AP1084

5A Low Dropout Positive Adjustable or Fixed-Mode Regulator

Anachip Corp.

www.anachip.com.tw

Rev.1.0 Aug.27, 2004

4/10

Functional Description

Introduction

The AP1084 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 1.25 to

5V. The AP1084 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 AP1084 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 AP1084 can be programmed to any voltages in

the range of 1.25V to 5V 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

AP1084

Vin

Vout

Vref

Adj

R1

R2

Iadj=55uA

The AP1084 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 AP1084 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 AP1084 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 AP1084 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.