Microchip MCP1825S Handleiding

Microchip Niet gecategoriseerd MCP1825S

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AN1025

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.

Author: Cliff Ellison

Microchip Technology Inc.

Converting A 5.0V Supply Rail To A Regulated 3.0V

AN1025

FIGURE 1: Basic LDO System Schematic.

Understanding LDO IGND Specifications

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.

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:

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.

FIGURE 2: LDO Efficiency Comparison.

System line and load step performance is greatly

improved on LDOs that have higher I

q. 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.

VREF

IGND

IOUT

IIN

CIN COUT RL

VIN

Efficiency

VOUT

VIN

----------------=

When: IGND << IOUT

0.01 0.10 1.00 10.00 100.00

Output Current (mA)

Efficiency (%)

MCP1700

TC1017

VIN = 5.0V

VOUT = 3.0V

AN1025

CHARGE PUMP

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.

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.

Regulated Buck Charge Pump Operation

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.

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

FLY 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.

FIGURE 3: MCP1252/3 Charge Pump System Schematic.

SW3 SW4

SW2SW1

CFLY

VREF

Switch Control

and Oscillator

VIN

COUT RL

CIN

Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	MCP1825S

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