Microchip AN1025 Converting a 5.0v supply rail to a regulated 3.0v Datasheet

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.
© 2006 Microchip Technology Inc.
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 output voltages which typically include 3.0V. However,
some LDOs are offered with an adjustable output voltage. This requires the designer to use an external feedback 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.
DS01025A-page 1
AN1025
IIN
VIN
IOUT
VREF
COUT
CIN
RL
IGND
FIGURE 1:
Basic LDO System Schematic.
Understanding LDO IGND Specifications
An LDO can form a very efficient step-down regulator.
When the LDO output current is much greater than the
device quiescent current, the system efficiency is found
by dividing the output voltage by the input voltage. This
is shown in Equation 1.
EQUATION 1:
V OUT
Efficiency = ---------------V
IN
When: IGND << IOUT
70
60
Efficiency (%)
There are three current elements, IIN, IOUT and IGND,
labeled in Figure 1. IGND is the current used by the LDO
to perform the regulating operation and is often referred
to as the quiescent current (Iq) for no load conditions.
Since the specified Iq varies greatly depending on the
specific LDO or particular manufacture, it is important
to understand how this one specification impacts the
system performance.
MCP1700
50
40
30
20
TC1017
VIN = 5.0V
VOUT = 3.0V
10
0
0.01
0.10
1.00
10.00
100.00
Output Current (mA)
FIGURE 2:
LDO Efficiency Comparison.
System line and load step performance is greatly
improved on LDOs that have higher Iq. Since the Iq is
used by the LDO to preform the regulating operation, it
can respond quicker to a sudden change in load
requirements or line voltage.
System efficiency at lighter load currents is one of the
impacts Iq has on the system performance. In basic
terms, an LDO with a low Iq will only be more efficient
at lighter loads. This is because as the load current
increases, the Iq is only a small percentage of the total
IIN. The efficiency of two Microchip LDOs, the
MCP1700 and TC1017, is shown in Figure 2. Notice
how the efficiency of the MCP1700 is vastly greater
than the TC1017 at light loads since the TC1017 has a
higher IQ.
DS01025A-page 2
© 2006 Microchip Technology Inc.
AN1025
CHARGE PUMP
Regulated Buck Charge Pump Operation
A charge pump is another regulator topology that can
be used to convert a 5.0V system rail voltage down to
a regulated 3.0V to be used by microcontrollers or
other logic. Charge pumps, also referred to as an
inductor-less DC-DC converter or a switched-capacitor
circuit, are just as easy to use as LDOs. Like an LDO,
a charge pump requires an input and output capacitor
and a feedback resistor divider network. However,
charge pumps require an additional charge storing
capacitor which is sometimes referred to as a fly
capacitor.
Microchip’s MCP1252/3 is a positive regulated charge
pump that, like most charge pumps, uses four
MOSFET switches to control the charge and discharge
of the fly capacitor and thereby regulates the output
voltage. However, unlike most charge pumps, the
MCP1252/3 allows for the source voltage to be lower or
higher that the output voltage by automatically
switching between buck/boost operation. For the
purpose of this application note, the Buck mode is the
only operating state that is discussed. Refer to the
MCP1252/3 Data Sheet (DS21752) for a full
description of the buck/boost operation.
There are many different types of charge pumps. Some
of the more common types are: voltage inverting,
voltage doubling, regulated buck, regulated boost and
regulated buck/boost. The regulated buck charge
pump is the only type that is discussed in this
application note. For information on the other types of
charge pumps, refer to the Microchip web site at
www.microchip.com.
In Figure 3, it can be seen that the internal comparator
U1, determines which mode the MCP1252/3 operates
in. While in Buck mode, the positive input node is
greater than the negative input node, switch SW1 is
always closed, and SW2 is always open. When the
MCP1252/3 is not in Shutdown mode and a steadystate condition has been reached, there are three
phases of operation. During the first phase, charge is
transferred from the input source to CFLY by closing
switch SW3 for half of the internal oscillator period.
Once the first phase is complete, all switches are
opened and the second phase (idle phase) is entered.
The MCP1252/3 compares the reference voltage,
VREF, with the feedback voltage. If the feedback voltage
is below the regulation point, the device transitions to
the third phase. The third phase transitions charge from
CFLY to the output capacitor, COUT, and the load by
closing switch SW4. If regulation is maintained, the
device returns to the idle phase. If the charge transfer
occurs for half of the internal oscillator period, more
charge is needed in CFLY and the MCP1252/3
transitions back to the first phase.
CFLY
SW3
VIN
SW4
U1
SW1
CIN
SW2
COUT
RL
Switch Control
and Oscillator
U2
VREF
FIGURE 3:
MCP1252/3 Charge Pump System Schematic.
© 2006 Microchip Technology Inc.
DS01025A-page 3
AN1025
Designing with a Charge Pump
Output voltage ripple and charge pump strength are
affected by the style and value of the capacitors used.
Typically, low ESR capacitors should be used for the
input and output capacitors. This helps minimize noise
and ripple in the system.
The value of the input capacitor is somewhat dictated
by the system voltage supply. If the source impedance
to the charge pump is very low, the input capacitor
might not be needed. However, if there is a large
source impedance, an input capacitor is needed to help
prevent ripple on the input voltage pin.
