01-Jun-2002 Rev. A

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Elektronische Bauelemente

Functional Description

Introduction

The SJ1084 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 2.5V. The SJ1084 only needs 1.5V differential

between Vin and Vout to maintain output regulation, the output voltage tolerances are also extremely tight and they include

the transient response as port 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 SJ1084 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 SJ1084 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:

The SJ1084 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 requirement of the GJ1084 is 10mA, R1 is typically

selected to be 121 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=121 and R2=200 the

error due to Iadj is only 0.3% of the nominal set point.

Load Regulation

Since the SJ1084 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 SJ1084 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 SJ1084 incorporates an internal shutdown that protects the device when the junction temperature exceeds the

maximum allowable junction temperatures. Although this device can operate with junction temperatures in the range of

150 ,

:::: it is 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.

SJ1084ADJ

SJ1084

5A Low Dropout Positive

Adjustable or Fixed-Mode Regulator