components is the power
transistor. The transistor can be replaced with one that has a
lower R
DS(ON)
 (resistance-drain-to-source when the transistor is
on) or two transistors can be paralleled. Another important
component is the output inductor. Because an inductor that
carries 15A is physically larger than an inductor that carries
10A, the layout must allow sufficient space. Finally, a provision
should be made to add extra decoupling capacitors. The
calculations in the examples starting on page 12 show how
many decoupling capacitors are needed for various cases.
Many of the components are common, including the
regulator/controller IC and the basic circuitry. Typically,
switching transistors and the output inductor need to change.
The output filter capacitance needs to be increased for the
higher currents.
Linear Regulator
The linear regulator relies on a linear series component to
continuously drive the power to a load. The series component is
considered a load, and the voltage drop between the input and
output represents the power loss. The higher the
input-to-output voltage ratio, the lower the conversion
efficiency. In order to meet the voltage requirement, output
feedback to the control unit is commonly used to obtain an
accurate (and adjustable) voltage output.
For a linear regulator, converting a 5-V source to 3.3V results in
a 66% conversion efficiency and a 34% power loss (See Figure 2
on page 6). The efficiency of the conversion gets worse if the
output voltage is lower than 3.3V. The low dropout (LDO) linear
regulator is a reasonable solution for providing the processor
core voltage in systems that already support 3.3V from the
silver-box power supply or in systems converted from an
existing 3.3V design to a lower voltage.
Heat is an additional consideration. The voltage drop between
the input and output multiplied by the current supplied is the
power that must be dissipated by the regulator. For example,
when converting 5V to 2.2V at 6A, the power dissipated is (5V–
2.2V) 
 6A = 16.8W. Therefore, linear regulators often have
large heat sinks. This heat raises the ambient air temperature,