APE1084

5A Low Dropout Positive Adjustable or Fixed-Mode Regulator

Advanced Power Electronics Corp.

www.a-power.com.tw

5/1

0

Pb Free Plating Product

Functional Description

Introduction

The APE1084 adjustable Low Dropout (LDO) regulator

is a 3 terminal device that can easily be programmed

with the addition of two external resistors to any

voltages within the range of 1.25V to Vin-1.4V. The

APE1084 only needs 1.4V differential between Vin

and Vout to maintain output regulation. In addition, the

output voltage tolerances are also extremely tight and

they include the transient response as part of the

specification. For example, Intel VRE specification

calls for a total of +/- 100mV including initial tolerance,

load regulation and 0 to 5.0A load step.

The APE1084 is specifically designed to meet the fast

current transient needs as well as providing an

accurate initial voltage, reducing the overall system

cost with the need for fewer output capacitors.

Output Voltage Setting

The APE1084 can be programmed to any voltages in

the range of 1.25V to Vin-1.4V with the addition of R1

and R2 external resistors according to the following

formula:

Vout = Vref (1+ R2/R1) +Iadj *R2 , where Vref = 1.25 typically, Iadj =

55uA typically R1&R2 as shown at below

APE1084

Vin

Vout

Vref

Adj

R1

R2

Iadj=55uA

The APE1084 keeps a constant 1.25V between the

output pin and the adjust pin. By placing a resistor R1

across these two pins a constant current flows through

R1, adding to the Iadj current and into the R2 resistor

producing a voltage equal to the (1.25/R1)*R2+Iadj*R2

which will be added to the 1.25V to set the output

voltage. This is summarized in the above equation.

Since the minimum load current requirement of the

APE1084 is 10mA, R1 is typically selected to be 121Ω

resistor so that it automatically satisfies the minimum

current requirement. Notice that since Iadj is typically

in the range of 55uA it only adds a small error to the

output voltage and should only be considered when a

very precise output voltage setting is required. For

example, in a typical 3.3V application where R1=121Ω

and R2=200Ω the error due to Iadj is only 0.3% of the

nominal set point.

Load Regulation

Since the APE1084 is only a 3 terminal device, it is not

possible to provide true remote sensing of the output

voltage at the load. The best load regulation is

achieved when the bottom side of R2 is connected to

the load and the top-side of R1 resistor is connected

directly to the case or the Vout pin of the regulator and

not to the load. It is important to note that for high

current applications, this can re-present a significant

percentage of

the overall load regulation and one must keep the path

from the regulator to the load as short as possible to

minimize this effect.

Stability

The APE1084 requires the use of an output capacitor

as part of the frequency compensation in order to

make the regulator stable. For most applications a

minimum of 10uF aluminum electrolytic capacitor

insures both stability and good transient response.

Thermal Design

The APE1084 incorporates an internal thermal

shutdown that protects the device when the junction

temperature exceeds the maximum allowable junction

temperature. Although this device can operate with

junction temperatures in the range of 150

recommended that the selected heat sink be chosen

such that during maximum continuous load operation

the junction temperature is kept below the temperature.

Layout Consideration

The output capacitors must be located as close to the

Vout terminal of the device as possible. It is

recommended to use a section of a layer of the PC

board as a plane to connect the Vout pin to the output

capacitors to prevent any high frequency oscillation

that may result due to excessive trace inductance.