AN2433 Freescale Semiconductor / Motorola, AN2433 Datasheet - Page 21

• Manufacturer Part Number: AN2433
• Description: 5V to 3V Design Considerations
• Manufacturer: Freescale Semiconductor / Motorola

Power Supplies for Low Voltage Systems
Linear Voltage
Regulators
MOTOROLA
The challenge is to provide a power supply that will satisfy the new technology
operating at 3.3 V and also provide power for legacy 5 V devices. The 3.3 V
power supply may be either a switching or a linear design. For most
applications, a linear regulated power supply minimizes the number of
additional components.
Linear voltage regulators are the simplest to design, as they do not require any
magnetic components and are much more forgiving from a standpoint of
printed circuit layout and grounding requirements. Linear voltage regulators are
not as efficient as switching voltage regulators. The linear voltage regulator
operates by reducing a higher input voltage to a lower output voltage by linearly
controlling the conduction of a series pass power device in response to
changes in the output load. This results in a voltage being dropped across the
series pass device with the load current passing through it. Because of the
voltage drop across the series pass device, the linear voltage regulator may be
only 30 to 50 percent efficient. Linear voltage regulators become somewhat
uneconomical above 10 W due to a significant increase in the heat sink
requirements.
For a hybrid 5 V–3 V system it is reasonable to use a 5 V regulator, which
supplies 5 V logic, and derive the 3.3 V for the rest of the system from it. As
there are only 1.7V of drop between the 5 V power supply and the 3.3 V power
supply, a low dropout regulator is a necessity. Typically low current linear
voltage regulators supplying less than 500 mA use PNP pass transistors. A
PNP pass transistor linear voltage regulator usually requires about 500 mV of
dropout. Modern designed low dropout voltage regulators utilize FET
transistors as the pass transistor, yielding dropout voltages of less than 100
mV. Higher current output linear voltage regulators typically use an NPN pass
transistor and their dropout voltage is typically greater than 1 V. An advantage
of deriving the 3.3 V necessary for the system from the 5 V regulated power
supply is the increased ripple rejection from the 5 V pre-regulation device.
A number of semiconductor suppliers manufacture both adjustable and fixed
linear voltage regulators. The following table is a partial list of 3.3 V linear
regulators:
Freescale Semiconductor, Inc.
For More Information On This Product,
5V to 3V Design Considerations
Go to: www.freescale.com
Power Supplies for Low Voltage Systems
AN2433/D
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