Output voltage ripple is controlled by the amount of
capacitance in the output capacitor. Large values of
output capacitance will reduce the output ripple at the
expense of a slower turn-on time from shutdown and a
higher in-rush current.
The fly capacitor controls the strength of the charge
pump. However, care must be taken when selecting the
value of this capacitor. Recall that the maximum charge
time for the fly capacitor is one half the charge pump
oscillator frequency and when charging, it is in series
with the ON resistance of two switches. The charging
time constant of this RC circuit should be less than the
maximum charge time.
VOUT
Q1
L1
VIN
D1
CIN
FIGURE 4:
Schematic.
COUT
RL
Buck Regulator System
Understanding the operation of the buck converter and
realizing that the volt-time across the inductor in the ON
time must equal the inductor volt-time in the OFF time
allows a relationship between the input voltage and
output voltage to be established. This input to output
voltage relationship is shown in Equation 2.
EQUATION 2:
V
OUT
DutyCycle = ---------------V IN
Where:
Duty Cycle
=
tON / (tON + tOFF)
BUCK SWITCHING REGULATOR
Synchronous Buck Converters
One of the simplest switch mode converters is the buck
converter. The buck converter is an inductor-based
converter used to step-down an input voltage to a lower
magnitude output voltage. It is similar to the LDO circuit
previously discussed, but with one main difference.
Instead of the pass transistor that functions as a
variable resistor in the LDO, the MOSFET in a buck
converter is either ON or OFF. The regulation of the
output voltage is achieved by controlling the ON and
OFF time of this MOSFET. This allows the buck
regulator to convert a high source voltage to a
regulated lower output voltage efficiently.
When a buck converter is used to generate low output
voltages, the recirculating diode, D1 in Figure 4, can be
replaced with another MOSFET and is switched out-ofphase with the main MOSFET. By doing so, the overall
system efficiency is improved. For example, a buck
converter is used to generate an output voltage of 3.0V
and D1 has a forward voltage drop, VFD, of 0.75V.
There would be approximately an initial 25% decrease
in the buck converters maximum efficiency because of
the diode’s VFD. The efficiency degradation would be
worse with a lower output voltage.
Buck Converter Operation
A basic buck regulator schematic is shown in Figure 4.
A typical buck regulator consist of a switching
MOSFET, an inductor, output capacitor and a
recirculating diode. During a switching cycle, the
MOSFET, Q1, transitions between an ON state and an
OFF state. Assume the buck regulator is operating in
steady-state and Q1 is in the ON state. The voltage
across the inductor, L1, is equal to the input voltage,
VIN, minus the output voltage, VOUT. Energy is being
stored in L1. At the end of the ON time, tON, Q1
transitions to an OFF state. The voltage across L1
collapses, changing polarity to a value equal to -VOUT.
The energy in L1 is now decreasing and suppling the
output requirements. Q1 remains OFF until the end of
the period. This complete cycle is then repeated.
DS01025A-page 4
Microchip offers a number of synchronous buck
converter regulators. Devices like the MCP1601 or
MCP1612 integrate both the main switching MOSFET
and the synchronous MOSFET. Figure 5 shows an
adjustable output voltage, synchronous buck converter.
The items in the dashed box are contained within the
buck IC. Another Microchip device, the TC1303,
contains both a synchronous buck regulator with
integrated MOSFETs and an LDO.
© 2006 Microchip Technology Inc.
AN1025
L1
Q1
VIN
COUT
CIN
RL
Q2
Switch Control
and Oscillator
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.
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-Regulated 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 Portable Applications: Understanding the Buck Switch
Mode Power Converter”, DS00793, Microchip Technology Inc., 2001
AN968 Application Note, “Simple Synchronous Buck
Converter Design - MCP1612”, DS00968, Microchip
Technology Inc., 2005
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.
AN960 Application Note, “New Components and
Design Methods Bring Intelligence to Battery Charger
Applications”, DS00960, Microchip Technology Inc.,
2004
Deciding which option to use when converting an existing 5.0V system rail to a regulated 3.0V ultimately lays
with the specific application requirements.
TC1303B Buck Regulator LDO Demo Board,
TC1303BDM-DDBK1, Microchip Technology Inc., 2005
REFERENCES
MCP1601 Buck Regulator Evaluation Board,
MCP1601EV, Microchip Technology Inc., 2004
MCP1612 Synchronous Buck Regulator Evaluation
Board, MCP1612EV, Microchip Technology Inc., 2005
TC1016/17 LDO Linear Regulator Evaluation Board,
TC1016/17EV, Microchip Technology Inc.,2005
MCP1601 Data Sheet, “500 mA Synchronous BUCK
Regulator”, DS21762, Microchip Technology Inc., 2003
MCP1612 Data Sheet, “Single 1A, 1.4 MHz Synchronous Buck Regulator”, DS21921, Microchip
Technology Inc., 2005
© 2006 Microchip Technology Inc.
DS01025A-page 5
AN1025
NOTES:
DS01025A-page 6
© 2006 Microchip Technology Inc.
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