AN1025_技术文档

AN1025

Converting A 5.0V Supply Rail To A Regulated 3.0V

LDO Operation

Author:

Cliff Ellison

Microchip Technology Inc.

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,

V_DROPOUT, will set how much higher the bus voltage

needs to be when compared to the output voltage in

order to regulate the output.

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.

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.

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

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 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.

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

I_IN

I_OUT

V_REF

V_IN

C_OUT

R_L

C_IN

I_GND

FIGURE 1:

Basic LDO System Schematic.

Understanding LDO I

Specifications

GND

70

60

50

40

30

20

10

0

There are three current elements, I_IN, I_OUTand I_GND

,

MCP1700

labeled in Figure 1. I_GNDis the current used by the LDO

to perform the regulating operation and is often referred

to as the quiescent current (I_q) for no load conditions.

Since the specified I_qvaries greatly depending on the

specific LDO or particular manufacture, it is important

to understand how this one specification impacts the

system performance.

TC1017

V_IN= 5.0V

V_OUT= 3.0V

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.

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 I_q. Since the I_qis

used by the LDO to preform the regulating operation, it

can respond quicker to a sudden change in load

requirements or line voltage.

EQUATION 1:

V

OUT

Efficiency = ----------------

V

IN

When: I_GND<< I_OUT

System efficiency at lighter load currents is one of the

impacts I_qhas on the system performance. In basic

terms, an LDO with a low I_qwill only be more efficient

at lighter loads. This is because as the load current

increases, the I_qis only a small percentage of the total

I_IN. 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 I_Q.

DS01025A-page 2

AN1025

Regulated Buck Charge Pump Operation

CHARGE PUMP

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

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.

MCP1252/3 Data Sheet (DS21752) for

description of the buck/boost operation.

a

full

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 steady-

state condition has been reached, there are three

phases of operation. During the first phase, charge is

transferred from the input source to C_FLYby 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,

V_REF, 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

C_FLYto the output capacitor, C_OUT, 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 C_FLYand the MCP1252/3

transitions back to the first phase.

C_FLY

SW3

SW4

SW2

U₁

V_IN

SW1

C_IN

C_OUT

R_L

Switch Control

and Oscillator

U₂

V_REF

FIGURE 3:

MCP1252/3 Charge Pump System Schematic.

DS01025A-page 3

AN1025

Designing with a Charge Pump

V_OUT

Q₁

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.

L₁

C_OUT

V_IN

R_L

D₁

C_IN

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.

FIGURE 4:

Schematic.

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.

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.

EQUATION 2:

V

OUT

DutyCycle = ----------------

V

IN

Where:

Duty Cycle

=

t_ON/ (t_ON+ t_OFF)

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, D₁in Figure 4, can be

replaced with another MOSFET and is switched out-of-

phase 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 D₁has a forward voltage drop, V_FD, of 0.75V.

There would be approximately an initial 25% decrease

in the buck converters maximum efficiency because of

the diode’s V_FD. The efficiency degradation would be

worse with a lower output voltage.

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.

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 Q₁is in the ON state. The voltage

across the inductor, L₁, is equal to the input voltage,

V_IN, minus the output voltage, V_OUT. Energy is being

stored in L₁. At the end of the ON time, t_ON, Q₁

transitions to an OFF state. The voltage across L₁

collapses, changing polarity to a value equal to -V_OUT

.

The energy in L₁is now decreasing and suppling the

output requirements. Q₁remains OFF until the end of

the period. This complete cycle is then repeated.

DS01025A-page 4

AN1025

L₁

Q₁

C_OUT

V_IN

C_IN

R_L

Q₂

Switch Control

and Oscillator

FIGURE 5:

Synchronous Buck Converter.

TC1303A/TC1303B — TC1303C/TC1304 Data Sheet,

“500 mA Synchronous Buck Regulator, + 300 mA LDO

with Power-Good Output”, DS21949, Microchip

Technology Inc., 2005

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.

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

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.

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

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.

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 V_INto much greater

than V_OUTand 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

MCP1601 Buck Regulator Evaluation Board,

MCP1601EV, Microchip Technology Inc., 2004

MCP1612 Synchronous Buck Regulator Evaluation

Board, MCP1612EV, Microchip Technology Inc., 2005

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.

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

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

DS01025A-page 5

AN1025

NOTES:

DS01025A-page 6

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DS01025A-page 8


型号：	AN1025
厂家：	MICROCHIP
描述：	Converting A 5.0V Supply Rail To A Regulated 3.0V 转换5.0V电源轨为稳定的3.0V
文件：	总8页 (文件大小：528K)
中文：	中文翻译
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AN1025 [MICROCHIP]

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