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Axelite Confidential Materials, do not copy or distribute without written consent.

Rev.1.7 Sep.19, 2019

AX1008



FUNCTION DESCRIPTIONS

Introduction

The AX1008 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-1.5V. The AX1008 only needs 1.5V 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 AX1008 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 AX1008 can be programmed to any voltages in the range of 1.25V to VIN-1.5V 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 = 50uA typically R1 and R2 as shown at below

AX1008

VIN

VOUT

VREF

Adj

R1

R2

Iadj=50uA

The AX1008 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

AX1008 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 50uA 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 AX1008 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