Intel 855PM Design Manual page 99

R
temperature of a device, such as a processor power delivery circuit-switching transistor, is a balance of
heat being generated by the device and its ability to shed heat either through radiation into the
surrounding air or by conduction into the circuit board. Increased power will effectively raise the
temperature of the processor power delivery circuits. Switching transistor die temperatures can exceed
the recommended operating value if the heat cannot be removed from the package effectively.
As the current demands for higher frequency and performance processors increases, the amount of
power dissipated, i.e., heat generated, in the processor power delivery circuit has become of concern for
mobile system, thermal, and electrical design engineers. The high input voltage, low duty factor inherent
in mobile power supply designs leads to increasing power dissipation losses in the output stage of the
traditional buck regulator topology used in the mobile industry today.
These losses can be attributed to three main areas of the processor power delivery circuit. The switching
MOSFET dissipates a significant amount of power during switching of the top control MOSFET, power
dissipation resulting from drain to source resistance (R
MOSFET, and the power dissipation generated through the magnetic core and windings of the main
power inductor.
There has been significant improvement in the switching MOSFET technology to lower gate charge of
the control MOSFET allowing them to switch faster thus reducing switching losses. Improvements in
lowering the R
Direct Current Resistance (DCR) of the power inductor has been reduced, as well, to lower the amount
of power dissipation in the circuit's magnetic.
These technology improvements by themselves are not sufficient to effectively remove the heat
generated during the high current demand and tighter voltage regulation required by today's mobile
processors. There are several mechanisms for effectively removing heat from the package of these
integrated devices. Some of the most common methods are listed below.
Attaching a heat spreader or heat pipe to the package with a low thermal co-efficient bonding
material
Adding and/or increasing the copper fill area attached to high current carrying leads
Adding or re-directing air flow to flow across the device
Utilize multiple devices in parallel, as allowed, to reduce package power dissipation
Utilizing newer/enhanced technology and devices to lower heat generation but with equal or better
performance.
For the mobile designer, these options are not always available or economically feasible. The most
effective method of thermal spreading and heat removal, from these devices, is to generate airflow
across the package AND add copper fill area to the current carrying leads of the package.
The processor power delivery topology can also be modified to improve the thermal spreading
characteristic of the circuit and dramatically reduce the power dissipation requirements of the switching
MOSFET and inductor. This topology referred to as multi-phase, provides an output stage of the
processor regulator consisting of several smaller buck inductor phases that are summed together at the
processor. Each phase can be designed to handle and source a much smaller current. This can reduce the
size, quantity, and rating of the components needed in the design. This can also decrease the cost and
PCB area needed for the total solution. The implementation options for this topology are discussed in
the next section.
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Intel 855PM Chipset Platform Design Guide
parametric of the synchronous MOSFET have resulted in reduced DC losses. The
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Platform Power Requirements
) DC losses across the bottom synchronous
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