AN1769 Freescale Semiconductor / Motorola, AN1769 Datasheet - Page 34

• Manufacturer Part Number: AN1769
• Description: Designing a Minimal PowerPC System
• Manufacturer: Freescale Semiconductor / Motorola

Part 6 Power
In order to increase speeds without excessive heat loss, the newest, fastest PowerPC processors have cores
which operate at low voltages. To remain compatible with external devices, the I/O cells have remained at
3.3V. This increases the complexity of a system somewhat by requiring multiple voltages levels.
Furthermore, as transistor counts rise in the processors, the static and transient current demands of the power
supplies rise as well. Consequently, a well-designed, quiet and responsive power supply is a critical Þrst step
to a well-designed PowerPC-based system. There are many ways to derive power, ranging from batteries to
radioisotope-thermocoupled generators. The two most popular methods are linear supplies and switching
supplies, which are considered in further detail in sections 6.1 and 6.2.
6.1 Linear Regulators
Linear regulators operate by dissipating unwanted energy in the form of heat. With proper thermal
management, linear regulators are very easy to design, inexpensive, and provide quiet, stable outputs. The
disadvantages are the heat and the inability to generate higher voltages. Figure 22 shows an example of a
linear 2.5-V power supply, similar to that used on the Excimer board.
6.2 Switchmode Regulators
An alternate method of providing other voltages is to use a switching power supply. These devices can
efÞciently convert high-voltage, low-current energy into low-voltage, high-current energy by storing it
magnetically in an inductor. Switching power supplies require more complicated logic and careful design,
but the rewards are that the efÞciencies are high and that thermal dissipation is of little issue. Because
switchers work basically by periodically dumping energy into a low-impedance load, clocking noise and
transient effects can make for a noisy supply unless components are carefully selected. Figure 23 shows an
example of a switching power supply.
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+5.0V
Freescale Semiconductor, Inc.
For More Information On This Product,
Figure 22. Excimer Linear Power Supply
Minimal PowerPC System Design
0.1µF
Go to: www.freescale.com
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4
Linear Tech
LT1584CT
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0.1µF
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1
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MOTOROLA