© 2006 Microchip Technology Inc.

DS01025A-page 1

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,

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