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Número de pieza AN1025
Descripción Converting A 5.0V Supply Rail To A Regulated 3.0V
Fabricantes Microchip 
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AN1025
Converting A 5.0V Supply Rail To A Regulated 3.0V
Author: Cliff Ellison
Microchip Technology Inc.
INTRODUCTION
As system designers are forced to produce products
with increased features while maintaining a flat or
decreasing product cost, advancements in device
technology must be considered. To produce Integrated
Circuits (IC) with increased functionality at a
reasonable cost, IC manufacturers need to reduce the
overall silicon area. However, the functional and cost
benefits associated with smaller areas can not be
achieved without some system design trade-offs.
These smaller geometry ICs typically have a maximum
voltage rating of 3.0V or below, instead of the existing
maximum 5.0V rating.
This application note is intended to provide the system
designer with an overview of different options that
could be used to down convert an existing 5.0V system
rail to a regulated 3.0V.
The approaches discussed in this application note are
the Low Dropout Regulator (LDO), charge pump and
buck switch mode converter. Other options exist, but
they do not provide a regulated 3.0V. A summary of
these options, as well as a reference section containing
detailed design application note titles and data sheets,
appears at the end of the document.
LOW DROPOUT REGULATOR
A simple way of converting the 5.0V bus voltage to the
required regulated 3.0V is by using a low dropout
regulator. An LDO is nothing more than a three terminal
linear system providing closed-loop control. The
solution is easy to implement, requiring only the device
itself and an input and output capacitor.
LDO Operation
In Figure 1, we can see that an LDO is built from four
main elements: 1) pass transistor, 2) bandgap
reference, 3) operational amplifier, and 4) feedback
resistors. An LDO can be thought of as a variable
resistor. The output voltage is divided down by the
resistor divider and compared to a fixed bandgap
reference voltage. The operational amplifier controls
the drive to the pass transistor accordingly to equalize
the voltage on its inputs. The difference between the
bus voltage and the required output voltage is dropped
across the pass transistor. When the pass transistor,
shown as a P-Channel MOSFET, is turned fully ON,
there will be some finite amount of resistance and
therefore a voltage drop. This minimum voltage drop,
VDROPOUT, will set how much higher the bus voltage
needs to be when compared to the output voltage in
order to regulate the output.
Designing With An LDO
Generating a well regulated 3.0V output is very easy
with an LDO. There are just a couple of specifications
that the circuit designer should take into consideration
when using an LDO. One specification is the output
voltage. Many LDOs are supplied in standard fixed out-
put voltages which typically include 3.0V. However,
some LDOs are offered with an adjustable output volt-
age. This requires the designer to use an external feed-
back resistor divider.
Another LDO specification is the typical dropout
voltage at load. The sum of the output voltage and the
typical dropout voltage must be less than the minimum
input voltage. If the sum is greater, the LDO will not be
able to regulate the output at minimum input voltages.
A very important specification that should not be over
looked is the requirements that some LDOs place on
the output capacitor. Certain LDOs require the output
capacitor to be either tantalum or aluminum electrolytic
to produce a stable system. These capacitors have a
large Equivalent Series Resistance (ESR) when
compared to ceramic capacitors. Tantalum or
aluminum electrolytic capacitors are normally cheaper
than ceramic capacitors when a large value of
capacitance is needed, but they are also usually larger
in size.
© 2006 Microchip Technology Inc.
DS01025A-page 1
Free Datasheet http://www.datasheet4u.com/

1 page




AN1025 pdf
VIN CIN
Q1
Switch Control
and Oscillator
Q2
L1
AN1025
COUT
RL
FIGURE 5:
Synchronous Buck Converter.
SUMMARY
This application note has provided the system designer
with an overview of different options used to produce a
regulated 3.0V from a 5.0V system rail. Key highlights
of each option were discussed, but often it is important
to compare the advantages of one particular solution
over another.
As a system designer, an LDO might be chosen
because of its lower cost, smaller size, ease-of-use, or
low system noise generation. However, under certain
conditions, the extra power that needs to be dissipated
in an LDO might over shadow these advantages.
The biggest advantage of using charge pumps is no
inductor is required. Regulation is accomplished by
transferring charge from the fly capacitor to the output.
The low output current capability of a charge pump
might prohibit a charge pump from being chosen for
heavy load applications.
A buck switch mode converter offers the advantages of
being the highest efficiency when VIN to much greater
than VOUT and capable of suppling higher output
current levels. With the integration of the MOSFETs
and control circuitry into a buck regulator IC, designing
a buck converter is relatively simple to accomplish.
However, an inductor and output capacitor are required
causing the parts count to be slightly higher than other
options.
Deciding which option to use when converting an exist-
ing 5.0V system rail to a regulated 3.0V ultimately lays
with the specific application requirements.
REFERENCES
MCP1601 Data Sheet, “500 mA Synchronous BUCK
Regulator”, DS21762, Microchip Technology Inc., 2003
MCP1612 Data Sheet, “Single 1A, 1.4 MHz Synchro-
nous Buck Regulator”, DS21921, Microchip
Technology Inc., 2005
TC1303A/TC1303B — TC1303C/TC1304 Data Sheet,
“500 mA Synchronous Buck Regulator, + 300 mA LDO
with Power-Good Output”, DS21949, Microchip
Technology Inc., 2005
MCP1252/53 Data Sheet, “Low Noise, Positive-Regu-
lated Charge Pump”, DS21752, Microchip Technology
Inc., 2002
MCP1700 Data Sheet, “Low Quiescent Current LDO”,
DS21826, Microchip Technology Inc., 2003
TC1017 Data Sheet, “150 mA, Tiny CMOS LDO With
Shutdown”, DS21813, Microchip Technology Inc., 2005
AN793 Application Note, “Power Management in Por-
table Applications: Understanding the Buck Switch
Mode Power Converter”, DS00793, Microchip Technol-
ogy Inc., 2001
AN968 Application Note, “Simple Synchronous Buck
Converter Design - MCP1612”, DS00968, Microchip
Technology Inc., 2005
AN960 Application Note, “New Components and
Design Methods Bring Intelligence to Battery Charger
Applications”, DS00960, Microchip Technology Inc.,
2004
MCP1601 Buck Regulator Evaluation Board,
MCP1601EV, Microchip Technology Inc., 2004
MCP1612 Synchronous Buck Regulator Evaluation
Board, MCP1612EV, Microchip Technology Inc., 2005
TC1303B Buck Regulator LDO Demo Board,
TC1303BDM-DDBK1, Microchip Technology Inc., 2005
TC1016/17 LDO Linear Regulator Evaluation Board,
TC1016/17EV, Microchip Technology Inc.,2005
© 2006 Microchip Technology Inc.
DS01025A-page 5
Free Datasheet http://www.datasheet4u.com/

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